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<div id="content">
<h1 class="title">README</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#sec-1">1 pidq - A Process Pool Library for Erlang </a>
<ul>
<li><a href="#sec-1_1">1.1 Use pidq to manage pools of processes (pids). </a></li>
<li><a href="#sec-1_2">1.2 Motivation </a></li>
<li><a href="#sec-1_3">1.3 Usage and API </a>
<ul>
<li><a href="#sec-1_3_1">1.3.1 Startup configuration </a></li>
<li><a href="#sec-1_3_2">1.3.2 Getting and returning pids </a></li>
<li><a href="#sec-1_3_3">1.3.3 Other things you can do </a></li>
</ul>
</li>
<li><a href="#sec-1_4">1.4 Details </a>
<ul>
<li><a href="#sec-1_4_1">1.4.1 Pool management </a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-1" class="outline-2">
<h2 id="sec-1"><span class="section-number-2">1</span> pidq - A Process Pool Library for Erlang </h2>
<div class="outline-text-2" id="text-1">
<p>
<b>Note:</b> 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.
</p>
</div>
<div id="outline-container-1_1" class="outline-3">
<h3 id="sec-1_1"><span class="section-number-3">1.1</span> Use pidq to manage pools of processes (pids). </h3>
<div class="outline-text-3" id="text-1_1">
<ul>
<li>
Protect the pids from being used concurrently. The main pidq
interface is <code>pidq:take_pid/0</code> and <code>pidq:return_pid/2</code>. The pidq
server will keep track of which pids are <b>in use</b> and which are
<b>free</b>.
</li>
<li>
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.
</li>
<li>
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.
</li>
</ul>
</div>
</div>
<div id="outline-container-1_2" class="outline-3">
<h3 id="sec-1_2"><span class="section-number-3">1.2</span> Motivation </h3>
<div class="outline-text-3" id="text-1_2">
<p>
The need for the pidq kit arose while writing an Erlang-based
application that uses <a href="https://wiki.basho.com/display/RIAK/">Riak</a> 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 <sup><a class="footref" name="fnr.1" href="#fn.1">1</a></sup> and <sup><a class="footref" name="fnr.2" href="#fn.2">2</a></sup>.
</p>
<p>
I wanted to avoid spinning up a new client for each request in the
application. Riak's protocol buffer client is a <code>gen_server</code> 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 <a href="http://haproxy.1wt.eu/">HAProxy</a> for that aspect, you might still want the client
pooling.
</p>
</div>
</div>
<div id="outline-container-1_3" class="outline-3">
<h3 id="sec-1_3"><span class="section-number-3">1.3</span> Usage and API </h3>
<div class="outline-text-3" id="text-1_3">
</div>
<div id="outline-container-1_3_1" class="outline-4">
<h4 id="sec-1_3_1"><span class="section-number-4">1.3.1</span> Startup configuration </h4>
<div class="outline-text-4" id="text-1_3_1">
<p>
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.
</p>
<p>
An example configuration looks like this:
</p>
<pre class="src src-erlang"><span style="color: #FF6400;">Pool1</span> = [{<span style="color: #D8FA3C;">name</span>, <span style="color: #61CE3C;">"node1"</span>},
{<span style="color: #D8FA3C;">max_pids</span>, 10},
{<span style="color: #D8FA3C;">min_free</span>, 2},
{<span style="color: #D8FA3C;">init_size</span>, 5}
{<span style="color: #D8FA3C;">pid_starter_args</span>, <span style="color: #FF6400;">Args1</span>}],
<span style="color: #FF6400;">Pool2</span> = [{<span style="color: #D8FA3C;">name</span>, <span style="color: #61CE3C;">"node2"</span>},
{<span style="color: #D8FA3C;">max_pids</span>, 100},
{<span style="color: #D8FA3C;">min_free</span>, 2},
{<span style="color: #D8FA3C;">init_size</span>, 50}
{<span style="color: #D8FA3C;">pid_starter_args</span>, <span style="color: #FF6400;">Args2</span>}],
<span style="color: #FF6400;">Config</span> = [{<span style="color: #D8FA3C;">pid_starter</span>, {<span style="color: #FF6400;">M</span>, <span style="color: #FF6400;">F</span>}},
{<span style="color: #D8FA3C;">pid_stopper</span>, {<span style="color: #FF6400;">M</span>, <span style="color: #FF6400;">F</span>}},
{<span style="color: #D8FA3C;">pools</span>, [<span style="color: #FF6400;">Pool1</span>, <span style="color: #FF6400;">Pool2</span>]}]
<span style="color: #AEAEAE; font-style: italic;">% </span><span style="color: #AEAEAE; font-style: italic;">either call this directly, or wire this
</span><span style="color: #AEAEAE; font-style: italic;">% </span><span style="color: #AEAEAE; font-style: italic;">call into your application's supervisor
</span><span style="color: #8DA6CE;">pidq</span>:<span style="color: #8DA6CE;">start</span>(<span style="color: #FF6400;">Config</span>)
</pre>
<p>
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 <code>init_size</code> value. If there are <code>min_free</code>
pids or fewer, pidq will add a pid as long as that doesn't bring the
total used + free count over <code>max_pids</code>.
</p>
<p>
Specifying a <code>pid_stopper</code> function is optional. If not specified,
<code>exit(pid, kill)</code> 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 <code>pid_starter</code> function.
</p>
</div>
</div>
<div id="outline-container-1_3_2" class="outline-4">
<h4 id="sec-1_3_2"><span class="section-number-4">1.3.2</span> Getting and returning pids </h4>
<div class="outline-text-4" id="text-1_3_2">
<p>
Once started, the main interaction you will have with pidq is through
two functions, <code>take_pid/0</code> and <code>return_pid/2</code>.
</p>
<p>
Call <code>pidq:take_pid()</code> to obtain a pid from the pool. When you are done
with it, return it to the pool using <code>pidq:return_pid(Pid, ok)</code>. If
you encountered an error using the pid, you can pass <code>fail</code> as the
second argument. In this case, pidq will permently remove that pid
from the pool and start a new pid to replace it.
</p>
</div>
</div>
<div id="outline-container-1_3_3" class="outline-4">
<h4 id="sec-1_3_3"><span class="section-number-4">1.3.3</span> Other things you can do </h4>
<div class="outline-text-4" id="text-1_3_3">
<p>
You can get the status for the system via <code>pidq:status()</code>. This will
return some informational details about the pools being managed.
</p>
<p>
You can also add or remove new pools while pidq is running using
<code>pidq:add_pool/1</code> and <code>pidq:remove_pool/1</code>. Each pid
</p>
</div>
</div>
</div>
<div id="outline-container-1_4" class="outline-3">
<h3 id="sec-1_4"><span class="section-number-3">1.4</span> Details </h3>
<div class="outline-text-3" id="text-1_4">
<p>
pidq is implemented as a <code>gen_server</code>. Server state consists of:
</p>
<ul>
<li>
A dict of pools keyed by pool name.
</li>
<li>
A dict mapping in use pids to their pool name.
</li>
<li>
A dict mapping consumer process pids to the pid they are using.
</li>
<li>
A module and function to use for starting new pids.
</li>
</ul>
<p>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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
The pid starter MFA should use spawn<sub>link</sub> 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.
</p>
<p>
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).
</p>
</div>
<div id="outline-container-1_4_1" class="outline-4">
<h4 id="sec-1_4_1"><span class="section-number-4">1.4.1</span> Pool management </h4>
<div class="outline-text-4" id="text-1_4_1">
<p>
It is an error to add a pool with a name that already exists.
</p>
<p>
Pool removal has two forms:
</p>
<ul>
<li>
<b>graceful</b> pids in the free list are killed (using exit(pid, kill)
unless a <code>pid_stopper</code> 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.
</li>
<li>
<b>immediate</b> all pids in free and in-use lists are shut down; the
pool is removed.
</li>
</ul>
<pre class="src src-erlang"><span style="color: #7fffd4;">-spec</span>(<span style="color: #8DA6CE;">take_pid</span>() -&gt; <span style="color: #94bff3;">pid</span>()).
<span style="color: #7fffd4;">-spec</span>(<span style="color: #8DA6CE;">return_pid</span>(<span style="color: #94bff3;">pid</span>(), <span style="color: #D8FA3C;">ok</span> | <span style="color: #D8FA3C;">fail</span>) -&gt; <span style="color: #D8FA3C;">ignore</span>).
<span style="color: #7fffd4;">-spec</span>(<span style="color: #8DA6CE;">status</span>() -&gt; [<span style="color: #8DA6CE;">term</span>()]).
<span style="color: #7fffd4;">-type</span>(<span style="color: #8DA6CE;">pid_type_opt</span>() ::
{<span style="color: #D8FA3C;">name</span>, <span style="color: #8DA6CE;">string</span>()} |
{<span style="color: #D8FA3C;">max_pids</span>, <span style="color: #8DA6CE;">int</span>()} |
{<span style="color: #D8FA3C;">min_free</span>, <span style="color: #8DA6CE;">int</span>()} |
{<span style="color: #D8FA3C;">init_size</span>, <span style="color: #8DA6CE;">int</span>()} |
{<span style="color: #D8FA3C;">pid_starter_args</span>, [<span style="color: #8DA6CE;">term</span>()]}).
<span style="color: #7fffd4;">-type</span>(<span style="color: #8DA6CE;">pid_type_spec</span>() :: [<span style="color: #8DA6CE;">pid_type_opt</span>()]).
<span style="color: #7fffd4;">-spec</span>(<span style="color: #8DA6CE;">add_type</span>(<span style="color: #8DA6CE;">pid_type_spec</span>()) -&gt; <span style="color: #D8FA3C;">ok</span> | {<span style="color: #D8FA3C;">error</span>, <span style="color: #FF6400;">Why</span>}).
<span style="color: #7fffd4;">-spec</span>(<span style="color: #8DA6CE;">remove_type</span>(<span style="color: #8DA6CE;">string</span>()) -&gt; <span style="color: #D8FA3C;">ok</span> | {<span style="color: #D8FA3C;">error</span>, <span style="color: #FF6400;">Why</span>}).
</pre>
</div>
</div>
</div>
</div>
<div id="footnotes">
<h2 class="footnotes">Footnotes: </h2>
<div id="text-footnotes">
<p class="footnote"><sup><a class="footnum" name="fn.1" href="#fnr.1">1</a></sup> <a href="http://lists.basho.com/pipermail/riak-users_lists.basho.com/2010-September/001900.html">http://lists.basho.com/pipermail/riak-users_lists.basho.com/2010-September/001900.html</a>
</p>
<p class="footnote"><sup><a class="footnum" name="fn.2" href="#fnr.2">2</a></sup> <a href="http://lists.basho.com/pipermail/riak-users_lists.basho.com/2010-September/001904.html">http://lists.basho.com/pipermail/riak-users_lists.basho.com/2010-September/001904.html</a>
</p>
</div>
</div>
<div id="postamble">
<p class="author"> Author: Seth Falcon
</p>
<p class="date"> Date: 2010-09-08 23:24:52 PDT</p>
<p class="creator">HTML generated by org-mode 7.01trans in emacs 23</p>
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