A pool of gen servers.

The goal of worker pool is pretty straightforward: to provide a transparent way to manage a pool of workers and do the best effort in balancing the load among them, distributing the tasks requested to the pool.

You can just replace your calls to the gen_server module with calls to the wpool module, i.e., wherever you had a gen_server:call/N now you put a wpool:call/N, and that’s it!

Worker Pool is available on Hex.pm. To install, just add it to your dependencies in rebar.config:

{deps, [{worker_pool, "~> 6.1"}]}.

or in mix.ers

defp deps() do
  [{:worker_pool, "~> 6.1"}]
end

The documentation can be generated from code using rebar3_ex_doc with rebar3 ex_doc. It is also available online in Hexdocs.

All user functions are exposed through the wpool module.

Detailed usage is also documented in the same wpool module summary.

Say your application needs a connection to a third-party service that is frequently used. You implement a gen_server called my_server that knows how to talk the third-party protocol and keeps the connection open, and your business logic uses this my_server as the API to interact with. But this server is not only a single-point-of-failure, but also a bottleneck.

Let's pool this server!

First we need to start the pool, instead of starting a single server. If your server was part of your supervision tree, and your supervisor had a child-spec like:

    ChildSpec = #{id => my_server_name,
      start => {my_server, start_link, Arguments},
      restart => permanent,
      shutdown => 5000,
      type => worker}.

You can now replace it by

    WPoolOpts = [{worker, {my_server, Arguments}}],
    ChildSpec = wpool:child_spec(my_server_name, WPoolOpts),

Now that the pool is in place, wherever you've called the server, now you can simply call the pool: all code like the following

    %% ...
    gen_server:call(my_server, Request),
    %% ...
    gen_server:cast(my_server, Notify),
    %% ...

can simply be replaced by

    %% ...
    wpool:call(my_server, Request),
    %% ...
    wpool:cast(my_server, Notify),
    %% ...

If you want all the workers to get notified of an event (for example for consistency reasons), you can use:

    wpool:broadcast(my_server, Request)

And if events have a partial ordering, that is, there is a subset of them were they should be processed in a strict ordering, for example requests by user X should be processed sequentially but how they interleave with other requests is irrelevant, you can use:

    wpool:call(my_server, Request, {hash_worker, X})

and requests for X will always be sent to the same worker.

And just like that, all your requests are now pooled!

By passing a more complex configuration in the WPoolOpts parameter, you can tweak many things, for example the number of workers (t:wpool:workers()), options to pass to OTP's the gen_server engine behind your code t:wpool:worker_opt(), the strategy to supervise all the workers (t:wpool:strategy()), register callbacks you want to be triggered on worker's events (t:wpool:callbacks()), and many more. See t:wpool:option() for all options available.

To see how wpool is used you can check the test folder where you'll find many different scenarios exercised in the different suites.

If you want to see worker_pool in a real life project, we recommend you to check sumo_db, another open-source library from Inaka that uses wpool intensively, or MongooseIM, an Erlang Solutions' Messaging server that uses wpool in many different ways.

wpool comes with a very basic benchmarker that let's you compare different strategies against the default wpool_worker. If you want to do the same in your project, you can use wpool_bench as a template and replace the worker and the tasks by your own ones.

If you find any bugs or have a problem while using this library, please open an issue in this repo (or a pull request :)).

Required OTP version 25 or higher. We only provide guarantees that the system runs on OTP25 since that's what we're testing it in, but the minimum_otp_vsn is "21" because some systems where worker_pool is integrated do require it.