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