Merge pull request #55 from uwiger/uw-standby-monitors

Uw standby monitors

README.md

@@ -77,8 +77,8 @@ alternative sources, and cache them for efficient lookup. Caching also provides
 a way to see which processes rely on certain configuration values, as well as
 which values they actually ended up using.
 
-See [`gproc:get_env/4`](http://github.com/esl/gproc/blob/master/doc/gproc.md#get_env-4), [`gproc:get_set_env/4`](http://github.com/esl/gproc/blob/master/doc/gproc.md#get_set_env-4) and
-[`gproc:set_env/5`](http://github.com/esl/gproc/blob/master/doc/gproc.md#set_env-5) for details.
+See [`gproc:get_env/4`](http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc.md#get_env-4), [`gproc:get_set_env/4`](http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc.md#get_set_env-4) and
+[`gproc:set_env/5`](http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc.md#set_env-5) for details.
 
 
 ## Testing ##
@@ -98,23 +98,23 @@ global gproc.
 By default, `./rebar doc` generates Github-flavored Markdown files.
 If you want to change this, remove the `edoc_opts` line from `rebar.config`.
 Gproc was first introduced at the ACM SIGPLAN Erlang Workshop in
-Freiburg 2007 ([Paper available here](http://github.com/esl/gproc/blob/master/doc/erlang07-wiger.pdf)).
+Freiburg 2007 ([Paper available here](http://github.com/esl/gproc/blob/uw-standby-monitors/doc/erlang07-wiger.pdf)).
 
 
 ## Modules ##
 
 
 <table width="100%" border="0" summary="list of modules">
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc.md" class="module">gproc</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_app.md" class="module">gproc_app</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_bcast.md" class="module">gproc_bcast</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_dist.md" class="module">gproc_dist</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_info.md" class="module">gproc_info</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_init.md" class="module">gproc_init</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_lib.md" class="module">gproc_lib</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_monitor.md" class="module">gproc_monitor</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_pool.md" class="module">gproc_pool</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_ps.md" class="module">gproc_ps</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_pt.md" class="module">gproc_pt</a></td></tr>
-<tr><td><a href="http://github.com/esl/gproc/blob/master/doc/gproc_sup.md" class="module">gproc_sup</a></td></tr></table>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc.md" class="module">gproc</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_app.md" class="module">gproc_app</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_bcast.md" class="module">gproc_bcast</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_dist.md" class="module">gproc_dist</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_info.md" class="module">gproc_info</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_init.md" class="module">gproc_init</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_lib.md" class="module">gproc_lib</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_monitor.md" class="module">gproc_monitor</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_pool.md" class="module">gproc_pool</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_ps.md" class="module">gproc_ps</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_pt.md" class="module">gproc_pt</a></td></tr>
+<tr><td><a href="http://github.com/esl/gproc/blob/uw-standby-monitors/doc/gproc_sup.md" class="module">gproc_sup</a></td></tr></table>
 

doc/gproc.md

@@ -163,6 +163,18 @@ keypat() = {<a href="#type-sel_type">sel_type()</a> | <a href="#type-sel_var">se
 
 
 
+### <a name="type-monitor_type">monitor_type()</a> ###
+
+
+
+<pre><code>
+monitor_type() = info | standby | follow
+</code></pre>
+
+
+
+
+
 ### <a name="type-pidpat">pidpat()</a> ###
 
 
@@ -282,8 +294,8 @@ unique_id() = {n | a, <a href="#type-scope">scope()</a>, any()}
 <table width="100%" border="1" cellspacing="0" cellpadding="2" summary="function index"><tr><td valign="top"><a href="#add_global_aggr_counter-1">add_global_aggr_counter/1</a></td><td>Registers a global (unique) aggregated counter.</td></tr><tr><td valign="top"><a href="#add_global_counter-2">add_global_counter/2</a></td><td>Registers a global (non-unique) counter.</td></tr><tr><td valign="top"><a href="#add_global_name-1">add_global_name/1</a></td><td>Registers a global (unique) name.</td></tr><tr><td valign="top"><a href="#add_global_property-2">add_global_property/2</a></td><td>Registers a global (non-unique) property.</td></tr><tr><td valign="top"><a href="#add_local_aggr_counter-1">add_local_aggr_counter/1</a></td><td>Registers a local (unique) aggregated counter.</td></tr><tr><td valign="top"><a href="#add_local_counter-2">add_local_counter/2</a></td><td>Registers a local (non-unique) counter.</td></tr><tr><td valign="top"><a href="#add_local_name-1">add_local_name/1</a></td><td>Registers a local (unique) name.</td></tr><tr><td valign="top"><a href="#add_local_property-2">add_local_property/2</a></td><td>Registers a local (non-unique) property.</td></tr><tr><td valign="top"><a href="#add_shared_local_counter-2">add_shared_local_counter/2</a></td><td>Registers a local shared (unique) counter.</td></tr><tr><td valign="top"><a href="#audit_process-1">audit_process/1</a></td><td></td></tr><tr><td valign="top"><a href="#await-1">await/1</a></td><td>Equivalent to <a href="#await-2"><tt>await(Key, infinity)</tt></a>.</td></tr><tr><td valign="top"><a href="#await-2">await/2</a></td><td>Wait for a local name to be registered.</td></tr><tr><td valign="top"><a href="#await-3">await/3</a></td><td>Wait for a local name to be registered on <code>Node</code>.</td></tr><tr><td valign="top"><a href="#bcast-2">bcast/2</a></td><td>Equivalent to <a href="#bcast-3"><tt>bcast(nodes(), Key, Msg)</tt></a>.</td></tr><tr><td valign="top"><a href="#bcast-3">bcast/3</a></td><td>Sends a message to processes corresponding to Key on Nodes.</td></tr><tr><td valign="top"><a href="#cancel_wait-2">cancel_wait/2</a></td><td>Cancels a previous call to nb_wait/1.</td></tr><tr><td valign="top"><a href="#cancel_wait-3">cancel_wait/3</a></td><td>Cancels a previous call to nb_wait/2.</td></tr><tr><td valign="top"><a href="#cancel_wait_or_monitor-1">cancel_wait_or_monitor/1</a></td><td></td></tr><tr><td valign="top"><a href="#default-1">default/1</a></td><td></td></tr><tr><td valign="top"><a href="#demonitor-2">demonitor/2</a></td><td>Remove a monitor on a registered name
 This function is the reverse of monitor/1.</td></tr><tr><td valign="top"><a href="#first-1">first/1</a></td><td>Behaves as ets:first(Tab) for a given type of registration.</td></tr><tr><td valign="top"><a href="#get_attribute-2">get_attribute/2</a></td><td>Get attribute value of <code>Attr</code> associated with <code>Key</code> for most likely Pid.</td></tr><tr><td valign="top"><a href="#get_attribute-3">get_attribute/3</a></td><td>Get the attribute value of <code>Attr</code> associated with <code>Key</code> for process Pid.</td></tr><tr><td valign="top"><a href="#get_attribute_shared-2">get_attribute_shared/2</a></td><td>Get the attribute value of <code>Attr</code> associated with the shared <code>Key</code>.</td></tr><tr><td valign="top"><a href="#get_attributes-1">get_attributes/1</a></td><td>Get attributes associated with registration.</td></tr><tr><td valign="top"><a href="#get_attributes-2">get_attributes/2</a></td><td>Returns the list of attributes associated with the registration.</td></tr><tr><td valign="top"><a href="#get_env-3">get_env/3</a></td><td>Equivalent to <a href="#get_env-4"><tt>get_env(Scope, App, Key, [app_env])</tt></a>.</td></tr><tr><td valign="top"><a href="#get_env-4">get_env/4</a></td><td>Read an environment value, potentially cached as a <code>gproc_env</code> property.</td></tr><tr><td valign="top"><a href="#get_set_env-3">get_set_env/3</a></td><td>Equivalent to <a href="#get_set_env-4"><tt>get_set_env(Scope, App, Key, [app_env])</tt></a>.</td></tr><tr><td valign="top"><a href="#get_set_env-4">get_set_env/4</a></td><td>Fetch and cache an environment value, if not already cached.</td></tr><tr><td valign="top"><a href="#get_value-1">get_value/1</a></td><td>Reads the value stored with a key registered to the current process.</td></tr><tr><td valign="top"><a href="#get_value-2">get_value/2</a></td><td>Reads the value stored with a key registered to the process Pid.</td></tr><tr><td valign="top"><a href="#get_value_shared-1">get_value_shared/1</a></td><td>Reads the value stored with a shared key.</td></tr><tr><td valign="top"><a href="#give_away-2">give_away/2</a></td><td>Atomically transfers the key <code>From</code> to the process identified by <code>To</code>.</td></tr><tr><td valign="top"><a href="#goodbye-0">goodbye/0</a></td><td>Unregister all items of the calling process and inform gproc
 to forget about the calling process.</td></tr><tr><td valign="top"><a href="#i-0">i/0</a></td><td>Similar to the built-in shell command <code>i()</code> but inserts information
-about names and properties registered in Gproc, where applicable.</td></tr><tr><td valign="top"><a href="#info-1">info/1</a></td><td>Similar to <code>process_info(Pid)</code> but with additional gproc info.</td></tr><tr><td valign="top"><a href="#info-2">info/2</a></td><td>Similar to process_info(Pid, Item), but with additional gproc info.</td></tr><tr><td valign="top"><a href="#last-1">last/1</a></td><td>Behaves as ets:last(Tab) for a given type of registration.</td></tr><tr><td valign="top"><a href="#lookup_global_aggr_counter-1">lookup_global_aggr_counter/1</a></td><td>Lookup a global (unique) aggregated counter and returns its value.</td></tr><tr><td valign="top"><a href="#lookup_global_counters-1">lookup_global_counters/1</a></td><td>Look up all global (non-unique) instances of a given Counter.</td></tr><tr><td valign="top"><a href="#lookup_global_name-1">lookup_global_name/1</a></td><td>Lookup a global unique name.</td></tr><tr><td valign="top"><a href="#lookup_global_properties-1">lookup_global_properties/1</a></td><td>Look up all global (non-unique) instances of a given Property.</td></tr><tr><td valign="top"><a href="#lookup_local_aggr_counter-1">lookup_local_aggr_counter/1</a></td><td>Lookup a local (unique) aggregated counter and returns its value.</td></tr><tr><td valign="top"><a href="#lookup_local_counters-1">lookup_local_counters/1</a></td><td>Look up all local (non-unique) instances of a given Counter.</td></tr><tr><td valign="top"><a href="#lookup_local_name-1">lookup_local_name/1</a></td><td>Lookup a local unique name.</td></tr><tr><td valign="top"><a href="#lookup_local_properties-1">lookup_local_properties/1</a></td><td>Look up all local (non-unique) instances of a given Property.</td></tr><tr><td valign="top"><a href="#lookup_pid-1">lookup_pid/1</a></td><td>Lookup the Pid stored with a key.</td></tr><tr><td valign="top"><a href="#lookup_pids-1">lookup_pids/1</a></td><td>Returns a list of pids with the published key Key.</td></tr><tr><td valign="top"><a href="#lookup_value-1">lookup_value/1</a></td><td>Lookup the value stored with a key.</td></tr><tr><td valign="top"><a href="#lookup_values-1">lookup_values/1</a></td><td>Retrieve the <code>{Pid,Value}</code> pairs corresponding to Key.</td></tr><tr><td valign="top"><a href="#monitor-1">monitor/1</a></td><td>monitor a registered name
-This function works much like erlang:monitor(process, Pid), but monitors
+about names and properties registered in Gproc, where applicable.</td></tr><tr><td valign="top"><a href="#info-1">info/1</a></td><td>Similar to <code>process_info(Pid)</code> but with additional gproc info.</td></tr><tr><td valign="top"><a href="#info-2">info/2</a></td><td>Similar to process_info(Pid, Item), but with additional gproc info.</td></tr><tr><td valign="top"><a href="#last-1">last/1</a></td><td>Behaves as ets:last(Tab) for a given type of registration.</td></tr><tr><td valign="top"><a href="#lookup_global_aggr_counter-1">lookup_global_aggr_counter/1</a></td><td>Lookup a global (unique) aggregated counter and returns its value.</td></tr><tr><td valign="top"><a href="#lookup_global_counters-1">lookup_global_counters/1</a></td><td>Look up all global (non-unique) instances of a given Counter.</td></tr><tr><td valign="top"><a href="#lookup_global_name-1">lookup_global_name/1</a></td><td>Lookup a global unique name.</td></tr><tr><td valign="top"><a href="#lookup_global_properties-1">lookup_global_properties/1</a></td><td>Look up all global (non-unique) instances of a given Property.</td></tr><tr><td valign="top"><a href="#lookup_local_aggr_counter-1">lookup_local_aggr_counter/1</a></td><td>Lookup a local (unique) aggregated counter and returns its value.</td></tr><tr><td valign="top"><a href="#lookup_local_counters-1">lookup_local_counters/1</a></td><td>Look up all local (non-unique) instances of a given Counter.</td></tr><tr><td valign="top"><a href="#lookup_local_name-1">lookup_local_name/1</a></td><td>Lookup a local unique name.</td></tr><tr><td valign="top"><a href="#lookup_local_properties-1">lookup_local_properties/1</a></td><td>Look up all local (non-unique) instances of a given Property.</td></tr><tr><td valign="top"><a href="#lookup_pid-1">lookup_pid/1</a></td><td>Lookup the Pid stored with a key.</td></tr><tr><td valign="top"><a href="#lookup_pids-1">lookup_pids/1</a></td><td>Returns a list of pids with the published key Key.</td></tr><tr><td valign="top"><a href="#lookup_value-1">lookup_value/1</a></td><td>Lookup the value stored with a key.</td></tr><tr><td valign="top"><a href="#lookup_values-1">lookup_values/1</a></td><td>Retrieve the <code>{Pid,Value}</code> pairs corresponding to Key.</td></tr><tr><td valign="top"><a href="#monitor-1">monitor/1</a></td><td>Equivalent to <a href="#monitor-2"><tt>monitor(Key, info)</tt></a>.</td></tr><tr><td valign="top"><a href="#monitor-2">monitor/2</a></td><td>monitor a registered name
+<code>monitor(Key, info)</code> works much like erlang:monitor(process, Pid), but monitors
 a unique name registered via gproc.</td></tr><tr><td valign="top"><a href="#mreg-3">mreg/3</a></td><td>Register multiple {Key,Value} pairs of a given type and scope.</td></tr><tr><td valign="top"><a href="#munreg-3">munreg/3</a></td><td>Unregister multiple Key items of a given type and scope.</td></tr><tr><td valign="top"><a href="#nb_wait-1">nb_wait/1</a></td><td>Wait for a local name to be registered.</td></tr><tr><td valign="top"><a href="#nb_wait-2">nb_wait/2</a></td><td>Wait for a local name to be registered on <code>Node</code>.</td></tr><tr><td valign="top"><a href="#next-2">next/2</a></td><td>Behaves as ets:next(Tab,Key) for a given type of registration.</td></tr><tr><td valign="top"><a href="#prev-2">prev/2</a></td><td>Behaves as ets:prev(Tab,Key) for a given type of registration.</td></tr><tr><td valign="top"><a href="#reg-1">reg/1</a></td><td>Equivalent to <a href="#reg-2"><tt>reg(Key, default(Key))</tt></a>.</td></tr><tr><td valign="top"><a href="#reg-2">reg/2</a></td><td>Register a name or property for the current process.</td></tr><tr><td valign="top"><a href="#reg_or_locate-1">reg_or_locate/1</a></td><td>Equivalent to <a href="#reg_or_locate-2"><tt>reg_or_locate(Key, default(Key))</tt></a>.</td></tr><tr><td valign="top"><a href="#reg_or_locate-2">reg_or_locate/2</a></td><td>Try registering a unique name, or return existing registration.</td></tr><tr><td valign="top"><a href="#reg_or_locate-3">reg_or_locate/3</a></td><td>Spawn a process with a registered name, or return existing registration.</td></tr><tr><td valign="top"><a href="#reg_shared-1">reg_shared/1</a></td><td>Register a resource, but don't tie it to a particular process.</td></tr><tr><td valign="top"><a href="#reg_shared-2">reg_shared/2</a></td><td>Register a resource, but don't tie it to a particular process.</td></tr><tr><td valign="top"><a href="#register_name-2">register_name/2</a></td><td>Behaviour support callback.</td></tr><tr><td valign="top"><a href="#reset_counter-1">reset_counter/1</a></td><td>Reads and resets a counter in a "thread-safe" way.</td></tr><tr><td valign="top"><a href="#select-1">select/1</a></td><td>Perform a select operation on the process registry.</td></tr><tr><td valign="top"><a href="#select-2">select/2</a></td><td>Perform a select operation with limited context on the process registry.</td></tr><tr><td valign="top"><a href="#select-3">select/3</a></td><td>Like <a href="#select-2"><code>select/2</code></a> but returns Limit objects at a time.</td></tr><tr><td valign="top"><a href="#select_count-1">select_count/1</a></td><td>Equivalent to <a href="#select_count-2"><tt>select_count(all, Pat)</tt></a>.</td></tr><tr><td valign="top"><a href="#select_count-2">select_count/2</a></td><td>Perform a select_count operation on the process registry.</td></tr><tr><td valign="top"><a href="#send-2">send/2</a></td><td>Sends a message to the process, or processes, corresponding to Key.</td></tr><tr><td valign="top"><a href="#set_attributes-2">set_attributes/2</a></td><td>Add/modify <code>{Key, Value}</code> attributes associated with a registration.</td></tr><tr><td valign="top"><a href="#set_attributes_shared-2">set_attributes_shared/2</a></td><td>Add/modify <code>{Key, Value}</code> attributes associated with a shared registration.</td></tr><tr><td valign="top"><a href="#set_env-5">set_env/5</a></td><td>Updates the cached value as well as underlying environment.</td></tr><tr><td valign="top"><a href="#set_value-2">set_value/2</a></td><td>Sets the value of the registration given by Key.</td></tr><tr><td valign="top"><a href="#set_value_shared-2">set_value_shared/2</a></td><td>Sets the value of the shared registration given by Key.</td></tr><tr><td valign="top"><a href="#start_link-0">start_link/0</a></td><td>Starts the gproc server.</td></tr><tr><td valign="top"><a href="#table-0">table/0</a></td><td>Equivalent to <a href="#table-1"><tt>table({all, all})</tt></a>.</td></tr><tr><td valign="top"><a href="#table-1">table/1</a></td><td>Equivalent to <a href="#table-2"><tt>table(Context, [])</tt></a>.</td></tr><tr><td valign="top"><a href="#table-2">table/2</a></td><td>QLC table generator for the gproc registry.</td></tr><tr><td valign="top"><a href="#unreg-1">unreg/1</a></td><td>Unregister a name or property.</td></tr><tr><td valign="top"><a href="#unreg_shared-1">unreg_shared/1</a></td><td>Unregister a shared resource.</td></tr><tr><td valign="top"><a href="#unregister_name-1">unregister_name/1</a></td><td>Equivalent to <tt>unreg / 1</tt>.</td></tr><tr><td valign="top"><a href="#update_counter-2">update_counter/2</a></td><td>Updates the counter registered as Key for the current process.</td></tr><tr><td valign="top"><a href="#update_counter-3">update_counter/3</a></td><td></td></tr><tr><td valign="top"><a href="#update_counters-2">update_counters/2</a></td><td>Update a list of counters.</td></tr><tr><td valign="top"><a href="#update_shared_counter-2">update_shared_counter/2</a></td><td>Updates the shared counter registered as Key.</td></tr><tr><td valign="top"><a href="#where-1">where/1</a></td><td>Returns the pid registered as Key.</td></tr><tr><td valign="top"><a href="#whereis_name-1">whereis_name/1</a></td><td>Equivalent to <tt>where / 1</tt>.</td></tr><tr><td valign="top"><a href="#wide_await-3">wide_await/3</a></td><td>Wait for a local name to be registered on any of <code>Nodes</code>.</td></tr></table>
 
 
@@ -1110,9 +1122,16 @@ object, the return value can be a list of any length.
 
 ### monitor/1 ###
 
+`monitor(Key) -> any()`
+
+Equivalent to [`monitor(Key, info)`](#monitor-2).
+<a name="monitor-2"></a>
+
+### monitor/2 ###
+
 
 <pre><code>
-monitor(Key::<a href="#type-key">key()</a>) -&gt; reference()
+monitor(Key::<a href="#type-key">key()</a>, Type::<a href="#type-monitor_type">monitor_type()</a>) -&gt; reference()
 </code></pre>
 
 <br></br>
@@ -1120,13 +1139,32 @@ monitor(Key::<a href="#type-key">key()</a>) -&gt; reference()
 
 
 monitor a registered name
-This function works much like erlang:monitor(process, Pid), but monitors
+`monitor(Key, info)` works much like erlang:monitor(process, Pid), but monitors
 a unique name registered via gproc. A message, `{gproc, unreg, Ref, Key}`
 will be sent to the requesting process, if the name is unregistered or
-the registered process dies.
+the registered process dies. If there is a standby monitor (see below), a
+message `{gproc, {failover, ToPid}, Ref, Key}` is sent to all monitors.
+If the name is passed to another process using [`give_away/2`](#give_away-2), the event
+`{gproc, {migrated, ToPid}, Ref, Key}` is sent to all monitors.
+
+
+
+`monitor(Key, standby)` sets up the monitoring process as a standby for the
+registered name. If the registered process dies, the first standby process
+inherits the name, and a message `{gproc, {failover, ToPid}, Ref, Key}` is
+sent to all monitors, including the one that inherited the name.
+
+
+
+If the name is not yet registered, the unreg event is sent immediately.
+If the calling process in this case tried to start a `standby` monitoring,
+it receives the registered name and the failover event immediately.
 
 
-If the name is not yet registered, the same message is sent immediately.
+`monitor(Key, follow)` keeps monitoring the registered name even if it is
+temporarily unregistered. The messages received are the same as for the other
+monitor types, but `{gproc, registered, Ref, Key}` is also sent when a new
+process registers the name.
 <a name="mreg-3"></a>
 
 ### mreg/3 ###

doc/gproc_dist.md

@@ -23,7 +23,7 @@ For a detailed description, see gproc/doc/erlang07-wiger.pdf.
 ## Function Index ##
 
 
-<table width="100%" border="1" cellspacing="0" cellpadding="2" summary="function index"><tr><td valign="top"><a href="#code_change-4">code_change/4</a></td><td></td></tr><tr><td valign="top"><a href="#elected-2">elected/2</a></td><td></td></tr><tr><td valign="top"><a href="#elected-3">elected/3</a></td><td></td></tr><tr><td valign="top"><a href="#from_leader-3">from_leader/3</a></td><td></td></tr><tr><td valign="top"><a href="#get_leader-0">get_leader/0</a></td><td>Returns the node of the current gproc leader.</td></tr><tr><td valign="top"><a href="#give_away-2">give_away/2</a></td><td></td></tr><tr><td valign="top"><a href="#handle_DOWN-3">handle_DOWN/3</a></td><td></td></tr><tr><td valign="top"><a href="#handle_call-4">handle_call/4</a></td><td></td></tr><tr><td valign="top"><a href="#handle_cast-3">handle_cast/3</a></td><td></td></tr><tr><td valign="top"><a href="#handle_info-2">handle_info/2</a></td><td></td></tr><tr><td valign="top"><a href="#handle_info-3">handle_info/3</a></td><td></td></tr><tr><td valign="top"><a href="#handle_leader_call-4">handle_leader_call/4</a></td><td></td></tr><tr><td valign="top"><a href="#handle_leader_cast-3">handle_leader_cast/3</a></td><td></td></tr><tr><td valign="top"><a href="#init-1">init/1</a></td><td></td></tr><tr><td valign="top"><a href="#leader_call-1">leader_call/1</a></td><td></td></tr><tr><td valign="top"><a href="#leader_cast-1">leader_cast/1</a></td><td></td></tr><tr><td valign="top"><a href="#mreg-2">mreg/2</a></td><td></td></tr><tr><td valign="top"><a href="#munreg-2">munreg/2</a></td><td></td></tr><tr><td valign="top"><a href="#reg-1">reg/1</a></td><td></td></tr><tr><td valign="top"><a href="#reg-2">reg/2</a></td><td>
+<table width="100%" border="1" cellspacing="0" cellpadding="2" summary="function index"><tr><td valign="top"><a href="#code_change-4">code_change/4</a></td><td></td></tr><tr><td valign="top"><a href="#demonitor-2">demonitor/2</a></td><td></td></tr><tr><td valign="top"><a href="#elected-2">elected/2</a></td><td></td></tr><tr><td valign="top"><a href="#elected-3">elected/3</a></td><td></td></tr><tr><td valign="top"><a href="#from_leader-3">from_leader/3</a></td><td></td></tr><tr><td valign="top"><a href="#get_leader-0">get_leader/0</a></td><td>Returns the node of the current gproc leader.</td></tr><tr><td valign="top"><a href="#give_away-2">give_away/2</a></td><td></td></tr><tr><td valign="top"><a href="#handle_DOWN-3">handle_DOWN/3</a></td><td></td></tr><tr><td valign="top"><a href="#handle_call-4">handle_call/4</a></td><td></td></tr><tr><td valign="top"><a href="#handle_cast-3">handle_cast/3</a></td><td></td></tr><tr><td valign="top"><a href="#handle_info-2">handle_info/2</a></td><td></td></tr><tr><td valign="top"><a href="#handle_info-3">handle_info/3</a></td><td></td></tr><tr><td valign="top"><a href="#handle_leader_call-4">handle_leader_call/4</a></td><td></td></tr><tr><td valign="top"><a href="#handle_leader_cast-3">handle_leader_cast/3</a></td><td></td></tr><tr><td valign="top"><a href="#init-1">init/1</a></td><td></td></tr><tr><td valign="top"><a href="#leader_call-1">leader_call/1</a></td><td></td></tr><tr><td valign="top"><a href="#leader_cast-1">leader_cast/1</a></td><td></td></tr><tr><td valign="top"><a href="#monitor-2">monitor/2</a></td><td></td></tr><tr><td valign="top"><a href="#mreg-2">mreg/2</a></td><td></td></tr><tr><td valign="top"><a href="#munreg-2">munreg/2</a></td><td></td></tr><tr><td valign="top"><a href="#reg-1">reg/1</a></td><td></td></tr><tr><td valign="top"><a href="#reg-2">reg/2</a></td><td>
 Class = n  - unique name
 | p  - non-unique property
 | c  - counter
@@ -42,6 +42,13 @@ Scope = l | g (global or local).</td></tr><tr><td valign="top"><a href="#reg_or_
 `code_change(FromVsn, S, Extra, E) -> any()`
 
 
+<a name="demonitor-2"></a>
+
+### demonitor/2 ###
+
+`demonitor(Key, Ref) -> any()`
+
+
 <a name="elected-2"></a>
 
 ### elected/2 ###
@@ -153,6 +160,13 @@ Returns the node of the current gproc leader.
 `leader_cast(Msg) -> any()`
 
 
+<a name="monitor-2"></a>
+
+### monitor/2 ###
+
+`monitor(Key, Type) -> any()`
+
+
 <a name="mreg-2"></a>
 
 ### mreg/2 ###

doc/gproc_lib.md

@@ -21,18 +21,18 @@ For a detailed description, see gproc/doc/erlang07-wiger.pdf.
 ## Function Index ##
 
 
-<table width="100%" border="1" cellspacing="0" cellpadding="2" summary="function index"><tr><td valign="top"><a href="#add_monitor-3">add_monitor/3</a></td><td></td></tr><tr><td valign="top"><a href="#await-3">await/3</a></td><td></td></tr><tr><td valign="top"><a href="#do_set_counter_value-3">do_set_counter_value/3</a></td><td></td></tr><tr><td valign="top"><a href="#do_set_value-3">do_set_value/3</a></td><td></td></tr><tr><td valign="top"><a href="#ensure_monitor-2">ensure_monitor/2</a></td><td></td></tr><tr><td valign="top"><a href="#insert_attr-4">insert_attr/4</a></td><td></td></tr><tr><td valign="top"><a href="#insert_many-4">insert_many/4</a></td><td></td></tr><tr><td valign="top"><a href="#insert_reg-4">insert_reg/4</a></td><td></td></tr><tr><td valign="top"><a href="#notify-2">notify/2</a></td><td></td></tr><tr><td valign="top"><a href="#notify-3">notify/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_many-4">remove_many/4</a></td><td></td></tr><tr><td valign="top"><a href="#remove_monitor-3">remove_monitor/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_monitors-3">remove_monitors/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_reg-3">remove_reg/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_reg-4">remove_reg/4</a></td><td></td></tr><tr><td valign="top"><a href="#remove_reverse_mapping-3">remove_reverse_mapping/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_wait-4">remove_wait/4</a></td><td></td></tr><tr><td valign="top"><a href="#update_aggr_counter-3">update_aggr_counter/3</a></td><td></td></tr><tr><td valign="top"><a href="#update_counter-3">update_counter/3</a></td><td></td></tr><tr><td valign="top"><a href="#valid_opts-2">valid_opts/2</a></td><td></td></tr></table>
+<table width="100%" border="1" cellspacing="0" cellpadding="2" summary="function index"><tr><td valign="top"><a href="#add_monitor-4">add_monitor/4</a></td><td></td></tr><tr><td valign="top"><a href="#await-3">await/3</a></td><td></td></tr><tr><td valign="top"><a href="#dbg-1">dbg/1</a></td><td></td></tr><tr><td valign="top"><a href="#do_set_counter_value-3">do_set_counter_value/3</a></td><td></td></tr><tr><td valign="top"><a href="#do_set_value-3">do_set_value/3</a></td><td></td></tr><tr><td valign="top"><a href="#ensure_monitor-2">ensure_monitor/2</a></td><td></td></tr><tr><td valign="top"><a href="#followers-1">followers/1</a></td><td></td></tr><tr><td valign="top"><a href="#insert_attr-4">insert_attr/4</a></td><td></td></tr><tr><td valign="top"><a href="#insert_many-4">insert_many/4</a></td><td></td></tr><tr><td valign="top"><a href="#insert_reg-4">insert_reg/4</a></td><td></td></tr><tr><td valign="top"><a href="#insert_reg-5">insert_reg/5</a></td><td></td></tr><tr><td valign="top"><a href="#monitors-1">monitors/1</a></td><td></td></tr><tr><td valign="top"><a href="#notify-2">notify/2</a></td><td></td></tr><tr><td valign="top"><a href="#notify-3">notify/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_many-4">remove_many/4</a></td><td></td></tr><tr><td valign="top"><a href="#remove_monitor-3">remove_monitor/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_monitor_pid-2">remove_monitor_pid/2</a></td><td></td></tr><tr><td valign="top"><a href="#remove_monitors-3">remove_monitors/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_reg-3">remove_reg/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_reg-4">remove_reg/4</a></td><td></td></tr><tr><td valign="top"><a href="#remove_reverse_mapping-3">remove_reverse_mapping/3</a></td><td></td></tr><tr><td valign="top"><a href="#remove_reverse_mapping-4">remove_reverse_mapping/4</a></td><td></td></tr><tr><td valign="top"><a href="#remove_wait-4">remove_wait/4</a></td><td></td></tr><tr><td valign="top"><a href="#standbys-1">standbys/1</a></td><td></td></tr><tr><td valign="top"><a href="#update_aggr_counter-3">update_aggr_counter/3</a></td><td></td></tr><tr><td valign="top"><a href="#update_counter-3">update_counter/3</a></td><td></td></tr><tr><td valign="top"><a href="#valid_opts-2">valid_opts/2</a></td><td></td></tr></table>
 
 
 <a name="functions"></a>
 
 ## Function Details ##
 
-<a name="add_monitor-3"></a>
+<a name="add_monitor-4"></a>
 
-### add_monitor/3 ###
+### add_monitor/4 ###
 
-`add_monitor(T, Pid, Ref) -> any()`
+`add_monitor(T, Pid, Ref, Type) -> any()`
 
 
 <a name="await-3"></a>
@@ -42,6 +42,13 @@ For a detailed description, see gproc/doc/erlang07-wiger.pdf.
 `await(Key, WPid, From) -> any()`
 
 
+<a name="dbg-1"></a>
+
+### dbg/1 ###
+
+`dbg(Mods) -> any()`
+
+
 <a name="do_set_counter_value-3"></a>
 
 ### do_set_counter_value/3 ###
@@ -63,6 +70,13 @@ For a detailed description, see gproc/doc/erlang07-wiger.pdf.
 `ensure_monitor(Pid, Scope) -> any()`
 
 
+<a name="followers-1"></a>
+
+### followers/1 ###
+
+`followers(Opts) -> any()`
+
+
 <a name="insert_attr-4"></a>
 
 ### insert_attr/4 ###
@@ -96,6 +110,20 @@ insert_reg(K::<a href="#type-key">key()</a>, Value::any(), Pid::pid() | shared,
 
 
 
+<a name="insert_reg-5"></a>
+
+### insert_reg/5 ###
+
+`insert_reg(K, Value, Pid, Scope, Event) -> any()`
+
+
+<a name="monitors-1"></a>
+
+### monitors/1 ###
+
+`monitors(Opts) -> any()`
+
+
 <a name="notify-2"></a>
 
 ### notify/2 ###
@@ -124,6 +152,13 @@ insert_reg(K::<a href="#type-key">key()</a>, Value::any(), Pid::pid() | shared,
 `remove_monitor(T, Pid, Ref) -> any()`
 
 
+<a name="remove_monitor_pid-2"></a>
+
+### remove_monitor_pid/2 ###
+
+`remove_monitor_pid(T, Pid) -> any()`
+
+
 <a name="remove_monitors-3"></a>
 
 ### remove_monitors/3 ###
@@ -152,6 +187,13 @@ insert_reg(K::<a href="#type-key">key()</a>, Value::any(), Pid::pid() | shared,
 `remove_reverse_mapping(Event, Pid, Key) -> any()`
 
 
+<a name="remove_reverse_mapping-4"></a>
+
+### remove_reverse_mapping/4 ###
+
+`remove_reverse_mapping(Event, Pid, Key, Opts) -> any()`
+
+
 <a name="remove_wait-4"></a>
 
 ### remove_wait/4 ###
@@ -159,6 +201,13 @@ insert_reg(K::<a href="#type-key">key()</a>, Value::any(), Pid::pid() | shared,
 `remove_wait(Key, Pid, Ref, Waiters) -> any()`
 
 
+<a name="standbys-1"></a>
+
+### standbys/1 ###
+
+`standbys(Opts) -> any()`
+
+
 <a name="update_aggr_counter-3"></a>
 
 ### update_aggr_counter/3 ###

src/gproc.erl

@@ -46,6 +46,8 @@
 %% @type reg_id() = {type(), scope(), any()}.
 %% @type unique_id() = {n | a, scope(), any()}.
 %%
+%% @type monitor_type() = info | standby | follow.
+%%
 %% @type sel_scope() = scope | all | global | local.
 %% @type sel_type() = type() | names | props | counters | aggr_counters.
 %% @type context() = {scope(), type()} | type(). {'all','all'} is the default
@@ -87,7 +89,7 @@
          nb_wait/1, nb_wait/2,
          cancel_wait/2, cancel_wait/3,
 	 cancel_wait_or_monitor/1,
-	 monitor/1,
+	 monitor/1, monitor/2,
 	 demonitor/2,
          lookup_pid/1,
          lookup_pids/1,
@@ -875,25 +877,46 @@ cancel_wait_or_monitor1({_,l,_} = Key) ->
     ok.
 
 
-%% @spec monitor(key()) -> reference()
+%% @equiv monitor(Key, info)
+monitor(Key) ->
+    ?CATCH_GPROC_ERROR(monitor1(Key, info), [Key]).
+
+%% @spec monitor(key(), monitor_type()) -> reference()
 %%
 %% @doc monitor a registered name
-%% This function works much like erlang:monitor(process, Pid), but monitors
+%% `monitor(Key, info)' works much like erlang:monitor(process, Pid), but monitors
 %% a unique name registered via gproc. A message, `{gproc, unreg, Ref, Key}'
 %% will be sent to the requesting process, if the name is unregistered or
-%% the registered process dies.
-%%
-%% If the name is not yet registered, the same message is sent immediately.
-%% @end
-monitor(Key) ->
-    ?CATCH_GPROC_ERROR(monitor1(Key), [Key]).
-
-monitor1({T,g,_} = Key) when T==n; T==a ->
+%% the registered process dies. If there is a standby monitor (see below), a
+%% message `{gproc, {failover, ToPid}, Ref, Key}' is sent to all monitors.
+%% If the name is passed to another process using {@link give_away/2}, the event
+%% `{gproc, {migrated, ToPid}, Ref, Key}' is sent to all monitors.
+%%
+%% `monitor(Key, standby)' sets up the monitoring process as a standby for the
+%% registered name. If the registered process dies, the first standby process
+%% inherits the name, and a message `{gproc, {failover, ToPid}, Ref, Key}' is
+%% sent to all monitors, including the one that inherited the name.
+%%
+%% If the name is not yet registered, the unreg event is sent immediately.
+%% If the calling process in this case tried to start a `standby' monitoring,
+%% it receives the registered name and the failover event immediately.
+%%
+%% `monitor(Key, follow)' keeps monitoring the registered name even if it is
+%% temporarily unregistered. The messages received are the same as for the other
+%% monitor types, but `{gproc, registered, Ref, Key}' is also sent when a new
+%% process registers the name.
+%% @end
+monitor(Key, Type) when Type==info;
+                        Type==follow;
+                        Type==standby ->
+    ?CATCH_GPROC_ERROR(monitor1(Key, Type), [Key, Type]).
+
+monitor1({T,g,_} = Key, Type) when T==n; T==a ->
     ?CHK_DIST,
-    call({monitor, Key, self()}, g);
-monitor1({T,l,_} = Key) when T==n; T==a ->
-    call({monitor, Key, self()}, l);
-monitor1(_) ->
+    gproc_dist:monitor(Key, Type);
+monitor1({T,l,_} = Key, Type) when T==n; T==a ->
+    call({monitor, Key, self(), Type}, l);
+monitor1(_, _) ->
     ?THROW_GPROC_ERROR(badarg).
 
 %% @spec demonitor(key(), reference()) -> ok
@@ -907,7 +930,7 @@ demonitor(Key, Ref) ->
 
 demonitor1({T,g,_} = Key, Ref) when T==n; T==a ->
     ?CHK_DIST,
-    call({demonitor, Key, Ref, self()}, g);
+    gproc_dist:demonitor(Key, Ref);
 demonitor1({T,l,_} = Key, Ref) when T==n; T==a ->
     call({demonitor, Key, Ref, self()}, l);
 demonitor1(_, _) ->
@@ -2054,18 +2077,42 @@ handle_call({reg_or_locate, {T,l,_} = Key, Val, P}, _, S) ->
 	[{_, OtherPid, OtherValue}] ->
 	    {reply, {OtherPid, OtherValue}, S}
     end;
-handle_call({monitor, {T,l,_} = Key, Pid}, _From, S)
+handle_call({monitor, {T,l,_} = Key, Pid, Type}, _From, S)
   when T==n; T==a ->
     Ref = make_ref(),
-    _ = case where(Key) of
-	    undefined ->
+    Lookup = ets:lookup(?TAB, {Key, T}),
+    IsRegged = is_regged(Lookup),
+    _ = case {IsRegged, Type} of
+	    {false, info} ->
 		Pid ! {gproc, unreg, Ref, Key};
-	    RegPid ->
+            {false, follow} ->
+		Pid ! {gproc, unreg, Ref, Key},
+                _ = gproc_lib:ensure_monitor(Pid, l),
+                case Lookup of
+                    [{K, Waiters}] ->
+                        NewWaiters = [{Pid,Ref,follow}|Waiters],
+                        ets:insert(?TAB, {K, NewWaiters}),
+                        ets:insert_new(?TAB, {{Pid,Key}, []});
+                    [] ->
+                        ets:insert(?TAB, {{Key,T}, [{Pid,Ref,follow}]}),
+                        ets:insert_new(?TAB, {{Pid,Key}, []})
+                end;
+            {false, standby} ->
+                Evt = {failover, Pid},
+                true = gproc_lib:insert_reg(Key, undefined, Pid, l, Evt),
+                Pid ! {gproc, Evt, Ref, Key},
+                _ = gproc_lib:ensure_monitor(Pid, l);
+	    {true, _} ->
+                [{_, RegPid, _}] = Lookup,
+                _ = gproc_lib:ensure_monitor(Pid, l),
 		case ets:lookup(?TAB, {RegPid, Key}) of
 		    [{K,r}] ->
-			ets:insert(?TAB, {K, [{monitor, [{Pid,Ref}]}]});
+			ets:insert(?TAB, {K, [{monitor, [{Pid,Ref,Type}]}]}),
+                        ets:insert_new({{Pid,Key}, []});
 		    [{K, Opts}] ->
-			ets:insert(?TAB, {K, gproc_lib:add_monitor(Opts, Pid, Ref)})
+			ets:insert(?TAB, {K, gproc_lib:add_monitor(
+                                               Opts, Pid, Ref, Type)}),
+                        ets:insert_new(?TAB, {{Pid,Key}, []})
 		end
 	end,
     {reply, Ref, S};
@@ -2233,8 +2280,8 @@ try_insert_reg({T,l,_} = Key, Val, Pid) ->
                         true ->
                             false;
                         false ->
-                            process_is_down(OtherPid),
-                            true = gproc_lib:insert_reg(Key, Val, Pid, l)
+                            process_is_down(OtherPid), % may result in failover
+                            try_insert_reg(Key, Val, Pid)
                     end;
                 [] ->
                     false
@@ -2274,17 +2321,26 @@ process_is_down(Pid) when is_pid(Pid) ->
               fun({{n,l,_}=K, R}) ->
                       Key = {K,n},
                       case ets:lookup(?TAB, Key) of
-                          [{_, Pid, _}] ->
+                          [{_, Pid, V}] ->
                               ets:delete(?TAB, Key),
-			      opt_notify(R, K);
+			      opt_notify(R, K, Pid, V);
                           [{_, Waiters}] ->
-                              case [W || {P,_} = W <- Waiters,
-                                         P =/= Pid] of
+                              case [W || W <- Waiters,
+                                         element(1,W) =/= Pid] of
                                   [] ->
                                       ets:delete(?TAB, Key);
                                   Waiters1 ->
                                       ets:insert(?TAB, {Key, Waiters1})
                               end;
+                          [{_, OtherPid, _}] when Pid =/= OtherPid ->
+                              case ets:lookup(?TAB, {OtherPid, K}) of
+                                  [{RK, Opts}] when is_list(Opts) ->
+                                      Opts1 = gproc_lib:remove_monitor_pid(
+                                                Opts, Pid),
+                                      ets:insert(?TAB, {RK, Opts1});
+                                  _ ->
+                                      true
+                              end;
                           [] ->
                               true
                       end;
@@ -2294,8 +2350,22 @@ process_is_down(Pid) when is_pid(Pid) ->
                       ets:delete(?TAB, Key),
                       gproc_lib:update_aggr_counter(l, C, -Value);
                  ({{a,l,_} = K, R}) ->
-                      ets:delete(?TAB, {K,a}),
-		      opt_notify(R, K);
+                      case ets:lookup(?TAB, {K,a}) of
+                          [{_, Pid, V}] ->
+                              ets:delete(?TAB, {K,a}),
+                              opt_notify(R, K, Pid, V);
+                          [{_, OtherPid, _}] when Pid =/= OtherPid ->
+                              case ets:lookup(?TAB, {OtherPid, K}) of
+                                  [{RK, Opts}] when is_list(Opts) ->
+                                      Opts1 = gproc_lib:remove_monitor_pid(
+                                                Opts, Pid),
+                                      ets:insert(?TAB, {RK, Opts1});
+                                  _ ->
+                                      true
+                              end;
+                          [] ->
+                              opt_notify(R, K, Pid, undefined)
+                      end;
                  ({{p,_,_} = K, _}) ->
                       ets:delete(?TAB, {K, Pid})
               end, Revs),
@@ -2304,10 +2374,52 @@ process_is_down(Pid) when is_pid(Pid) ->
             ok
     end.
 
-opt_notify(r, _) ->
+opt_notify(r, _, _, _) ->
     ok;
-opt_notify(Opts, Key) ->
-    gproc_lib:notify(Key, Opts).
+opt_notify(Opts, {T,_,_} = Key, Pid, Value) ->
+    case gproc_lib:standbys(Opts) of
+        [] ->
+            keep_followers(Opts, Key),
+            gproc_lib:notify(unreg, Key, Opts);
+        SBs ->
+            case pick_standby(SBs) of
+                false ->
+                    keep_followers(Opts, Key),
+                    gproc_lib:notify(unreg, Key, Opts),
+                    ok;
+                {ToPid, Ref} ->
+                    ets:insert(?TAB, [{{Key,T}, ToPid, Value},
+                                      {{ToPid, Key},
+                                       gproc_lib:remove_monitor(
+                                         Opts, ToPid, Ref)}]),
+                    _ = gproc_lib:remove_reverse_mapping(
+                          {failover,ToPid}, Pid, Key),
+                    _ = gproc_lib:ensure_monitor(ToPid, l),
+                    ok
+            end
+    end.
+
+keep_followers(Opts, {T,_,_} = Key) ->
+    case gproc_lib:followers(Opts) of
+        [] ->
+            ok;
+        [_|_] = F ->
+            ets:insert(?TAB, {{Key,T}, F})
+    end.
+
+pick_standby([{Pid, Ref, standby}|T]) when node(Pid) =:= node() ->
+    case is_process_alive(Pid) of
+        true ->
+            {Pid, Ref};
+        false ->
+            pick_standby(T)
+    end;
+pick_standby([_|T]) ->
+    pick_standby(T);
+pick_standby([]) ->
+    false.
+
+
 
 
 do_give_away({T,l,_} = K, To, Pid) when T==n; T==a ->
@@ -2743,3 +2855,8 @@ qlc_select(false, {Objects, Cont}) ->
 is_unique(n) -> true;
 is_unique(a) -> true;
 is_unique(_) -> false.
+
+is_regged([{_, _, _}]) ->
+    true;
+is_regged(_) ->
+    false.

src/gproc_dist.erl

@@ -26,6 +26,8 @@
          reg/1, reg/2, unreg/1,
 	 reg_or_locate/3,
 	 reg_shared/2, unreg_shared/1,
+         monitor/2,
+         demonitor/2,
 	 set_attributes/2,
 	 set_attributes_shared/2,
          mreg/2,
@@ -126,6 +128,19 @@ reg_shared({_,g,_} = Key, Value) ->
 reg_shared(_, _) ->
     ?THROW_GPROC_ERROR(badarg).
 
+monitor({_,g,_} = Key, Type) when Type==info;
+                                  Type==follow;
+                                  Type==standby ->
+    leader_call({monitor, Key, self(), Type});
+monitor(_, _) ->
+    ?THROW_GPROC_ERROR(badarg).
+
+demonitor({_,g,_} = Key, Ref) ->
+    leader_call({demonitor, Key, self(), Ref});
+demonitor(_, _) ->
+    ?THROW_GPROC_ERROR(badarg).
+
+
 set_attributes({_,g,_} = Key, Attrs) ->
     leader_call({set_attributes, Key, Attrs, self()});
 set_attributes(_, _) ->
@@ -336,6 +351,68 @@ handle_leader_call({reg, {_C,g,_Name} = K, Value, Pid}, _From, S, _E) ->
             %%     end,
             {reply, true, [{insert, Vals}], S}
     end;
+handle_leader_call({monitor, {T,g,_} = K, MPid, Type}, _From, S, _E) when T==n;
+                                                                          T==a ->
+    case ets:lookup(?TAB, {K, T}) of
+        [{_, Pid, _}] ->
+            Opts = get_opts(Pid, K),
+            Ref = make_ref(),
+            Opts1 = gproc_lib:add_monitor(Opts, MPid, Ref, Type),
+            _ = gproc_lib:ensure_monitor(MPid, g),
+            Obj = {{Pid,K}, Opts1},
+            Rev = {{MPid,K}, []},
+            ets:insert(?TAB, [Obj, Rev]),
+            {reply, Ref, [{insert, [Obj, Rev]}], S};
+        LookupRes ->
+            Ref = make_ref(),
+            case Type of
+                standby ->
+                    Event = {failover, MPid},
+                    Msgs = insert_reg(LookupRes, K, undefined, MPid, Event),
+                    Obj = {{K,T}, MPid, undefined},
+                    Rev = {{MPid,K}, []},
+                    ets:insert(?TAB, [Obj, Rev]),
+                    MPid ! {gproc, {failover,MPid}, Ref, K},
+                    {reply, Ref, [{insert, [Obj, Rev]},
+                                  {notify, Msgs}], S};
+                follow ->
+                    case LookupRes of
+                        [{_, Waiters}] ->
+                            add_follow_to_waiters(Waiters, K, MPid, Ref, S);
+                        [] ->
+                            add_follow_to_waiters([], K, MPid, Ref, S);
+                        [{_, Pid, _}] ->
+                            case ets:lookup(?TAB, {Pid,K}) of
+                                [{_, Opts}] when is_list(Opts) ->
+                                    Opts1 = gproc_lib:add_monitor(
+                                              Opts, MPid, Ref, follow),
+                                    ets:insert(?TAB, {{Pid,K}, Opts1}),
+                                    {reply, Ref,
+                                     [{insert, [{{Pid,K}, Opts1}]}], S}
+                            end
+                    end;
+                _ ->
+                    MPid ! {gproc, unreg, Ref, K},
+                    {reply, Ref, S}
+            end
+    end;
+handle_leader_call({demonitor, {T,g,_} = K, MPid, Ref}, _From, S, _E) ->
+    case ets:lookup(?TAB, {K,T}) of
+        [{_, Pid, _}] ->
+            Opts = get_opts(Pid, K),
+            Opts1 = gproc_lib:remove_monitors(Opts, MPid, Ref),
+            Obj = {{Pid,K}, Opts1},
+            ets:insert(?TAB, Obj),
+            ets:delete(?TAB, {MPid, K}),
+            {reply, ok, [{delete, [{MPid,K}]},
+                         {insert, [Obj]}], S};
+        [{Key, Waiters}] ->
+            NewWaiters = [W || W <- Waiters,
+                               W =/= {MPid, Ref, follow}],
+            {reply, ok, [{insert, [{Key, NewWaiters}]}], S};
+        _ ->
+            {reply, ok, S}
+    end;
 handle_leader_call({set_attributes, {_,g,_} = K, Attrs, Pid}, _From, S, _E) ->
     case gproc_lib:insert_attr(K, Attrs, Pid, g) of
 	false ->
@@ -369,39 +446,6 @@ handle_leader_call({reg_or_locate, {n,g,_} = K, Value, P},
 	[{_, OtherPid, OtherVal}] ->
 	    {reply, {OtherPid, OtherVal}, S}
     end;
-handle_leader_call({monitor, {T,g,_} = Key, Pid}, _From, S, _E)
-  when T==n; T==a ->
-    Ref = make_ref(),
-    case gproc:where(Key) of
-	undefined ->
-	    Pid ! {gproc, unreg, Ref, Key},
-	    {reply, Ref, [], S};
-	RegPid ->
-	    NewRev =
-		case ets:lookup_element(?TAB, K = {RegPid, Key}, 2) of
-		    r ->
-			{K, [{monitor, [{Pid,Ref}]}]};
-		    Opts ->
-			{K, gproc_lib:add_monitor(Opts, Pid, Ref)}
-		end,
-	    ets:insert(?TAB, NewRev),
-	    {reply, Ref, [{insert, [NewRev]}], S}
-    end;
-handle_leader_call({demonitor, {T,g,_} = Key, Ref, Pid}, _From, S, _E)
-  when T==n; T==a ->
-    case gproc:where(Key) of
-	undefined ->
-	    {reply, ok, [], S};
-	RegPid ->
-	    case ets:lookup_element(?TAB, K = {RegPid, Key}, 2) of
-		r ->
-		    {reply, ok, [], S};
-		Opts ->
-		    NewRev = {K, gproc_lib:remove_monitor(Opts, Pid, Ref)},
-		    ets:insert(?TAB, NewRev),
-		    {reply, Ref, [{insert, [NewRev]}], S}
-	    end
-    end;
 handle_leader_call({update_counter, {T,g,_Ctr} = Key, Incr, Pid}, _From, S, _E)
   when is_integer(Incr), T==c;
        is_integer(Incr), T==n ->
@@ -451,13 +495,14 @@ handle_leader_call({unreg, {T,g,Name} = K, Pid}, _From, S, _E) ->
                     case ets:lookup(?TAB, {{a,g,Name},a}) of
                         [Aggr] ->
                             %% updated by remove_reg/3
-                            {reply, true, [{delete,[Key, {Pid,K}], unreg},
+                            {reply, true, [{delete,[Key, {Pid,K}]},
                                            {insert, [Aggr]}], S};
                         [] ->
-                            {reply, true, [{delete, [Key, {Pid,K}], unreg}], S}
+                            {reply, true, [{delete, [Key, {Pid,K}]}], S}
                     end;
                true ->
-                    {reply, true, [{delete, [Key, {Pid,K}], unreg}], S}
+                    {reply, true, [{notify, [{K, Pid, unreg}]},
+                                   {delete, [Key, {Pid,K}]}], S}
             end;
         false ->
             {reply, badarg, S}
@@ -467,19 +512,27 @@ handle_leader_call({give_away, {T,g,_} = K, To, Pid}, _From, S, _E)
     Key = {K, T},
     case ets:lookup(?TAB, Key) of
         [{_, Pid, Value}] ->
+            Opts = get_opts(Pid, K),
             case pid_to_give_away_to(To) of
                 Pid ->
                     {reply, Pid, S};
                 ToPid when is_pid(ToPid) ->
                     ets:insert(?TAB, [{Key, ToPid, Value},
-                                      {{ToPid,K}, []}]),
+                                      {{ToPid,K}, Opts}]),
                     _ = gproc_lib:ensure_monitor(ToPid, g),
-                    {reply, ToPid, [{delete, [Key, {Pid,K}], {migrated,ToPid}},
-				    {insert, [{Key, ToPid, Value}]}], S};
+                    Rev = {Pid, K},
+                    ets:delete(?TAB, Rev),
+                    gproc_lib:notify({migrated, ToPid}, K, Opts),
+                    {reply, ToPid, [{insert, [{Key, ToPid, Value}]},
+                                    {notify, [{K, Pid, {migrated, ToPid}}]},
+				    {delete, [Rev]}], S};
                 undefined ->
                     ets:delete(?TAB, Key),
-		    Rev = gproc_lib:remove_reverse_mapping(unreg, Pid, K),
-                    {reply, undefined, [{delete, [Key, Rev], unreg}], S}
+                    Rev = {Pid, K},
+                    ets:delete(?TAB, Rev),
+                    gproc_lib:notify(unreg, K, Opts),
+                    {reply, undefined, [{notify, [{K, Pid, unreg}]},
+                                        {delete, [Key, Rev]}], S}
             end;
         _ ->
             {reply, badarg, S}
@@ -499,7 +552,7 @@ handle_leader_call({munreg, T, g, L, Pid}, _From, S, _E) ->
         [] ->
             {reply, true, S};
         Objs ->
-            {reply, true, [{delete, Objs, unreg}], S}
+            {reply, true, [{delete, Objs}], S}
     catch
         error:_ -> {reply, badarg, S}
     end;
@@ -570,7 +623,7 @@ handle_leader_cast({add_globals, Missing}, S, _E) ->
     Update = insert_globals(Missing),
     {ok, [{insert, Update}], S};
 handle_leader_cast({remove_globals, Globals}, S, _E) ->
-    delete_globals(Globals, []),
+    delete_globals(Globals),
     {ok, S};
 handle_leader_cast({cancel_wait, Pid, {T,_,_} = Key, Ref}, S, _E) ->
     case ets:lookup(?TAB, {Key, T}) of
@@ -617,36 +670,84 @@ mk_broadcast_insert_vals(Objs) ->
 
 
 process_globals(Globals) ->
-    Modified =
+    {Modified, Notifications} =
         lists:foldl(
-          fun({{T,_,_} = Key, Pid}, A) ->
-                  A1 = case T of
-                           c ->
-                               Incr = ets:lookup_element(?TAB, {Key,Pid}, 3),
-                               update_aggr_counter(Key, -Incr) ++ A;
-                           _ ->
-                               A
-                       end,
-                  K = ets_key(Key, Pid),
-                  ets:delete(?TAB, K),
-                  remove_rev_entry(Pid, Key, unreg),
-                  A1
-          end, [], Globals),
+          fun({{T,_,_} = Key, Pid}, A) when T==n; T==a ->
+                  case ets:lookup(?TAB, {Pid,Key}) of
+                      [{_, Opts}] when is_list(Opts) ->
+                          maybe_failover(Key, Pid, Opts, A);
+                      _ ->
+                          A
+                  end;
+             ({{T,_,_} = Key, Pid}, {MA,NA}) ->
+                  MA1 = case T of
+                            c ->
+                                Incr = ets:lookup_element(?TAB, {Key,Pid}, 3),
+                                update_aggr_counter(Key, -Incr) ++ MA;
+                            _ ->
+                               MA
+                        end,
+                  N = remove_entry(Key, Pid, unreg),
+                  {MA1, N ++ NA}
+          end, {[],[]}, Globals),
     [{insert, Modified} || Modified =/= []] ++
-	[{delete, Globals, unreg} || Globals =/= []].
+        [{notify, Notifications} || Notifications =/= []] ++
+	[{delete, Globals} || Globals =/= []].
+
+maybe_failover({T,_,_} = Key, Pid, Opts, {MAcc, NAcc}) ->
+    Opts = get_opts(Pid, Key),
+    case filter_standbys(gproc_lib:standbys(Opts)) of
+        [] ->
+            Notify = remove_entry(Key, Pid, unreg),
+            {MAcc, Notify ++ NAcc};
+        [{ToPid,Ref,_}|_] ->
+            Value = case ets:lookup(?TAB, {Key,T}) of
+                        [{_, _, V}] -> V;
+                        _ -> undefined
+                    end,
+            Notify = remove_rev_entry(Opts, Pid, Key, {failover, ToPid}),
+            Opts1 = gproc_lib:remove_monitor(Opts, ToPid, Ref),
+            _ = gproc_lib:ensure_monitor(ToPid, g),
+            NewReg = {{Key,T}, ToPid, Value},
+            NewRev = {{ToPid, Key}, Opts1},
+            ets:insert(?TAB, [NewReg, NewRev]),
+            {[NewReg, NewRev | MAcc], Notify ++ NAcc}
+    end.
+
+filter_standbys(SBs) ->
+    filter_standbys(SBs, [node()|nodes()]).
+
+filter_standbys([{Pid,_,_} = H|T], Nodes) ->
+    case lists:member(node(Pid), Nodes) of
+        true ->
+            [H|T];
+        false ->
+            filter_standbys(T, Nodes)
+    end;
+filter_standbys([], _) ->
+    [].
 
-remove_rev_entry(Pid, {T,g,_} = K, Event) when T==n; T==a ->
-    Key = {Pid, K},
-    _ = case ets:lookup(?TAB, Key) of
-	    []       -> ok;
-	    [{_, r}] -> ok;
-	    [{_, Opts}] when is_list(Opts) ->
-		gproc_lib:notify(Event, K, Opts)
-	end,
-    ets:delete(?TAB, Key);
-remove_rev_entry(Pid, K, _Event) ->
-    ets:delete(?TAB, {Pid, K}).
 
+remove_entry(Key, Pid, Event) ->
+    K = ets_key(Key, Pid),
+    ets:delete(?TAB, K),
+    remove_rev_entry(get_opts(Pid, Key), Pid, Key, Event).
+
+remove_rev_entry(Opts, Pid, {T,g,_} = K, Event) when T==n; T==a ->
+    Key = {Pid, K},
+    gproc_lib:notify(Event, K, Opts),
+    ets:delete(?TAB, Key),
+    [{K, Pid, Event}];
+remove_rev_entry(_, Pid, K, _Event) ->
+    ets:delete(?TAB, {Pid, K}),
+    [].
+
+get_opts(Pid, K) ->
+    case ets:lookup(?TAB, {Pid, K}) of
+        [] -> [];
+        [{_, r}] -> [];
+        [{_, Opts}] -> Opts
+    end.
 
 code_change(_FromVsn, S, _Extra, _E) ->
     {ok, S}.
@@ -659,23 +760,22 @@ from_leader({sync, Ref}, S, _E) ->
     {ok, S};
 from_leader(Ops, S, _E) ->
     lists:foreach(
-      fun({delete, Globals, Event}) ->
-              delete_globals(Globals, Event);
+      fun({delete, Globals}) ->
+              delete_globals(Globals);
          ({insert, Globals}) ->
-	      _ = insert_globals(Globals)
+	      _ = insert_globals(Globals);
+         ({notify, Events}) ->
+              do_notify(Events)
       end, Ops),
     {ok, S}.
 
 insert_globals(Globals) ->
-    ets:insert(?TAB, Globals),
     lists:foldl(
-      fun({{{_,_,_} = Key,Pid}, Pid, _}, A) ->
+      fun({{{_,_,_} = Key,_}, Pid, _} = Obj, A) ->
+              ets:insert(?TAB, Obj),
 	      ets:insert_new(?TAB, {{Pid,Key}, []}),
 	      gproc_lib:ensure_monitor(Pid,g),
 	      A;
-	 ({{{_,_,_}, n}, Pid, _}, A) ->
-	      gproc_lib:ensure_monitor(Pid,g),
-	      A;
 	 ({{P,_K}, Opts} = Obj, A) when is_pid(P), is_list(Opts),Opts =/= [] ->
 	      ets:insert(?TAB, Obj),
 	      gproc_lib:ensure_monitor(P,g),
@@ -685,20 +785,30 @@ insert_globals(Globals) ->
       end, Globals, Globals).
 
 
-delete_globals(Globals, Event) ->
+delete_globals(Globals) ->
     lists:foreach(
       fun({{_,g,_},T} = K) when is_atom(T) ->
               ets:delete(?TAB, K);
          ({Key, Pid}) when is_pid(Pid); Pid==shared ->
-              K = ets_key(Key,Pid),
-              ets:delete(?TAB, K),
-	      remove_rev_entry(Pid, Key, Event);
+              ets:delete(?TAB, {Pid, Key});
          ({Pid, Key}) when is_pid(Pid); Pid==shared ->
-	      K = ets_key(Key, Pid),
-	      ets:delete(?TAB, K),
-	      remove_rev_entry(Pid, Key, Event)
+	      ets:delete(?TAB, {Pid, Key})
       end, Globals).
 
+do_notify([{P, Msg}|T]) when is_pid(P) ->
+    P ! Msg,
+    do_notify(T);
+do_notify([{K, P, E}|T]) ->
+    case ets:lookup(?TAB, {P,K}) of
+        [{_, Opts}] when is_list(Opts) ->
+            gproc_lib:notify(E, K, Opts);
+        _ ->
+            do_notify(T)
+    end;
+do_notify([]) ->
+    ok.
+
+
 ets_key({T,_,_} = K, _) when T==n; T==a ->
     {K, T};
 ets_key(K, Pid) ->
@@ -846,3 +956,37 @@ pid_to_give_away_to({T,g,_} = Key) when T==n; T==a ->
         _ ->
             undefined
     end.
+
+insert_reg([{_, Waiters}], K, Val, Pid, Event) ->
+    gproc_lib:insert_reg(K, Val, Pid, g, []),
+    tell_waiters(Waiters, K, Pid, Val, Event).
+
+tell_waiters([{P,R}|T], K, Pid, V, Event) ->
+    Msg = {gproc, R, registered, {K, Pid, V}},
+    if node(P) == node() ->
+            P ! Msg;
+       true ->
+            [{P, Msg} | tell_waiters(T, K, Pid, V, Event)]
+    end;
+tell_waiters([{P,R,follow}|T], K, Pid, V, Event) ->
+    Msg = {gproc, Event, R, K},
+    if node(P) == node() ->
+            P ! Msg;
+       true ->
+            [{P, Msg} | tell_waiters(T, K, Pid, V, Event)]
+    end;
+tell_waiters([], _, _, _, _) ->
+    [].
+
+add_follow_to_waiters(Waiters, {T,_,_} = K, Pid, Ref, S) ->
+    Obj = {{K,T}, [{Pid, Ref, follow}|Waiters]},
+    Rev = {{Pid,K}, []},
+    ets:insert(?TAB, [Obj, Rev]),
+    Msg = {gproc, unreg, Ref, K},
+    if node(Pid) == node() ->
+            Pid ! Msg,
+            {reply, Ref, [{insert, [Obj, Rev]}], S};
+       true ->
+            {reply, Ref, [{insert, [Obj, Rev]},
+                          {notify, [{Pid, Msg}]}]}
+    end.

src/gproc_lib.erl

@@ -26,23 +26,35 @@
          do_set_value/3,
          ensure_monitor/2,
          insert_many/4,
-         insert_reg/4,
+         insert_reg/4, insert_reg/5,
 	 insert_attr/4,
          remove_many/4,
          remove_reg/3, remove_reg/4,
-	 add_monitor/3,
+         monitors/1,
+         standbys/1,
+         followers/1,
+         remove_monitor_pid/2,
+	 add_monitor/4,
 	 remove_monitor/3,
 	 remove_monitors/3,
-	 remove_reverse_mapping/3,
+	 remove_reverse_mapping/3, remove_reverse_mapping/4,
 	 notify/2, notify/3,
 	 remove_wait/4,
          update_aggr_counter/3,
          update_counter/3,
 	 valid_opts/2]).
 
+-export([dbg/1]).
+
 -include("gproc_int.hrl").
 -include("gproc.hrl").
 
+dbg(Mods) ->
+    dbg:tracer(),
+    [dbg:tpl(M,x) || M <- Mods],
+    dbg:tp(ets,'_',[{[gproc,'_'], [], [{message,{exception_trace}}]}]),
+    dbg:p(all,[c]).
+
 %% We want to store names and aggregated counters with the same
 %% structure as properties, but at the same time, we must ensure
 %% that the key is unique. We replace the Pid in the key part
@@ -51,8 +63,10 @@
 %% symmetric.
 %%
 -spec insert_reg(key(), any(), pid() | shared, scope()) -> boolean().
+insert_reg(K, Value, Pid, Scope) ->
+    insert_reg(K, Value, Pid, Scope, registered).
 
-insert_reg({T,_,Name} = K, Value, Pid, Scope) when T==a; T==n ->
+insert_reg({T,_,Name} = K, Value, Pid, Scope, Event) when T==a; T==n ->
     MaybeScan = fun() ->
                         if T==a ->
                                 Initial = scan_existing_counters(Scope, Name),
@@ -61,23 +75,23 @@ insert_reg({T,_,Name} = K, Value, Pid, Scope) when T==a; T==n ->
                                 true
                         end
                 end,
-    Info = [{{K, T}, Pid, Value}, {{Pid,K}, []}],
-    case ets:insert_new(?TAB, Info) of
+    case ets:insert_new(?TAB, {{K,T}, Pid, Value}) of
         true ->
+            _ = ets:insert_new(?TAB, {{Pid,K}, []}),
             MaybeScan();
         false ->
-            if T==n ->
-                    maybe_waiters(K, Pid, Value, T, Info);
+            if T==n; T==a ->
+                    maybe_waiters(K, Pid, Value, T, Event);
                true ->
                     false
             end
     end;
-insert_reg({p,Scope,_} = K, Value, shared, Scope)
+insert_reg({p,Scope,_} = K, Value, shared, Scope, _E)
   when Scope == g; Scope == l ->
     %% shared properties are unique
     Info = [{{K, shared}, shared, Value}, {{shared,K}, []}],
     ets:insert_new(?TAB, Info);
-insert_reg({c,Scope,Ctr} = Key, Value, Pid, Scope) when Scope==l; Scope==g ->
+insert_reg({c,Scope,Ctr} = Key, Value, Pid, Scope, _E) when Scope==l; Scope==g ->
     %% Non-unique keys; store Pid in the key part
     K = {Key, Pid},
     Kr = {Pid, Key},
@@ -89,7 +103,7 @@ insert_reg({c,Scope,Ctr} = Key, Value, Pid, Scope) when Scope==l; Scope==g ->
             ignore
     end,
     Res;
-insert_reg({_,_,_} = Key, Value, Pid, _Scope) when is_pid(Pid) ->
+insert_reg({_,_,_} = Key, Value, Pid, _Scope, _E) when is_pid(Pid) ->
     %% Non-unique keys; store Pid in the key part
     K = {Key, Pid},
     Kr = {Pid, Key},
@@ -150,7 +164,7 @@ insert_objects(Objs) ->
               case Existing of
                   [] -> ok;
                   [{_, Waiters}] ->
-                      notify_waiters(Waiters, Id, Pid, V)
+                      notify_waiters(Waiters, Id, Pid, V, registered)
               end
       end, Objs).
 
@@ -182,30 +196,30 @@ await({T,C,_} = Key, WPid, {_Pid, Ref} = From) ->
             {reply, Ref, [W,Rev]}
     end.
 
-
-
-maybe_waiters(K, Pid, Value, T, Info) ->
+maybe_waiters(_, _, _, _, []) ->
+    false;
+maybe_waiters(K, Pid, Value, T, Event) ->
     case ets:lookup(?TAB, {K,T}) of
         [{_, Waiters}] when is_list(Waiters) ->
-            ets:insert(?TAB, Info),
-            notify_waiters(Waiters, K, Pid, Value),
+            Followers = [F || {_,_,follow} = F <- Waiters],
+            ets:insert(?TAB, [{{K,T}, Pid, Value},
+                              {{Pid,K}, [{monitor, Followers}
+                                         || Followers =/= []]}]),
+            notify_waiters(Waiters, K, Pid, Value, Event),
             true;
-        [_] ->
+        _ ->
             false
     end.
 
-
--spec notify_waiters([{pid(), reference()}], key(), pid(), any()) -> ok.
-
-notify_waiters(Waiters, K, Pid, V) ->
-    _ = [begin
-             P ! {gproc, Ref, registered, {K, Pid, V}},
-             case P of
-		 Pid -> ignore;
-		 _ ->
-		     ets:delete(?TAB, {P, K})
-	     end
-         end || {P, Ref} <- Waiters],
+-spec notify_waiters([{pid(), reference()}], key(), pid(), any(), any()) -> ok.
+notify_waiters([{P, Ref}|T], K, Pid, V, E) ->
+    P ! {gproc, Ref, registered, {K, Pid, V}},
+    notify_waiters(T, K, Pid, V, E);
+notify_waiters([{P, Ref, follow}|T], K, Pid, V, E) ->
+    %% This is really a monitor, lurking in the Waiters list
+    P ! {gproc, E, Ref, K},
+    notify_waiters(T, K, Pid, V, E);
+notify_waiters([], _, _, _, _) ->
     ok.
 
 remove_wait({T,_,_} = Key, Pid, Ref, Waiters) ->
@@ -230,10 +244,15 @@ remove_wait({T,_,_} = Key, Pid, Ref, Waiters) ->
     end.
 
 remove_from_waiters(Waiters, Pid, all) ->
-    [{P,R} || {P,R} <- Waiters,
-	      P =/= Pid];
+    [W || W <- Waiters,
+	      element(1,W) =/= Pid];
 remove_from_waiters(Waiters, Pid, Ref) ->
-    Waiters -- [{Pid, Ref}].
+    [W || W <- Waiters, not is_waiter(W, Pid, Ref)].
+
+is_waiter({Pid, Ref}   , Pid, Ref) -> true;
+is_waiter({Pid, Ref, _}, Pid, Ref) -> true;
+is_waiter(_, _, _) ->
+    false.
 
 remove_monitors(Key, Pid, MPid) ->
     case ets:lookup(?TAB, {Pid, Key}) of
@@ -303,7 +322,8 @@ remove_reverse_mapping(Event, Pid, Key) ->
     remove_reverse_mapping(Event, Pid, Key, Opts).
 
 remove_reverse_mapping(Event, Pid, Key, Opts) when Event==unreg;
-						   element(1,Event)==migrated ->
+						   element(1,Event)==migrated;
+                                                   element(1,Event)==failover ->
     Rev = {Pid, Key},
     _ = notify(Event, Key, Opts),
     ets:delete(?TAB, Rev),
@@ -312,31 +332,77 @@ remove_reverse_mapping(Event, Pid, Key, Opts) when Event==unreg;
 notify(Key, Opts) ->
     notify(unreg, Key, Opts).
 
-notify([], _, _) ->
-    [];
-notify(Event, Key, Opts) ->
+monitors(Opts) ->
     case lists:keyfind(monitor, 1, Opts) of
 	false ->
 	    [];
 	{_, Mons} ->
-	    [begin P ! {gproc, Event, Ref, Key}, P end || {P, Ref} <- Mons,
-							  node(P) == node()]
+            Mons
     end.
 
-add_monitor([{monitor, Mons}|T], Pid, Ref) ->
-    [{monitor, [{Pid,Ref}|Mons]}|T];
-add_monitor([H|T], Pid, Ref) ->
-    [H|add_monitor(T, Pid, Ref)];
-add_monitor([], Pid, Ref) ->
-    [{monitor, [{Pid, Ref}]}].
+standbys(Opts) ->
+    select_monitors(monitors(Opts), standby, []).
+
+followers(Opts) ->
+    select_monitors(monitors(Opts), follow, []).
+
+select_monitors([{_,_,Type}=H|T], Type, Acc) ->
+    select_monitors(T, Type, [H|Acc]);
+select_monitors([_|T], Type, Acc) ->
+    select_monitors(T, Type, Acc);
+select_monitors([], _, Acc) ->
+    Acc.
+
+remove_monitor_pid([{monitor, Mons}|T], Pid) ->
+    [{monitors, [M || M <- Mons,
+                      element(1, M) =/= Pid]}|T];
+remove_monitor_pid([H|T], Pid) ->
+    [H | remove_monitor_pid(T, Pid)];
+remove_monitor_pid([], _) ->
+    [].
+
+
+notify([], _, _) ->
+    ok;
+notify(Event, Key, Opts) ->
+    notify_(monitors(Opts), Event, Key).
+
+%% Also handle old-style monitors
+notify_([{Pid,Ref}|T], Event, Key) ->
+    Pid ! {gproc, Event, Ref, Key},
+    notify_(T, Event, Key);
+notify_([{Pid,Ref,_}|T], Event, {_,l,_} = Key) ->
+    Pid ! {gproc, Event, Ref, Key},
+    notify_(T, Event, Key);
+notify_([{Pid,Ref,_}|T], Event, {_,g,_} = Key) when node(Pid) == node() ->
+    Pid ! {gproc, Event, Ref, Key},
+    notify_(T, Event, Key);
+notify_([_|T], Event, Key) ->
+    notify_(T, Event, Key);
+notify_([], _, _) ->
+    ok.
+
+
+
+add_monitor([{monitor, Mons}|T], Pid, Ref, Type) ->
+    [{monitor, [{Pid,Ref,Type}|Mons]}|T];
+add_monitor([H|T], Pid, Ref, Type) ->
+    [H|add_monitor(T, Pid, Ref, Type)];
+add_monitor([], Pid, Ref, Type) ->
+    [{monitor, [{Pid, Ref, Type}]}].
 
 remove_monitor([{monitor, Mons}|T], Pid, Ref) ->
-    [{monitor, Mons -- [{Pid, Ref}]}|T];
+    [{monitor, [Mon || Mon <- Mons, not is_mon(Mon,Pid,Ref)]} | T];
 remove_monitor([H|T], Pid, Ref) ->
     [H|remove_monitor(T, Pid, Ref)];
 remove_monitor([], _Pid, _Ref) ->
     [].
 
+is_mon({Pid,Ref,_}, Pid, Ref) -> true;
+is_mon({Pid,Ref},   Pid, Ref) -> true;
+is_mon(_, _, _) ->
+    false.
+
 remove_many(T, Scope, L, Pid) ->
     lists:flatmap(fun(K) ->
                           Key = {T, Scope, K},

test/gproc_dist_tests.erl

@@ -88,6 +88,12 @@ dist_test_() ->
                                        ?debugVal(t_monitor(Ns))
                                end,
                                fun() ->
+                                       ?debugVal(t_standby_monitor(Ns))
+                               end,
+                               fun() ->
+                                       ?debugVal(t_follow_monitor(Ns))
+                               end,
+                               fun() ->
                                        ?debugVal(t_subscribe(Ns))
                                end
                            ]
@@ -312,6 +318,31 @@ t_monitor([A,B|_]) ->
     ?assertMatch(true, t_call(Pc, {apply, gproc, unreg, [Na]})),
     ?assertMatch({gproc,unreg,Ref1,Na}, got_msg(Pb, gproc)).
 
+t_standby_monitor([A,B|_] = Ns) ->
+    Na = ?T_NAME,
+    Pa = t_spawn_reg(A, Na),
+    Pb = t_spawn(B, _Selective = true),
+    Ref = t_call(Pb, {apply, gproc, monitor, [Na, standby]}),
+    ?assert(is_reference(Ref)),
+    ?assertMatch(ok, t_call(Pa, die)),
+    ?assertMatch({gproc,{failover,Pb},Ref,Na}, got_msg(Pb, gproc)),
+    ?assertMatch(ok, t_lookup_everywhere(Na, Ns, Pb)),
+    Pc = t_spawn(A, true),
+    Ref1 = t_call(Pc, {apply, gproc, monitor, [Na, standby]}),
+    ?assertMatch(true, t_call(Pb, {apply, gproc, unreg, [Na]})),
+    ?assertMatch({gproc,unreg,Ref1,Na}, got_msg(Pc, gproc)),
+    ?assertMatch(ok, t_lookup_everywhere(Na, Ns, undefined)).
+
+t_follow_monitor([A,B|_] = Ns) ->
+    Na = ?T_NAME,
+    Pa = t_spawn(A, _Selective = true),
+    Ref = t_call(Pa, {apply, gproc, monitor, [Na, follow]}),
+    {gproc,unreg,Ref,Na} = got_msg(Pa),
+    Pb = t_spawn_reg(A, Na),
+    {gproc,registered,Ref,Na} = got_msg(Pa),
+    ok = t_call(Pb, die),
+    ok = t_call(Pa, die).
+
 t_subscribe([A,B|_] = Ns) ->
     Na = ?T_NAME,
     Pb = t_spawn(B, _Selective = true),
@@ -327,17 +358,17 @@ t_subscribe([A,B|_] = Ns) ->
     ?assertEqual({gproc_monitor,Na,undefined}, got_msg(Pb, gproc_monitor)),
     ok.
 
-got_msg(Pb, Tag) ->
-    t_call(Pb,
-	   {apply_fun,
-	    fun() ->
-		    receive
-			M when element(1, M) == Tag ->
-			    M
-		    after 1000 ->
-			    erlang:error({timeout, got_msg, [Pb, Tag]})
-		    end
-	    end}).
+%% got_msg(Pb, Tag) ->
+%%     t_call(Pb,
+%% 	   {apply_fun,
+%% 	    fun() ->
+%% 		    receive
+%% 			M when element(1, M) == Tag ->
+%% 			    M
+%% 		    after 1000 ->
+%% 			    erlang:error({timeout, got_msg, [Pb, Tag]})
+%% 		    end
+%% 	    end}).
 
 %% Verify that the gproc_dist:sync() call returns true even if a candidate dies
 %% while the sync is underway. This test makes use of sys:suspend() to ensure that
@@ -418,97 +449,17 @@ t_read_everywhere(Key, Pid, Nodes, Exp, I) ->
 read_result({badrpc, {'EXIT', {badarg, _}}}) -> badarg;
 read_result(R) -> R.
 
-t_spawn(Node) ->
-    t_spawn(Node, false).
-
-t_spawn(Node, Selective) when is_boolean(Selective) ->
-    Me = self(),
-    P = spawn(Node, fun() ->
-			    Me ! {self(), ok},
-			    t_loop(Selective)
-		    end),
-    receive
-	{P, ok} -> P
-    after 1000 ->
-            erlang:error({timeout, t_spawn, [Node, Selective]})
-    end.
-
-t_spawn_reg(Node, Name) ->
-    t_spawn_reg(Node, Name, default_value(Name)).
-
-t_spawn_reg(Node, Name, Value) ->
-    Me = self(),
-    P = spawn(Node, fun() ->
-                            ?assertMatch(true, gproc:reg(Name, Value)),
-                            Me ! {self(), ok},
-                            t_loop()
-                    end),
-    receive
-	{P, ok} -> P
-    after 1000 ->
-            erlang:error({timeout, t_spawn_reg, [Node, Name, Value]})
-    end.
-
-t_spawn_reg_shared(Node, Name, Value) ->
-    Me = self(),
-    P = spawn(Node, fun() ->
-                            ?assertMatch(true, gproc:reg_shared(Name, Value)),
-                            Me ! {self(), ok},
-                            t_loop()
-                    end),
-    receive
-	{P, ok} -> P
-    after 1000 ->
-              erlang:error({timeout, t_spawn_reg_shared, [Node,Name,Value]})
-    end.
-
-default_value({c,_,_}) -> 0;
-default_value(_) -> undefined.
-
-t_spawn_mreg(Node, KVL) ->
-    Me = self(),
-    P = spawn(Node, fun() ->
-                            ?assertMatch(true, gproc:mreg(n, g, KVL)),
-                            Me ! {self(), ok},
-                            t_loop()
-                    end),
-    receive
-	{P, ok} -> P
-    end.
+t_spawn(Node) -> gproc_test_lib:t_spawn(Node).
+t_spawn(Node, Selective) -> gproc_test_lib:t_spawn(Node, Selective).
+t_spawn_mreg(Node, KVL) -> gproc_test_lib:t_spawn_mreg(Node, KVL).
+t_spawn_reg(Node, N) -> gproc_test_lib:t_spawn_reg(Node, N).
+t_spawn_reg(Node, N, V) -> gproc_test_lib:t_spawn_reg(Node, N, V).
+t_spawn_reg_shared(Node, N, V) -> gproc_test_lib:t_spawn_reg_shared(Node, N, V).
+got_msg(P) -> gproc_test_lib:got_msg(P).
+got_msg(P, Tag) -> gproc_test_lib:got_msg(P, Tag).
 
 t_call(P, Req) ->
-    Ref = erlang:monitor(process, P),
-    P ! {self(), Ref, Req},
-    receive
-	{P, Ref, Res} ->
-	    erlang:demonitor(Ref, [flush]),
-	    Res;
-	{'DOWN', Ref, _, _, Error} ->
-	    erlang:error({'DOWN', P, Error})
-    after 1000 ->
-            erlang:error({timeout,t_call,[P,Req]})
-    end.
-
-t_loop() ->
-    t_loop(false).
-
-t_loop(Selective) when is_boolean(Selective) ->
-    receive
-	{From, Ref, die} ->
-	    From ! {self(), Ref, ok};
-	{From, Ref, {selective, Bool}} when is_boolean(Bool) ->
-	    From ! {self(), Ref, ok},
-	    t_loop(Bool);
-	{From, Ref, {apply, M, F, A}} ->
-	    From ! {self(), Ref, apply(M, F, A)},
-	    t_loop(Selective);
-	{From, Ref, {apply_fun, F}} ->
-	    From ! {self(), Ref, F()},
-	    t_loop(Selective);
-	Other when not Selective ->
-	    ?debugFmt("got unknown msg: ~p~n", [Other]),
-	    exit({unknown_msg, Other})
-    end.
+    gproc_test_lib:t_call(P, Req).
 
 start_slaves(Ns) ->
     [H|T] = Nodes = [start_slave(N) || N <- Ns],

test/gproc_test_lib.erl

@@ -0,0 +1,126 @@
+-module(gproc_test_lib).
+
+-export([t_spawn/1, t_spawn/2,
+         t_spawn_reg/2, t_spawn_reg/3,
+         t_spawn_reg_shared/3,
+         t_spawn_mreg/2,
+         t_call/2,
+         t_loop/0, t_loop/1,
+         got_msg/1, got_msg/2]).
+
+-include_lib("eunit/include/eunit.hrl").
+
+t_spawn(Node) ->
+    t_spawn(Node, false).
+
+t_spawn(Node, Selective) when is_boolean(Selective) ->
+    Me = self(),
+    P = spawn(Node, fun() ->
+			    Me ! {self(), ok},
+			    t_loop(Selective)
+		    end),
+    receive
+	{P, ok} -> P
+    after 1000 ->
+            erlang:error({timeout, t_spawn, [Node, Selective]})
+    end.
+
+t_spawn_reg(Node, Name) ->
+    t_spawn_reg(Node, Name, default_value(Name)).
+
+t_spawn_reg(Node, Name, Value) ->
+    Me = self(),
+    P = spawn(Node, fun() ->
+                            ?assertMatch(true, gproc:reg(Name, Value)),
+                            Me ! {self(), ok},
+                            t_loop()
+                    end),
+    receive
+	{P, ok} -> P
+    after 1000 ->
+            erlang:error({timeout, t_spawn_reg, [Node, Name, Value]})
+    end.
+
+t_spawn_mreg(Node, KVL) ->
+    Me = self(),
+    P = spawn(Node, fun() ->
+                            ?assertMatch(true, gproc:mreg(n, g, KVL)),
+                            Me ! {self(), ok},
+                            t_loop()
+                    end),
+    receive
+	{P, ok} -> P
+    end.
+
+
+t_spawn_reg_shared(Node, Name, Value) ->
+    Me = self(),
+    P = spawn(Node, fun() ->
+                            ?assertMatch(true, gproc:reg_shared(Name, Value)),
+                            Me ! {self(), ok},
+                            t_loop()
+                    end),
+    receive
+	{P, ok} -> P
+    after 1000 ->
+              erlang:error({timeout, t_spawn_reg_shared, [Node,Name,Value]})
+    end.
+
+default_value({c,_,_}) -> 0;
+default_value(_) -> undefined.
+
+t_call(P, Req) ->
+    Ref = erlang:monitor(process, P),
+    P ! {self(), Ref, Req},
+    receive
+	{P, Ref, Res} ->
+	    erlang:demonitor(Ref, [flush]),
+	    Res;
+	{'DOWN', Ref, _, _, Error} ->
+	    erlang:error({'DOWN', P, Error})
+    after 1000 ->
+            erlang:error({timeout,t_call,[P,Req]})
+    end.
+
+t_loop() ->
+    t_loop(false).
+
+t_loop(Selective) when is_boolean(Selective) ->
+    receive
+	{From, Ref, die} ->
+	    From ! {self(), Ref, ok};
+	{From, Ref, {selective, Bool}} when is_boolean(Bool) ->
+	    From ! {self(), Ref, ok},
+	    t_loop(Bool);
+	{From, Ref, {apply, M, F, A}} ->
+	    From ! {self(), Ref, apply(M, F, A)},
+	    t_loop(Selective);
+	{From, Ref, {apply_fun, F}} ->
+	    From ! {self(), Ref, F()},
+	    t_loop(Selective);
+	Other when not Selective ->
+	    ?debugFmt("got unknown msg: ~p~n", [Other]),
+	    exit({unknown_msg, Other})
+    end.
+
+got_msg(Pb) ->
+    t_call(Pb,
+           {apply_fun,
+            fun() ->
+                    receive M -> M
+                    after 1000 ->
+                            erlang:error({timeout, got_msg, [Pb]})
+                    end
+            end}).
+
+got_msg(Pb, Tag) ->
+    t_call(Pb,
+	   {apply_fun,
+	    fun() ->
+		    receive
+			M when element(1, M) == Tag ->
+			    M
+		    after 1000 ->
+			    erlang:error({timeout, got_msg, [Pb, Tag]})
+		    end
+	    end}).

test/gproc_tests.erl

@@ -22,6 +22,8 @@
 -include_lib("eunit/include/eunit.hrl").
 -include_lib("stdlib/include/qlc.hrl").
 
+-define(T_NAME, {n, l, {?MODULE, ?LINE, erlang:now()}}).
+
 conf_test_() ->
     {foreach,
      fun() ->
@@ -130,6 +132,10 @@ reg_test_() ->
       , ?_test(t_is_clean())
       , {spawn, ?_test(?debugVal(t_monitor_give_away()))}
       , ?_test(t_is_clean())
+      , {spawn, ?_test(?debugVal(t_monitor_standby()))}
+      , ?_test(t_is_clean())
+      , {spawn, ?_test(?debugVal(t_monitor_follow()))}
+      , ?_test(t_is_clean())
       , {spawn, ?_test(?debugVal(t_subscribe()))}
       , ?_test(t_is_clean())
       , {spawn, ?_test(?debugVal(t_gproc_info()))}
@@ -381,7 +387,7 @@ t_give_away_to_pid() ->
     ?assertEqual(Me, gproc:where(From)),
     ?assertEqual(P, gproc:give_away(From, P)),
     ?assertEqual(P, gproc:where(From)),
-    ?assertEqual(ok, t_call(P, die)).
+    ?assertEqual(ok, t_lcall(P, die)).
 
 t_give_away_to_self() ->
     From = {n, l, foo},
@@ -416,9 +422,9 @@ t_give_away_and_back() ->
     ?assertEqual(Me, gproc:where(From)),
     ?assertEqual(P, gproc:give_away(From, P)),
     ?assertEqual(P, gproc:where(From)),
-    ?assertEqual(ok, t_call(P, {give_away, From})),
+    ?assertEqual(ok, t_lcall(P, {give_away, From})),
     ?assertEqual(Me, gproc:where(From)),
-    ?assertEqual(ok, t_call(P, die)).
+    ?assertEqual(ok, t_lcall(P, die)).
 
 t_select() ->
     ?assertEqual(true, gproc:reg({n, l, {n,1}}, x)),
@@ -657,7 +663,7 @@ t_get_env_inherit() ->
     ?assertEqual(bar, gproc:get_env(l, gproc, foo, [{inherit, P}])),
     ?assertEqual(bar, gproc:get_env(l, gproc, foo,
 				    [{inherit, {n,l,get_env_p}}])),
-    ?assertEqual(ok, t_call(P, die)).
+    ?assertEqual(ok, t_lcall(P, die)).
 
 %% What we test here is that we return the same current_function as the
 %% process_info() BIF. As we parse the backtrace dump, we check with some
@@ -704,7 +710,7 @@ t_monitor() ->
 	    ok
     end,
     Ref = gproc:monitor({n,l,a}),
-    ?assertEqual(ok, t_call(P, die)),
+    ?assertEqual(ok, t_lcall(P, die)),
     receive
 	M ->
 	    ?assertEqual({gproc,unreg,Ref,{n,l,a}}, M)
@@ -721,12 +727,48 @@ t_monitor_give_away() ->
 	    ok
     end,
     Ref = gproc:monitor({n,l,a}),
-    ?assertEqual(ok, t_call(P, {give_away, {n,l,a}})),
+    ?assertEqual(ok, t_lcall(P, {give_away, {n,l,a}})),
     receive
 	M ->
 	    ?assertEqual({gproc,{migrated,Me},Ref,{n,l,a}}, M)
     end,
-    ?assertEqual(ok, t_call(P, die)).
+    ?assertEqual(ok, t_lcall(P, die)).
+
+t_monitor_standby() ->
+    Me = self(),
+    P = spawn(fun() ->
+                      gproc:reg({n,l,a}),
+                      Me ! continue,
+                      t_loop()
+              end),
+    receive continue ->
+	    ok
+    end,
+    Ref = gproc:monitor({n,l,a}, standby),
+    exit(P, kill),
+    receive
+	M ->
+	    ?assertEqual({gproc,{failover,Me},Ref,{n,l,a}}, M)
+    end,
+    gproc:unreg({n,l,a}),
+    ok.
+
+t_monitor_follow() ->
+    Name = ?T_NAME,
+    P1 = t_spawn(_Selective = true),
+    Ref = t_call(P1, {apply, gproc, monitor, [Name, follow]}),
+    {gproc,unreg,Ref,Name} = got_msg(P1),
+    %% gproc_lib:dbg([gproc,gproc_lib]),
+    P2 = t_spawn_reg(Name),
+    {gproc,registered,Ref,Name} = got_msg(P1),
+    exit(P2, kill),
+    {gproc,unreg,Ref,Name} = got_msg(P1),
+    P3 = t_spawn(true),
+    Ref3 = t_call(P3, {apply, gproc, monitor, [Name, standby]}),
+    {gproc,{failover,P3},Ref,Name} = got_msg(P1),
+    {gproc,{failover,P3},Ref3,Name} = got_msg(P3),
+    [exit(P,kill) || P <- [P1,P3]],
+    ok.
 
 t_subscribe() ->
     Key = {n,l,a},
@@ -737,11 +779,11 @@ t_subscribe() ->
 			   t_loop()
 		   end),
     ?assertEqual({gproc_monitor, Key, P}, get_msg()),
-    ?assertEqual(ok, t_call(P, {give_away, Key})),
+    ?assertEqual(ok, t_lcall(P, {give_away, Key})),
     ?assertEqual({gproc_monitor, Key, {migrated,self()}}, get_msg()),
     gproc:give_away(Key, P),
     ?assertEqual({gproc_monitor, Key, {migrated,P}}, get_msg()),
-    ?assertEqual(ok, t_call(P, die)),
+    ?assertEqual(ok, t_lcall(P, die)),
     ?assertEqual({gproc_monitor, Key, undefined}, get_msg()),
     ?assertEqual(ok, gproc_monitor:unsubscribe(Key)).
 
@@ -752,6 +794,14 @@ get_msg() ->
 	    timeout
     end.
 
+%% t_spawn()      -> gproc_test_lib:t_spawn(node()).
+t_spawn(Sel)   -> gproc_test_lib:t_spawn(node(), Sel).
+t_spawn_reg(N) -> gproc_test_lib:t_spawn_reg(node(), N).
+t_call(P, Req) -> gproc_test_lib:t_call(P, Req).
+%% got_msg(P, M)  -> gproc_test_lib:got_msg(P, M).
+got_msg(P)     -> gproc_test_lib:got_msg(P).
+
+
 t_loop() ->
     receive
 	{From, {give_away, Key}} ->
@@ -762,7 +812,7 @@ t_loop() ->
 	    From ! {self(), ok}
     end.
 
-t_call(P, Msg) ->
+t_lcall(P, Msg) ->
     P ! {self(), Msg},
     receive
 	{P, Reply} ->