@author Ulf Wiger <ulf.wiger@erlang-solutions.com>
@author Joseph Wayne Norton <norton@geminimobile.com>

@doc Extended process dictionary

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<h2>Note</h2>

Gproc has two dependencies: `gen_leader' and `edown'. Since most people don't
actively use either, they are no longer fetched by default.

<ul>
<li>To enable fetching of `gen_leader', export the OS environment variable
 `GPROC_DIST=true' (this can be done e.g. from a GNU Makefile)</li>
<li>`edown' is fetched on-demand whenver `rebar get-deps doc' is called (which
  happens when you call `make doc')</li>
</ul>

<h2>Introduction</h2>

Gproc is a process dictionary for Erlang, which provides a number of useful features beyond what the built-in dictionary has:

<ul>
<li>Use any term as a process alias</li>
<li>Register a process under several aliases</li>
<li>Non-unique properties can be registered simultaneously by many processes</li>
<li>QLC and match specification interface for efficient queries on the
  dictionary</li>
<li>Await registration, let's you wait until a process registers itself</li>
<li>Atomically give away registered names and properties to another process</li>
<li>Counters, and aggregated counters, which automatically maintain the
  total of all counters with a given name</li>
<li>Global registry, with all the above functions applied to a network of nodes</li>
</ul>

<h3>Use case: System inspection</h3>

Gproc was designed to work as a central index for "process metadata", i.e.
properties that describe the role and characteristics of each process. Having
a single registry that is flexible enough to hold important types of property
makes it easier to (a) find processes of a certain type, and (b) query and
browse key data in a running system.

<h3>Use case: Pub/Sub patterns</h3>

An interesting application of gproc is building publish/subscribe patterns.
Example:

<pre lang="erlang">
subscribe(EventType) ->
    %% Gproc notation: {p, l, Name} means {(p)roperty, (l)ocal, Name}
    gproc:reg({p, l, {?MODULE, EventType}}).

notify(EventType, Msg) ->
    Key = {?MODULE, EventType},
    gproc:send({p, l, Key}, {self(), Key, Msg}).
</pre>

<h3>Use case: Environment handling</h3>

Gproc provides a set of functions to read environment variables, possibly from
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 {@link gproc:get_env/4}, {@link gproc:get_set_env/4} and
{@link gproc:set_env/5} for details.

<h2>Testing</h2>

Gproc has a QuickCheck test suite, covering a fairly large part of the local
gproc functionality, although none of the global registry. It requires a
commercial EQC license, but rebar is smart enough to detect whether EQC is
available, and if it isn't, the code in gproc_eqc.erl will be "defined away".

There is also an eunit suite, covering the basic operations for local and
global gproc.

<h2>Building Edoc</h2>
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 (<a href="erlang07-wiger.pdf">Paper available here</a>).

@end