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(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) -
edownis fetched on-demand wheneverrebar get-deps docis called (which
happens when you callmake 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".
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).