# The gproc application # __Authors:__ Ulf Wiger ([`ulf.wiger@erlang-solutions.com`](mailto:ulf.wiger@erlang-solutions.com)), Joseph Wayne Norton ([`norton@geminimobile.com`](mailto: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 variable`GPROC_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: ```erlang 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.md#get_env-4), [`gproc:get_set_env/4`](gproc.md#get_set_env-4) and [`gproc:set_env/5`](gproc.md#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](erlang07-wiger.pdf)). ## Modules ##