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pooler - An OTP Process Pool Application

The pooler application allows you to manage pools of OTP behaviors such as gen_servers, gen_fsms, or supervisors, and provide consumers with exclusive access to pool members using pooler:take_member.

What pooler does

Protects the members of a pool from being used concurrently

The main pooler interface is pooler:take_member/1 and pooler:return_member/3. The pooler server will keep track of which members are in use and which are free. There is no need to call pooler:return_member if the consumer is a short-lived process; in this case, pooler will detect the consumer’s normal exit and reclaim the member. To achieve this, pooler tracks the calling process of take_member as the consumer of the pool member. Thus pooler assumes that there is no middle-man process calling take_member and handing out the member pid to another worker process.

Maintains the size of the pool

You specify an initial and a maximum number of members in the pool. Pooler will create new members on demand until the maximum member count is reached. New pool members are added to replace members that crash. If a consumer crashes, the member it was using will be destroyed and replaced. You can configure Pooler to periodically check for and remove members that have not been used recently to reduce the member count back to its initial size.

Manage multiple pools

You can use pooler to manage multiple independent pools and multiple grouped pools. Independent pools allow you to pool clients for different backend services (e.g. postgresql and redis). Grouped pools can optionally be accessed using pooler:take_group_member/1 to provide load balancing of the pools in the group. A typical use of grouped pools is to have each pool contain clients connected to a particular node in a cluster (think database read slaves). Pooler’s take_group_member function will randomly select a pool in the group to fetch a member from. If the randomly selected pool has no free members, pooler will attempt to obtain a member from each pool in the group. If there is no pool with available members, pooler will return error_no_members.


The need for pooler arose while writing an Erlang-based application that uses Riak for data storage. Riak’s protocol buffer client is a gen_server process that initiates a connection to a Riak node. A pool is needed to avoid spinning up a new client for each request in the application. Reusing clients also has the benefit of keeping the vector clocks smaller since each client ID corresponds to an entry in the vector clock.

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 explanation, see post 1 and post 2. Note that concurrent access to Riak’s pb client is actual ok as long as you avoid updating the same key at the same time. So the pool needs to have checkout/checkin semantics that give consumers exclusive access to a client.

On top of that, in order to evenly load a Riak cluster and be able to continue in the face of Riak node failures, consumers should spread their requests across clients connected to each node. The client pool provides an easy way to load balance.

Since writing pooler, I’ve seen it used to pool database connections for PostgreSQL, MySQL, and Redis. These uses led to a redesign to better support multiple independent pools.

Usage and API

Pool Configuration via application environment

Pool configuration is specified in the pooler application’s environment. This can be provided in a config file using -config or set at startup using application:set_env(pooler, pools, Pools). Here’s an example config file that creates two pools of Riak pb clients each talking to a different node in a local cluster and one pool talking to a Postgresql database:

% pooler.config
% Start Erlang as: erl -config pooler
% -*- mode: erlang -*-
% pooler app config
 {pooler, [
         {pools, [
                  #{name => rc8081,
                    group => riak,
                    max_count => 5,
                    init_count => 2,
                    start_mfa =>
                     {riakc_pb_socket, start_link, ["localhost", 8081]}},

                  #{name => rc8082,
                    group => riak,
                    max_count => 5,
                    init_count => 2,
                    start_mfa =>
                     {riakc_pb_socket, start_link, ["localhost", 8082]}},

                  #{name => pg_db1,
                    max_count => 10,
                    init_count => 2,
                    start_mfa =>
                     {epgsql, connect, [#{host => "localhost", username => "user", database => "base"}]}}
           %% if you want to enable metrics, set this to a module with
           %% an API conformant to the folsom_metrics module.
           %% If this config is missing, then no metrics are sent.
           %% {metrics_module, folsom_metrics}

Each pool has a unique name, specified as an atom, an initial and maximum number of members, and an {M, F, A} describing how to start members of the pool. When pooler starts, it will create members in each pool according to init_count. Optionally, you can indicate that a pool is part of a group. You can use pooler to load balance across pools labeled with the same group tag.

Culling stale members

The cull_interval and max_age pool configuration parameters allow you to control how (or if) the pool should be returned to its initial size after a traffic burst. Both parameters specify a time value which is specified as a tuple with the intended units. The following examples are valid:

%% two minutes, your way
{2, min}
{120, sec}
{120000, ms}

The cull_interval determines the schedule when a check will be made for stale members. Checks are scheduled using erlang:send_after/3 which provides a light-weight timing mechanism. The next check is scheduled after the prior check completes.

During a check, pool members that have not been used in more than max_age minutes will be removed until the pool size reaches init_count.

The default value for cull_interval is {1, min}. You can disable culling by specifying a value os {0, min}. The max_age parameter defaults to {30, sec}.

Pool Configuration via pooler:new_pool

You can create pools using pooler:new_pool/1 when accepts a map of pool configuration. Here’s an example:

PoolConfig = #{
    name => rc8081,
    group => riak,
    max_count => 5,
    init_count => 2,
    start_mfa => {riakc_pb_socket, start_link, ["localhost", 8081]}

Dynamic pool reconfiguration

Pool configuration can be changed in runtime

pooler:pool_reconfigure(rc8081, PoolConfig#{max_count => 10, init_count => 4}).

It will update the pool’s state and will start/stop workers if necessary, join/leave group, reschedule the cull timer etc. The only parameters that can’t be updated are name and start_mfa.

However, updated configuration won’t survive pool crash (it will be restarted with old config by supervisor). But this should not normally happen.

Using pooler

Here’s an example session:

P = pooler:take_member(mysql),
% use P
pooler:return_member(mysql, P, ok).

Once started, the main interaction you will have with pooler is through two functions, take_member/1 and return_member/3 (or return_member/2).

Call pooler:take_member(Pool) to obtain the pid belonging to a member of the pool Pool. When you are done with it, return it to the pool using pooler:return_member(Pool, Pid, ok). If you encountered an error using the member, you can pass fail as the second argument. In this case, pooler will permanently remove that member from the pool and start a new member to replace it. If your process is short lived, you can omit the call to return_member. In this case, pooler will detect the normal exit of the consumer and reclaim the member.

If you would like to obtain a member from a randomly selected pool in a group, call pooler:take_group_member(Group). This will return a Pid which must be returned using pooler:return_group_member/2 or pooler:return_group_member/3.

pooler as an included application

In order for pooler to start properly, all applications required to start a pool member must be start before pooler starts. Since pooler does not depend on members and since OTP may parallelize application starts for applications with no detectable dependencies, this can cause problems. One way to work around this is to specify pooler as an included application in your app. This means you will call pooler’s top-level supervisor in your app’s top-level supervisor and can regain control over the application start order. To do this, you would remove pooler from the list of applications in and add it to the included_application key:

{application, your_app,
  {description, "Your App"},
  {vsn, "0.1"},
  {registered, []},
  {applications, [kernel,
  {included_applications, [pooler]},
  {mod, {your_app, []}}

Then start pooler’s top-level supervisor with something like the following in your app’s top-level supervisor:

PoolerSup = {pooler_sup, {pooler_sup, start_link, []},
             permanent, infinity, supervisor, [pooler_sup]},
{ok, {{one_for_one, 5, 10}, [PoolerSup]}}.


You can enable metrics collection by adding a metrics_module entry to pooler’s app config. Metrics are disabled by default. The module specified must have an API matching that of the folsom_metrics module in folsom (to use folsom, specify {metrics_module, folsom_metrics}} and ensure that folsom is in your code path and has been started.

When enabled, the following metrics will be tracked:

Metric LabelDescription
pooler.POOL_NAME.take_ratemeter recording rate at which take_member is called
pooler.error_no_members_countcounter indicating how many times take_member has returned error_no_members
pooler.killed_free_countcounter how many members have been killed when in the free state
pooler.killed_in_use_countcounter how many members have been killed when in the in_use state
pooler.eventhistory various error conditions

Demo Quick Start

  1. Clone the repo:
    git clone
  2. Build and run tests:
    cd pooler; make && make test
  3. Start a demo
    make run
    Erlang R16B03 (erts-5.10.4) [source] [64-bit] [smp:8:8] [async-threads:10] [kernel-poll:false]
    Eshell V5.10.4  (abort with ^G)
    1> pooler:start().
    2> M = pooler:take_member(pool1).
    3> pooled_gs:get_id(M).
    4> M2 = pooler:take_member(pool1).
    5> pooled_gs:get_id(M2).
    6> pooler:return_member(pool1, M, ok).
    7> pooler:return_member(pool1, M2, ok).

Implementation Notes

Overview of supervision


The top-level supervisor is pooler_sup. It supervises one supervisor for each pool configured in pooler’s app config.

At startup, a pooler_NAME_pool_sup is started for each pool described in pooler’s app config with NAME matching the name attribute of the config.

The pooler_NAME_pool_sup starts the gen_server that will register with pooler_NAME_pool as well as a pooler_NAME_member_sup that will be used to start and supervise the members of this pool. The pooler_starter_sup is used to start temporary workers used for managing async member start.

pooler_sup: one_for_one pooler_NAME_pool_sup: all_for_one pooler_NAME_member_sup: simple_one_for_one pooler_starter_sup: simple_one_for_one

Groups of pools are managed using the pg (OTP-23+) or pg2 (OTP below 23) application. This imposes a requirement to set a configuration parameter on the kernel application in an OTP release. Like this in sys.config:

{kernel, [{start_pg, true}]}
{kernel, [{start_pg2, true}]}


All contributions are welcome!

Pooler uses rebar3 fmt code formatter. Please make sure to apply make format before committing any code.

In pooler we are trying to maintain high test coverage. Run make test to ensure code coverage does not fall below a threshold (it is automatically validated).

Pooler is quite critical to performance regressions. We do not run benchmarks in CI, so, to make sure your change does not make pooler slower, please run the benchmarks before and after your changes and make sure there are no major regressions on the most recent OTP release. The workflow is:

$ git checkout master
$ rebar3 bench --save-baseline master  # run benchmarks, save results to `master` file
$ git checkout -b <my feature branch>

# <do your code changes>

$ rebar3 bench --baseline master  # run benchmarks on updated code, compare results with `master` results
$ git commit ... && git push ...

Please attach the output of rebar3 bench --baseline master after your changes to the PR description in order to prove that there were no performance regressions. Please attach the OTP version you run the benchmarks on.

New release

Our goal is to allow the hot code upgrade of pooler, so it is shipped with .appup file and hot upgrade procedure is tested in CI.

To cut a new release, do the following steps:

  1. In src/ update the vsn
  2. In src/pooler.appup.src: replace the contents with upgrade instructions for a new release
  3. In test/relx-base.config: update the pooler’s app version to a previous release (or leave it without version)
  4. In test/relx-current.config: update the pooler’s app version to a new one
  5. In .github/workflows/hot_upgrade.yml: update from_version to a previous release, maybe bump OTP version as well
  6. Push, wait for the green build, tag


Pooler is licensed under the Apache License Version 2.0. See the LICENSE file for details.