Erlang: via

20 Apr 2023 13:02 erlang

Here’s how you can use via in a name specification when starting a gen_server (or gen_statem, etc.) to register your process with a process registry.

Anonymous process

Ordinarily, when you start an Erlang gen_server, you’ll use something like this:

% etc.

start_link() ->
    gen_server:start_link(?MODULE, [], [])

This tells Erlang to use ?MODULE as the server implementation, doesn’t pass any arguments to it, and doesn’t pass any gen_server options.

This is an anonymous process. We’ll only be able to find it again if we store the process ID (pid). For example:

{ok, Pid} = anon_server:start_link(),
% ...

Then we can call it like using the pid, for example:

gen_server:call(Pid, hello).

Alternatively – if it’s supervised – we can dig around in the supervisor tree.

Named process

If we want a named process, we can do it like this:

start_link() ->
    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).

This puts the process in the built-in local process registry (it calls erlang:register/2). In most cases, you’ll use the module name as the server name, as here.

We can call it by name:

gen_server:call(?SERVER, hello).

Global process

Erlang also provides a cluster-wide process registry, named global. We can register with that as follows:

start_link() ->
    gen_server:start_link({global, ?MODULE}, ?MODULE, [], []).

I’ll write something further about the global registry at some point. For example, it’s not clear to me the best way to ensure that your singleton process is always running somewhere in the cluster.

Custom process registries

But sometimes you want to use a custom process registry. You could do this by having your processes explicitly register themselves. Here’s an example using gproc:

% gen_server, e.g.
init([]) ->
    Name = %...
    true = gproc:reg({n, l, Name}),
    {ok, #state{}}.

Here’s another (possibly badly-remembered) example from a project I worked on a few years ago:

% gen_server, e.g.
init([]) ->
    Name = {device_id, DeviceId},
    ei_registry:register_name(device_registry, Name),
    {ok, #state{}}.

I feel obliged to explain why we didn’t use via in that case. First: I didn’t know it existed at the time. Second: our registry (similarly to gproc) allowed multiple names and properties per process, which doesn’t neatly fit into how via is defined.


Or you can do it by using via:

start_link() ->
    Name = %...
    gen_server:start_link({via, gproc, Name}, ?MODULE, [], []).

This takes advantage of the fact that gproc exports the functions needed to work with via.

Note that the name can be any Erlang term.