#The gproc application#

Authors: Ulf Wiger (ulf.wiger@erlang-solutions.com), Joseph Wayne Norton (norton@geminimobile.com).

Extended process dictionary

##Introduction##

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

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

###Use case: System inspection##

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.

###Use case: Pub/Sub patterns##

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

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}).

###Use case: Environment handling##

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 gproc:get_env/4, gproc:get_set_env/4 and gproc:set_env/5 for details.

##Testing##

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".