gproc/doc/gproc.md

#Module gproc#

• Description
• Data Types
• Function Index
• Function Details

Extended process registry
This module implements an extended process registry.

Behaviours: gen_server.

Authors: Ulf Wiger (ulf.wiger@erlang-consulting.com).

##Description##

For a detailed description, see erlang07-wiger.pdf.

##Tuning Gproc performance##

Gproc relies on a central server and an ordered-set ets table. Effort is made to perform as much work as possible in the client without sacrificing consistency. A few things can be tuned by setting the following application environment variables in the top application of gproc (usually gproc):

• {ets_options, list()} - Currently, the options {write_concurrency, F} and {read_concurrency, F} are allowed. The default is [{write_concurrency, true}, {read_concurrency, true}]
• {server_options, list()} - These will be passed as spawn options when starting the gproc and gproc_dist servers. Default is []. It is likely that {priority, high | max} and/or increasing min_heap_size will improve performance.

##Data Types##

###context()##

context() = {scope(), type()} | type()

{'all','all'} is the default

###headpat()##

headpat() = {keypat(), pidpat(), ValPat}

###key()##

key() = {type(), scope(), any()}

###keypat()##

keypat() = {sel_type() | sel_var(), l | g | sel_var(), any()}

###pidpat()##

pidpat() = pid() | sel_var()

###reg_id()##

reg_id() = {type(), scope(), any()}

###scope()##

scope() = l | g

l = local registration; g = global registration

###sel_pattern()##

sel_pattern() = [{headpat(), Guards, Prod}]

###sel_scope()##

sel_scope() = scope | all | global | local

###sel_type()##

sel_type() = type() | names | props | counters | aggr_counters

###sel_var()##

sel_var() = DollarVar | '_'

###type()##

type() = n | p | c | a

n = name; p = property; c = counter; a = aggregate_counter

###unique_id()##

unique_id() = {n | a, scope(), any()}

##Function Index##

add_global_aggr_counter/1	Registers a global (unique) aggregated counter.
add_global_counter/2	Registers a global (non-unique) counter.
add_global_name/1	Registers a global (unique) name.
add_global_property/2	Registers a global (non-unique) property.
add_local_aggr_counter/1	Registers a local (unique) aggregated counter.
add_local_counter/2	Registers a local (non-unique) counter.
add_local_name/1	Registers a local (unique) name.
add_local_property/2	Registers a local (non-unique) property.
add_shared_local_counter/2	Registers a local shared (unique) counter.
audit_process/1
await/1	Equivalent to await(Key, infinity).
await/2	Wait for a local name to be registered.
cancel_wait/2	Cancels a previous call to nb_wait/1.
cancel_wait_or_monitor/1
default/1
demonitor/2	Remove a monitor on a registered name This function is the reverse of monitor/1.
first/1	Behaves as ets:first(Tab) for a given type of registration object.
get_env/3	Equivalent to get_env(Scope, App, Key, [app_env]).
get_env/4	Read an environment value, potentially cached as a gproc_env property.
get_set_env/3	Equivalent to get_set_env(Scope, App, Key, [app_env]).
get_set_env/4	Fetch and cache an environment value, if not already cached.
get_value/1	Reads the value stored with a key registered to the current process.
get_value/2	Reads the value stored with a key registered to the process Pid.
give_away/2	Atomically transfers the key From to the process identified by To.
goodbye/0	Unregister all items of the calling process and inform gproc to forget about the calling process.
i/0	Similar to the built-in shell command i() but inserts information about names and properties registered in Gproc, where applicable.
info/1	Similar to process_info(Pid) but with additional gproc info.
info/2	Similar to process_info(Pid, Item), but with additional gproc info.
last/1	Behaves as ets:last(Tab) for a given type of registration object.
lookup_global_aggr_counter/1	Lookup a global (unique) aggregated counter and returns its value.
lookup_global_counters/1	Look up all global (non-unique) instances of a given Counter.
lookup_global_name/1	Lookup a global unique name.
lookup_global_properties/1	Look up all global (non-unique) instances of a given Property.
lookup_local_aggr_counter/1	Lookup a local (unique) aggregated counter and returns its value.
lookup_local_counters/1	Look up all local (non-unique) instances of a given Counter.
lookup_local_name/1	Lookup a local unique name.
lookup_local_properties/1	Look up all local (non-unique) instances of a given Property.
lookup_pid/1	Lookup the Pid stored with a key.
lookup_pids/1	Returns a list of pids with the published key Key.
lookup_value/1	Lookup the value stored with a key.
lookup_values/1	Retrieve the {Pid,Value} pairs corresponding to Key.
monitor/1	monitor a registered name This function works much like erlang:monitor(process, Pid), but monitors a unique name registered via gproc.
mreg/3	Register multiple {Key,Value} pairs of a given type and scope.
munreg/3	Unregister multiple Key items of a given type and scope.
nb_wait/1	Wait for a local name to be registered.
next/2	Behaves as ets:next(Tab,Key) for a given type of registration object.
prev/2	Behaves as ets:prev(Tab,Key) for a given type of registration object.
reg/1	Equivalent to reg(Key, default(Key)).
reg/2	Register a name or property for the current process.
reg_shared/1	Register a resource, but don't tie it to a particular process.
reg_shared/2	Register a resource, but don't tie it to a particular process.
register_name/2	Behaviour support callback.
reset_counter/1	Reads and resets a counter in a "thread-safe" way.
select/1	see http://www.erlang.org/doc/man/ets.html#select-1.
select/2	Perform a select operation on the process registry.
select/3	Like select/2 but returns Limit objects at a time.
select_count/1	Equivalent to select_count(all, Pat).
select_count/2	Perform a select_count operation on the process registry.
send/2	Sends a message to the process, or processes, corresponding to Key.
set_env/5	Updates the cached value as well as underlying environment.
set_value/2	Sets the value of the registeration entry given by Key.
start_link/0	Starts the gproc server.
table/0	Equivalent to table({all, all}).
table/1	Equivalent to table(Context, []).
table/2	QLC table generator for the gproc registry.
unreg/1	Unregister a name or property.
unreg_shared/1	Unregister a shared resource.
unregister_name/1	Equivalent to unreg / 1.
update_counter/2	Updates the counter registered as Key for the current process.
update_shared_counter/2
where/1	Returns the pid registered as Key.
whereis_name/1	Equivalent to where / 1.

##Function Details##

###add_global_aggr_counter/1##

add_global_aggr_counter(Name) -> any()

Equivalent to reg({a, g, Name}).

Registers a global (unique) aggregated counter.

###add_global_counter/2##

add_global_counter(Name, Initial) -> any()

Registers a global (non-unique) counter. @equiv reg({c,g,Name},Value)

###add_global_name/1##

add_global_name(Name) -> any()

Registers a global (unique) name. @equiv reg({n,g,Name})

###add_global_property/2##

add_global_property(Name, Value) -> any()

Registers a global (non-unique) property. @equiv reg({p,g,Name},Value)

###add_local_aggr_counter/1##

add_local_aggr_counter(Name) -> any()

Equivalent to reg({a, l, Name}).

Registers a local (unique) aggregated counter.

###add_local_counter/2##

add_local_counter(Name, Initial) -> any()

Registers a local (non-unique) counter. @equiv reg({c,l,Name},Value)

###add_local_name/1##

add_local_name(Name) -> any()

Registers a local (unique) name. @equiv reg({n,l,Name})

###add_local_property/2##

add_local_property(Name, Value) -> any()

Registers a local (non-unique) property. @equiv reg({p,l,Name},Value)

###add_shared_local_counter/2##

add_shared_local_counter(Name, Initial) -> any()

Equivalent to reg_shared({c, l, Name}, Value).

Registers a local shared (unique) counter.

###audit_process/1##

audit_process(Pid::pid()) -> ok

###await/1##

await(Key::key()) -> {pid(), Value}

Equivalent to await(Key, infinity).

###await/2##

await(Key::key(), Timeout) -> {pid(), Value}

• Timeout = integer() | infinity

Wait for a local name to be registered. The function raises an exception if the timeout expires. Timeout must be either an interger > 0 or 'infinity'. A small optimization: we first perform a lookup, to see if the name is already registered. This way, the cost of the operation will be roughly the same as of where/1 in the case where the name is already registered (the difference: await/2 also returns the value).

###cancel_wait/2##

cancel_wait(Key::key(), Ref) -> ok

• Ref = all | reference()

Cancels a previous call to nb_wait/1

If Ref = all, all wait requests on Key from the calling process are canceled.

###cancel_wait_or_monitor/1##

cancel_wait_or_monitor(Key) -> any()

###default/1##

default(X1) -> any()

###demonitor/2##

demonitor(Key::key(), Ref::reference()) -> ok

Remove a monitor on a registered name This function is the reverse of monitor/1. It removes a monitor previously set on a unique name. This function always succeeds given legal input.

###first/1##

first(Context::context()) -> key() | '$end_of_table'

Behaves as ets:first(Tab) for a given type of registration object.

See http://www.erlang.org/doc/man/ets.html#first-1. The registry behaves as an ordered_set table.

###get_env/3##

get_env(Scope::scope(), App::atom(), Key::atom()) -> term()

Equivalent to get_env(Scope, App, Key, [app_env]).

###get_env/4##

get_env(Scope::scope(), App::atom(), Key::atom(), Strategy) -> term()

• Strategy = [Alternative]

• Alternative = app_env | os_env | inherit | {inherit, pid()} | {inherit, unique_id()} | init_arg | {mnesia, ActivityType, Oid, Pos} | {default, term()} | error

Read an environment value, potentially cached as a gproc_env property.

This function first tries to read the value of a cached property, {p, Scope, {gproc_env, App, Key}}. If this fails, it will try the provided alternative strategy. Strategy is a list of alternatives, tried in order.
Each alternative can be one of:

• app_env - try application:get_env(App, Key)
• os_env - try os:getenv(ENV), where ENV is Key converted into an uppercase string
• {os_env, ENV} - try os:getenv(ENV)
• inherit - inherit the cached value, if any, held by the parent process.
• {inherit, Pid} - inherit the cached value, if any, held by Pid.
• {inherit, Id} - inherit the cached value, if any, held by the process registered in gproc as Id.
• init_arg - try init:get_argument(Key); expects a single value, if any.
• {mnesia, ActivityType, Oid, Pos} - try mnesia:activity(ActivityType, fun() -> mnesia:read(Oid) end); retrieve the value in position Pos if object found.
• {default, Value} - set a default value to return once alternatives have been exhausted; if not set, undefined will be returned.
• error - raise an exception, erlang:error(gproc_env, [App, Key, Scope]).

While any alternative can occur more than once, the only one that might make sense to use multiple times is {default, Value}.

The return value will be one of:

• The value of the first matching alternative, or error eception, whichever comes first
• The last instance of {default, Value}, or undefined, if there is no matching alternative, default or error entry in the list.

The error option can be used to assert that a value has been previously cached. Alternatively, it can be used to assert that a value is either cached or at least defined somewhere, e.g. get_env(l, mnesia, dir, [app_env, error]).

###get_set_env/3##

get_set_env(Scope::scope(), App::atom(), Key::atom()) -> term()

Equivalent to get_set_env(Scope, App, Key, [app_env]).

###get_set_env/4##

get_set_env(Scope::scope(), App::atom(), Key::atom(), Strategy) -> Value

Fetch and cache an environment value, if not already cached.

This function does the same thing as get_env/4, but also updates the cache. Note that the cache will be updated even if the result of the lookup is undefined.

See also: get_env/4.

###get_value/1##

get_value(Key) -> Value

Reads the value stored with a key registered to the current process.

If no such key is registered to the current process, this function exits.

###get_value/2##

get_value(Key, Pid) -> Value

Reads the value stored with a key registered to the process Pid.

If Pid == shared, the value of a shared key (see reg_shared/1) will be read.

###give_away/2##

give_away(From::key(), To::pid() | key()) -> undefined | pid()

Atomically transfers the key From to the process identified by To.

This function transfers any gproc key (name, property, counter, aggr counter)
from one process to another, and returns the pid of the new owner.

To must be either a pid or a unique name (name or aggregated counter), but does not necessarily have to resolve to an existing process. If there is no process registered with the To key, give_away/2 returns undefined, and the From key is effectively unregistered.

It is allowed to give away a key to oneself, but of course, this operation
will have no effect.

Fails with badarg if the calling process does not have a From key registered.

###goodbye/0##

goodbye() -> ok

Unregister all items of the calling process and inform gproc
to forget about the calling process.

This function is more efficient than letting gproc perform these cleanup operations.

###i/0##

i() -> ok

Similar to the built-in shell command i() but inserts information about names and properties registered in Gproc, where applicable.

###info/1##

info(Pid::pid()) -> ProcessInfo

• ProcessInfo = [{gproc, [{Key, Value}]} | ProcessInfo]

Similar to process_info(Pid) but with additional gproc info.

Returns the same information as process_info(Pid), but with the addition of a gproc information item, containing the {Key,Value} pairs registered to the process.

###info/2##

info(Pid::pid(), Item::atom()) -> {Item, Info}

Similar to process_info(Pid, Item), but with additional gproc info.

For Item = gproc, this function returns a list of {Key, Value} pairs registered to the process Pid. For other values of Item, it returns the same as http://www.erlang.org/doc/man/erlang.html#process_info-2.

###last/1##

last(Context::context()) -> key() | '$end_of_table'

Behaves as ets:last(Tab) for a given type of registration object.

See http://www.erlang.org/doc/man/ets.html#last-1. The registry behaves as an ordered_set table.

###lookup_global_aggr_counter/1##

lookup_global_aggr_counter(Name::any()) -> integer()

Equivalent to where({a, g, Name}).

Lookup a global (unique) aggregated counter and returns its value. Fails if there is no such object.

###lookup_global_counters/1##

lookup_global_counters(Counter::any()) -> [{pid(), Value::integer()}]

Equivalent to lookup_values({c, g, Counter}).

Look up all global (non-unique) instances of a given Counter. Returns a list of {Pid, Value} tuples for all matching objects.

###lookup_global_name/1##

lookup_global_name(Name::any()) -> pid()

Equivalent to where({n, g, Name}).

Lookup a global unique name. Fails if there is no such name.

###lookup_global_properties/1##

lookup_global_properties(Property::any()) -> [{pid(), Value}]

Equivalent to lookup_values({p, g, Property}).

Look up all global (non-unique) instances of a given Property. Returns a list of {Pid, Value} tuples for all matching objects.

###lookup_local_aggr_counter/1##

lookup_local_aggr_counter(Name::any()) -> integer()

Equivalent to where({a, l, Name}).

Lookup a local (unique) aggregated counter and returns its value. Fails if there is no such object.

###lookup_local_counters/1##

lookup_local_counters(Counter::any()) -> [{pid(), Value::integer()}]

Equivalent to lookup_values({c, l, Counter}).

Look up all local (non-unique) instances of a given Counter. Returns a list of {Pid, Value} tuples for all matching objects.

###lookup_local_name/1##

lookup_local_name(Name::any()) -> pid()

Equivalent to where({n, l, Name}).

Lookup a local unique name. Fails if there is no such name.

###lookup_local_properties/1##

lookup_local_properties(Property::any()) -> [{pid(), Value}]

Equivalent to lookup_values({p, l, Property}).

Look up all local (non-unique) instances of a given Property. Returns a list of {Pid, Value} tuples for all matching objects.

###lookup_pid/1##

lookup_pid(Key) -> Pid

Lookup the Pid stored with a key.

###lookup_pids/1##

lookup_pids(Key::key()) -> [pid()]

Returns a list of pids with the published key Key

If the type of registration entry is either name or aggregated counter, this function will return either an empty list, or a list of one pid. For non-unique types, the return value can be a list of any length.

###lookup_value/1##

lookup_value(Key) -> Value

Lookup the value stored with a key.

###lookup_values/1##

lookup_values(Key::key()) -> [{pid(), Value}]

Retrieve the {Pid,Value} pairs corresponding to Key.

Key refer to any type of registry object. If it refers to a unique object, the list will be of length 0 or 1. If it refers to a non-unique object, the return value can be a list of any length.

###monitor/1##

monitor(Key::key()) -> reference()

monitor a registered name This function 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.

###mreg/3##

mreg(T::type(), X2::scope(), KVL::[{Key::any(), Value::any()}]) -> true

Register multiple {Key,Value} pairs of a given type and scope.

This function is more efficient than calling reg/2 repeatedly. It is also atomic in regard to unique names; either all names are registered or none are.

###munreg/3##

munreg(T::type(), X2::scope(), L::[Key::any()]) -> true

Unregister multiple Key items of a given type and scope.

This function is usually more efficient than calling unreg/1 repeatedly.

###nb_wait/1##

nb_wait(Key::key()) -> Ref

Wait for a local name to be registered. The caller can expect to receive a message, {gproc, Ref, registered, {Key, Pid, Value}}, once the name is registered.

###next/2##

next(Context::context(), Key::key()) -> key() | '$end_of_table'

Behaves as ets:next(Tab,Key) for a given type of registration object.

See http://www.erlang.org/doc/man/ets.html#next-2. The registry behaves as an ordered_set table.

###prev/2##

prev(Context::context(), Key::key()) -> key() | '$end_of_table'

Behaves as ets:prev(Tab,Key) for a given type of registration object.

See http://www.erlang.org/doc/man/ets.html#prev-2. The registry behaves as an ordered_set table.

###reg/1##

reg(Key::key()) -> true

Equivalent to reg(Key, default(Key)).

###reg/2##

reg(Key::key(), Value) -> true

Register a name or property for the current process

###reg_shared/1##

reg_shared(Key::key()) -> true

Register a resource, but don't tie it to a particular process.

reg_shared({c,l,C}) -> reg_shared({c,l,C}, 0). reg_shared({a,l,A}) -> reg_shared({a,l,A}, undefined).

###reg_shared/2##

reg_shared(Key::key(), Value) -> true

Register a resource, but don't tie it to a particular process.

Shared resources are all unique. They remain until explicitly unregistered (using unreg_shared/1). The types of shared resources currently supported are counter and aggregated counter. In listings and query results, shared resources appear as other similar resources, except that Pid == shared. To wit, update_counter({c,l,myCounter}, 1, shared) would increment the shared counter myCounter with 1, provided it exists.

A shared aggregated counter will track updates in exactly the same way as an aggregated counter which is owned by a process.

###register_name/2##

register_name(Name::key(), Pid::pid()) -> yes | no

Behaviour support callback

###reset_counter/1##

reset_counter(Key) -> {ValueBefore, ValueAfter}

• Key = {c, Scope, Name}

• Scope = l | g

• ValueBefore = integer()

• ValueAfter = integer()

Reads and resets a counter in a "thread-safe" way

This function reads the current value of a counter and then resets it to its initial value. The reset operation is done using update_counter/2, which allows for concurrent calls to update_counter/2 without losing updates. Aggregated counters are updated accordingly.

###select/1##

select(Continuation::term()) -> {[Match], Continuation} | '$end_of_table'

see http://www.erlang.org/doc/man/ets.html#select-1

###select/2##

select(Context::context(), Pat::sel_pattern()) -> [{Key, Pid, Value}]

Perform a select operation on the process registry.

The physical representation in the registry may differ from the above, but the select patterns are transformed appropriately.

###select/3##

select(Context::context(), Pat::sel_patten(), Limit::integer()) -> {[Match], Continuation} | '$end_of_table'

Like select/2 but returns Limit objects at a time.

See http://www.erlang.org/doc/man/ets.html#select-3.

###select_count/1##

select_count(Pat::select_pattern()) -> [sel_object()]

Equivalent to select_count(all, Pat).

###select_count/2##

select_count(Context::context(), Pat::sel_pattern()) -> [{Key, Pid, Value}]

Perform a select_count operation on the process registry.

The physical representation in the registry may differ from the above, but the select patterns are transformed appropriately.

###send/2##

send(Key::key(), Msg::any()) -> Msg

Sends a message to the process, or processes, corresponding to Key.

If Key belongs to a unique object (name or aggregated counter), this function will send a message to the corresponding process, or fail if there is no such process. If Key is for a non-unique object type (counter or property), Msg will be send to all processes that have such an object.

###set_env/5##

set_env(Scope::scope(), App::atom(), Key::atom(), Value::term(), Strategy) -> Value

• Strategy = [Alternative]

• Alternative = app_env | os_env | {os_env, VAR} | {mnesia, ActivityType, Oid, Pos}

Updates the cached value as well as underlying environment.

This function should be exercised with caution, as it affects the larger
environment outside gproc. This function modifies the cached value, and then
proceeds to update the underlying environment (OS environment variable or
application environment variable).

When the mnesia alternative is used, gproc will try to update any existing object, changing only the Pos position. If no such object exists, it will create a new object, setting any other attributes (except Pos and the key) to undefined.

###set_value/2##

set_value(Key::key(), Value) -> true

Sets the value of the registeration entry given by Key

Key is assumed to exist and belong to the calling process.
If it doesn't, this function will exit.

Value can be any term, unless the object is a counter, in which case it must be an integer.

###start_link/0##

start_link() -> {ok, pid()}

Starts the gproc server.

This function is intended to be called from gproc_sup, as part of starting the gproc application.

###table/0##

table() -> any()

Equivalent to table({all, all}).

###table/1##

table(Context::context()) -> any()

Equivalent to table(Context, []).

###table/2##

table(Context::context(), Opts) -> any()

QLC table generator for the gproc registry. Context specifies which subset of the registry should be queried. See http://www.erlang.org/doc/man/qlc.html.

###unreg/1##

unreg(Key::key()) -> true

Unregister a name or property.

###unreg_shared/1##

unreg_shared(Key::key()) -> true

Unregister a shared resource.

###unregister_name/1##

unregister_name(Key) -> any()

Equivalent to unreg / 1.

###update_counter/2##

update_counter(Key::key(), Incr::integer()) -> integer()

Updates the counter registered as Key for the current process.

This function works like ets:update_counter/3 (see http://www.erlang.org/doc/man/ets.html#update_counter-3), but will fail if the type of object referred to by Key is not a counter.

###update_shared_counter/2##

update_shared_counter(Key, Incr) -> any()

###where/1##

where(Key::key()) -> pid()

Returns the pid registered as Key

The type of registration entry must be either name or aggregated counter. Otherwise this function will exit. Use lookup_pids/1 in these cases.

###whereis_name/1##

whereis_name(Key) -> any()

Equivalent to where / 1.