* 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=.

#+ATTR_HTML: alt="Build status images" title="Build status on Travis-CI"

** 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

** Motivation

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:

#+BEGIN_SRC erlang
  % 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},
                      {riakc_pb_socket, start_link, ["localhost", 8081]}}],

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

                    [{name, pg_db1},
                     {max_count, 10},
                     {init_count, 2},
                      {my_pg_sql_driver, start_link, ["db_host"]}}]
             %% 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:

#+BEGIN_SRC erlang
%% two minutes, your way
{2, min}
{120, sec}
{1200, 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

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
proplist of pool configuration. Here's an example:
#+BEGIN_SRC erlang
PoolConfig = [{name, rc8081},
              {group, riak},
              {max_count, 5},
              {init_count, 2},
                start_link, ["localhost", 8081]}}],
*** Using pooler

Here's an example session:

#+BEGIN_SRC erlang
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

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 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:

#+BEGIN_SRC erlang
{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:

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

*** Metrics
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 Label                  | Description                                                                 |
| pooler.POOL_NAME.take_rate    | meter recording rate at which take_member is called                         |
| pooler.error_no_members_count | counter indicating how many times take_member has returned error_no_members |
| pooler.killed_free_count      | counter how many members have been killed when in the free state            |
| pooler.killed_in_use_count    | counter how many members have been killed when in the in_use state          |
| pooler.event                  | history 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
   erl -pa .eunit ebin -config demo

   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> application:start(pooler).
   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

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 pg2 application. This imposes a
requirement to set a configuration parameter on the kernel application
in an OTP release. Like this in sys.config:
#+begin_src erlang
{kernel, [{start_pg2, true}]}

** License
Pooler is licensed under the Apache License Version 2.0.  See the
[[file:LICENSE][LICENSE]] file for details.

#+OPTIONS: ^:{}