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Extended process registry for Erlang

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README.md

The gproc application

Authors: Ulf Wiger (ulf.wiger@erlang-solutions.com), 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 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_ps
gproc_sup
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