The gproc application

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

Extended process dictionary

Note

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

To enable fetching of gen_leader, export the OS environment variableGPROC_DIST=true (this can be done e.g. from a GNU Makefile)
edown is fetched on-demand whenever rebar get-deps doc is called (which happens when you call make doc)

Installation

You can get gproc from the Hex package manager

That means declaring dependency on {gproc, "0.5.0"} in your rebar3-based applications or {:gproc, "~> 0.5.0"} in your mix based applications.

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".