A Process Pool Library for Erlang
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pidq - A Process Pool Library for Erlang

Note: this is all work very much in progress. If you are interested, drop me a note. Right now, it is really just a readme and no working code.

Use pidq to manage pools of processes (pids).

  • Protect the pids from being used concurrently. The main pidq interface is pidq:take_pid/0 and pidq:return_pid/2. The pidq server will keep track of which pids are in use and which are free.
  • Maintain the size of the pid pool. Specify a maximum number of pids in the pool. Trigger pid creation when the free count drops below a minimum level or when a pid is marked as failing.
  • Organize pids by type and randomly load-balance pids by type. This is useful when the pids represent client processes connected to a particular node in a cluster (think database read slaves). Separate pools are maintained for each type and a request for a pid will randomly select a type.


The need for the pidq kit arose while writing an Erlang-based application that uses Riak for data storage. When using the Erlang protocol buffer client for Riak, one should avoid accessing a given client concurrently. This is because each client is associated with a unique client ID that corresponds to an element in an object’s vector clock. Concurrent action from the same client ID defeats the vector clock. For some further explaination, see [1] and [2].

I wanted to avoid spinning up a new client for each request in the application. Riak’s protocol buffer client is a gen_server process and my intuition is that one doesn’t want to pay for the startup time for every request you send to an app. This suggested a pool of clients with some management to avoid concurrent use of a given client. On top of that, it seemed convenient to add the ability to load balance between clients connected to different nodes in the Riak cluster. The load-balancing is a secondary feature; even if you end up setting up HAProxy for that aspect, you might still want the client pooling.

[1] http://lists.basho.com/pipermail/riak-users_lists.basho.com/2010-September/001900.html [2] http://lists.basho.com/pipermail/riak-users_lists.basho.com/2010-September/001904.html

Usage and API

Startup configuration

The idea is that you would wire up pidq to be a supervised process in your application. When you start pidq, you specify a module and function to use for creating new pids. You also specify the properties for each pool that you want pidq to manage, including the arguments to pass to the pid starter function.

An example configuration looks like this:

Pool1 = [{name, "node1"},
         {max_pids, 10},
         {min_free, 2},
         {init_size, 5}
         {pid_starter_args, Args1}],

Pool2 = [{name, "node2"},
         {max_pids, 100},
         {min_free, 2},
         {init_size, 50}
         {pid_starter_args, Args2}],

Config = [{pid_starter, {M, F}},
          {pid_stopper, {M, F}},
          {pools, [Pool1, Pool2]}]

% either call this directly, or wire this
% call into your application's supervisor  

Each pool has a unique name, a maximum number of pids, an initial number of pids, and a minimum free pids count. When pidq starts, it will create pids to match the init_size value. If there are min_free pids or fewer, pidq will add a pid as long as that doesn’t bring the total used + free count over max_pids.

Specifying a pid_stopper function is optional. If not specified, exit(pid, kill) will be used to shutdown pids in the case of error, pidq shutdown, or pool removal. The function specified will be passed a pid as returned by the pid_starter function.

Getting and returning pids

Once started, the main interaction you will have with pidq is through two functions, take_pid/0 and return_pid/2.

Call pidq:take_pid() to obtain a pid from the pool. When you are done with it, return it to the pool using pidq:return_pid(Pid, ok). If you encountered an error using the pid, you can pass fail as the second argument. In this case, pidq will permently remove that pid from the pool and start a new pid to replace it.

Other things you can do

You can get the status for the system via pidq:status(). This will return some informational details about the pools being managed.

You can also add or remove new pools while pidq is running using pidq:add_pool/1 and pidq:remove_pool/1. Each pid


pidq is implemented as a gen_server. Server state consists of:

  • A dict of pools keyed by pool name.
  • A dict mapping in use pids to their pool name.
  • A dict mapping consumer process pids to the pid they are using.
  • A module and function to use for starting new pids.

Each pool keeps track of its parameters, such as max pids to allow, initial pids to start, number of pids in use, and a list of free pids.

Since our motivating use-case is Riak’s pb client, we opt to reuse a given client as much as possible to avoid unnecessary vector clock growth; pids are taken from the head of the free list and returned to the head of the free list.

pidq is a system process and traps exits. Before giving out a pid, it links to the requesting consumer process. This way, if the consumer process crashes, pidq can recover the pid. When the pid is returned, the requesting process will be unlinked. Since the state of the pid is unknown in the case of a crashing consumer, we will destroy the pid and add a fresh one to the pool.

The pid starter MFA should use spawn_link so that pidq will be linked to the pids (is it confusing that we’ve taken the term “pid” and turned it into a noun of this system?). This way, when pids crash, pidq will be notified and can refill the pool with new pids.

Also note that an alternative to a consumer explicitly returning a pid is for the consumer to exit normally. pidq will receive the normal exit and can reclaim the pid. In fact, we might want to implement pid return as “fake death” by sending pidq exit(PidqPid, normal).

Pool management

It is an error to add a pool with a name that already exists.

Pool removal has two forms:

  • graceful pids in the free list are killed (using exit(pid, kill) unless a pid_stopper is specified in the pool parameters. No pids will be handed out from this pool’s free list. As pids are returned, they are shut down. When the pool is empty, it is removed.
  • immediate all pids in free and in-use lists are shut down; the pool is removed.
-spec(take_pid() -> pid()).

-spec(return_pid(pid(), ok | fail) -> ignore).

-spec(status() -> [term()]).

-type(pid_type_opt() ::
      {name, string()} |
      {max_pids, int()} |
      {min_free, int()} |
      {init_size, int()} |
      {pid_starter_args, [term()]}).

-type(pid_type_spec() :: [pid_type_opt()]).
-spec(add_type(pid_type_spec()) -> ok | {error, Why}).
-spec(remove_type(string()) -> ok | {error, Why}).