The gproc application

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

Extended process dictionary

Note

This branch of Gproc uses the locks_leader version of leader election. The locks application is still in beta, so production use of this branch is not yet recommended. Please feel free to test it and report how it goes.

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

• To enable fetching of locks_leader, export the OS environment variableGPROC_DIST=true (this can be done e.g. from a GNU Makefile)

• edown is fetched on-demand whenver rebar get-deps doc is called (which happens when you call make doc)

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".

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

Building Edoc

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 (Paper available here).

Modules

gproc

gproc_app

gproc_bcast

gproc_dist

gproc_info

gproc_init

gproc_lib

gproc_monitor

gproc_pool

gproc_ps

gproc_pt

gproc_sup