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<h1>5 Supervisor Behaviour</h1>
<p>This section should be read in conjunction with
<span class="code">supervisor(3)</span>, where all details about the supervisor
behaviour is given.</p>
<h3><a name="id71210">5.1
Supervision Principles</a></h3>
<p>A supervisor is responsible for starting, stopping and
monitoring its child processes. The basic idea of a supervisor is
that it should keep its child processes alive by restarting them
when necessary.</p>
<p>Which child processes to start and monitor is specified by a
list of <span class="bold_code"><a href="#spec">child specifications</a></span>.
The child processes are started in the order specified by this
list, and terminated in the reversed order.</p>
<h3><a name="id71235">5.2
Example</a></h3>
<p>The callback module for a supervisor starting the server from
the <span class="bold_code"><a href="gen_server_concepts.html#ex">gen_server chapter</a></span>
could look like this:</p>
<a name="ex"></a>
<div class="example"><pre>
-module(ch_sup).
-behaviour(supervisor).
-export([start_link/0]).
-export([init/1]).
start_link() ->
supervisor:start_link(ch_sup, []).
init(_Args) ->
{ok, {{one_for_one, 1, 60},
[{ch3, {ch3, start_link, []},
permanent, brutal_kill, worker, [ch3]}]}}.</pre></div>
<p><span class="code">one_for_one</span> is the <span class="bold_code"><a href="#strategy">restart strategy</a></span>.</p>
<p>1 and 60 defines the <span class="bold_code"><a href="#frequency">maximum restart frequency</a></span>.</p>
<p>The tuple <span class="code">{ch3, ...}</span> is a <span class="bold_code"><a href="#spec">child specification</a></span>.</p>
<h3><a name="id71306">5.3
Restart Strategy</a></h3>
<a name="strategy"></a>
<h4>one_for_one</h4>
<p>If a child process terminates, only that process is restarted.</p>
<a name="sup4"></a>
<img alt="IMAGE MISSING" src="../design_principles/sup4.gif"><br>
<em>Figure
5.1:
One_For_One Supervision</em>
<h4>one_for_all</h4>
<p>If a child process terminates, all other child processes are
terminated and then all child processes, including
the terminated one, are restarted.</p>
<a name="sup5"></a>
<img alt="IMAGE MISSING" src="../design_principles/sup5.gif"><br>
<em>Figure
5.2:
One_For_All Supervision</em>
<h4>rest_for_one</h4>
<p>If a child process terminates, the 'rest' of the child
processes -- i.e. the child processes after the terminated
process in start order -- are terminated. Then the terminated
child process and the rest of the child processes are restarted.</p>
<h3><a name="id71384">5.4
Maximum Restart Frequency</a></h3>
<a name="frequency"></a>
<p>The supervisors have a built-in mechanism to limit the number of
restarts which can occur in a given time interval. This is
determined by the values of the two parameters <span class="code">MaxR</span> and
<span class="code">MaxT</span> in the start specification returned by the callback
function <span class="code">init</span>:</p>
<div class="example"><pre>
init(...) ->
{ok, {{RestartStrategy, MaxR, MaxT},
[ChildSpec, ...]}}.</pre></div>
<p>If more than <span class="code">MaxR</span> number of restarts occur in the last
<span class="code">MaxT</span> seconds, then the supervisor terminates all the child
processes and then itself.</p>
<p>When the supervisor terminates, then the next higher level
supervisor takes some action. It either restarts the terminated
supervisor, or terminates itself.</p>
<p>The intention of the restart mechanism is to prevent a situation
where a process repeatedly dies for the same reason, only to be
restarted again.</p>
<h3><a name="id71906">5.5
Child Specification</a></h3>
<a name="spec"></a>
<p>This is the type definition for a child specification:</p>
<div class="example"><pre>
{Id, StartFunc, Restart, Shutdown, Type, Modules}
Id = term()
StartFunc = {M, F, A}
M = F = atom()
A = [term()]
Restart = permanent | transient | temporary
Shutdown = brutal_kill | integer()>0 | infinity
Type = worker | supervisor
Modules = [Module] | dynamic
Module = atom()</pre></div>
<ul>
<li>
<p><span class="code">Id</span> is a name that is used to identify the child
specification internally by the supervisor.</p>
</li>
<li>
<p><span class="code">StartFunc</span> defines the function call used to start
the child process. It is a module-function-arguments tuple
used as <span class="code">apply(M, F, A)</span>.</p>
<p>It should be (or result in) a call to
<span class="code">supervisor:start_link</span>, <span class="code">gen_server:start_link</span>,
<span class="code">gen_fsm:start_link</span> or <span class="code">gen_event:start_link</span>.
(Or a function compliant with these functions, see
<span class="code">supervisor(3)</span> for details.</p>
</li>
<li>
<p><span class="code">Restart</span> defines when a terminated child process should
be restarted.</p>
<ul>
<li>A <span class="code">permanent</span> child process is always restarted.</li>
<li>A <span class="code">temporary</span> child process is never restarted
(not even when the supervisor's restart strategy
is <span class="code">rest_for_one</span> or <span class="code">one_for_all</span> and a sibling's
death causes the temporary process to be terminated).</li>
<li>A <span class="code">transient</span> child process is restarted only if it
terminates abnormally, i.e. with another exit reason than
<span class="code">normal</span>, <span class="code">shutdown</span> or <span class="code">{shutdown,Term}</span>.</li>
</ul>
</li>
<li>
<a name="shutdown"></a>
<p><span class="code">Shutdown</span> defines how a child process should be
terminated.</p>
<ul>
<li>
<span class="code">brutal_kill</span> means the child process is
unconditionally terminated using <span class="code">exit(Child, kill)</span>.</li>
<li>An integer timeout value means that the supervisor tells
the child process to terminate by calling
<span class="code">exit(Child, shutdown)</span> and then waits for an exit
signal back. If no exit signal is received within
the specified time, the child process is unconditionally
terminated using <span class="code">exit(Child, kill)</span>.</li>
<li>If the child process is another supervisor, it should be
set to <span class="code">infinity</span> to give the subtree enough time to
shutdown. It is also allowed to set it to <span class="code">infinity</span>, if the
child process is a worker.</li>
</ul>
<div class="warning">
<div class="label">Warning</div>
<div class="content"><p>
<p>Be careful by setting the <span class="code">Shutdown</span> strategy to
<span class="code">infinity</span> when the child process is a worker. Because, in this
situation, the termination of the supervision tree depends on the
child process, it must be implemented in a safe way and its cleanup
procedure must always return.</p>
</p></div>
</div>
</li>
<li>
<p><span class="code">Type</span> specifies if the child process is a supervisor or
a worker.</p>
</li>
<li>
<p><span class="code">Modules</span> should be a list with one element
<span class="code">[Module]</span>, where <span class="code">Module</span> is the name of
the callback module, if the child process is a supervisor,
gen_server or gen_fsm. If the child process is a gen_event,
<span class="code">Modules</span> should be <span class="code">dynamic</span>.</p>
<p>This information is used by the release handler during
upgrades and downgrades, see
<span class="bold_code"><a href="release_handling.html">Release Handling</a></span>.</p>
</li>
</ul>
<p>Example: The child specification to start the server <span class="code">ch3</span>
in the example above looks like:</p>
<div class="example"><pre>
{ch3,
{ch3, start_link, []},
permanent, brutal_kill, worker, [ch3]}</pre></div>
<p>Example: A child specification to start the event manager from
the chapter about
<span class="bold_code"><a href="events.html#mgr">gen_event</a></span>:</p>
<div class="example"><pre>
{error_man,
{gen_event, start_link, [{local, error_man}]},
permanent, 5000, worker, dynamic}</pre></div>
<p>Both the server and event manager are registered processes which
can be expected to be accessible at all times, thus they are
specified to be <span class="code">permanent</span>.</p>
<p><span class="code">ch3</span> does not need to do any cleaning up before
termination, thus no shutdown time is needed but
<span class="code">brutal_kill</span> should be sufficient. <span class="code">error_man</span> may
need some time for the event handlers to clean up, thus
<span class="code">Shutdown</span> is set to 5000 ms.</p>
<p>Example: A child specification to start another supervisor:</p>
<div class="example"><pre>
{sup,
{sup, start_link, []},
transient, infinity, supervisor, [sup]}</pre></div>
<h3><a name="id72235">5.6
Starting a Supervisor</a></h3>
<a name="super_tree"></a>
<p>In the example above, the supervisor is started by calling
<span class="code">ch_sup:start_link()</span>:</p>
<div class="example"><pre>
start_link() ->
supervisor:start_link(ch_sup, []).</pre></div>
<p><span class="code">ch_sup:start_link</span> calls the function
<span class="code">supervisor:start_link/2</span>. This function spawns and links to
a new process, a supervisor.</p>
<ul>
<li>The first argument, <span class="code">ch_sup</span>, is the name of
the callback module, that is the module where the <span class="code">init</span>
callback function is located.</li>
<li>The second argument, [], is a term which is passed as-is to
the callback function <span class="code">init</span>. Here, <span class="code">init</span> does not
need any indata and ignores the argument.</li>
</ul>
<p>In this case, the supervisor is not registered. Instead its pid
must be used. A name can be specified by calling
<span class="code">supervisor:start_link({local, Name}, Module, Args)</span> or
<span class="code">supervisor:start_link({global, Name}, Module, Args)</span>.</p>
<p>The new supervisor process calls the callback function
<span class="code">ch_sup:init([])</span>. <span class="code">init</span> is expected to return
<span class="code">{ok, StartSpec}</span>:</p>
<div class="example"><pre>
init(_Args) ->
{ok, {{one_for_one, 1, 60},
[{ch3, {ch3, start_link, []},
permanent, brutal_kill, worker, [ch3]}]}}.</pre></div>
<p>The supervisor then starts all its child processes according to
the child specifications in the start specification. In this case
there is one child process, <span class="code">ch3</span>.</p>
<p>Note that <span class="code">supervisor:start_link</span> is synchronous. It does
not return until all child processes have been started.</p>
<h3><a name="id72349">5.7
Adding a Child Process</a></h3>
<p>In addition to the static supervision tree, we can also add
dynamic child processes to an existing supervisor with
the following call:</p>
<div class="example"><pre>
supervisor:start_child(Sup, ChildSpec)</pre></div>
<p><span class="code">Sup</span> is the pid, or name, of the supervisor.
<span class="code">ChildSpec</span> is a <span class="bold_code"><a href="#spec">child specification</a></span>.</p>
<p>Child processes added using <span class="code">start_child/2</span> behave in
the same manner as the other child processes, with the following
important exception: If a supervisor dies and is re-created, then
all child processes which were dynamically added to the supervisor
will be lost.</p>
<h3><a name="id72393">5.8
Stopping a Child Process</a></h3>
<p>Any child process, static or dynamic, can be stopped in
accordance with the shutdown specification:</p>
<div class="example"><pre>
supervisor:terminate_child(Sup, Id)</pre></div>
<p>The child specification for a stopped child process is deleted
with the following call:</p>
<div class="example"><pre>
supervisor:delete_child(Sup, Id)</pre></div>
<p><span class="code">Sup</span> is the pid, or name, of the supervisor.
<span class="code">Id</span> is the id specified in the <span class="bold_code"><a href="#spec">child specification</a></span>.</p>
<p>As with dynamically added child processes, the effects of
deleting a static child process is lost if the supervisor itself
restarts.</p>
<h3><a name="id72443">5.9
Simple-One-For-One Supervisors</a></h3>
<p>A supervisor with restart strategy <span class="code">simple_one_for_one</span> is
a simplified one_for_one supervisor, where all child processes are
dynamically added instances of the same process.</p>
<p>Example of a callback module for a simple_one_for_one supervisor:</p>
<div class="example"><pre>
-module(simple_sup).
-behaviour(supervisor).
-export([start_link/0]).
-export([init/1]).
start_link() ->
supervisor:start_link(simple_sup, []).
init(_Args) ->
{ok, {{simple_one_for_one, 0, 1},
[{call, {call, start_link, []},
temporary, brutal_kill, worker, [call]}]}}.</pre></div>
<p>When started, the supervisor will not start any child processes.
Instead, all child processes are added dynamically by calling:</p>
<div class="example"><pre>
supervisor:start_child(Sup, List)</pre></div>
<p><span class="code">Sup</span> is the pid, or name, of the supervisor.
<span class="code">List</span> is an arbitrary list of terms which will be added to
the list of arguments specified in the child specification. If
the start function is specified as <span class="code">{M, F, A}</span>, then
the child process is started by calling
<span class="code">apply(M, F, A++List)</span>.</p>
<p>For example, adding a child to <span class="code">simple_sup</span> above:</p>
<div class="example"><pre>
supervisor:start_child(Pid, [id1])</pre></div>
<p>results in the child process being started by calling
<span class="code">apply(call, start_link, []++[id1])</span>, or actually:</p>
<div class="example"><pre>
call:start_link(id1)</pre></div>
<p>A child under a <span class="code">simple_one_for_one</span> supervisor can be terminated
with</p>
<div class="example"><pre>
supervisor:terminate_child(Sup, Pid)</pre></div>
<p>where <span class="code">Sup</span> is the pid, or name, of the supervisor and
<span class="code">Pid</span> is the pid of the child.</p>
<p>Because a <span class="code">simple_one_for_one</span> supervisor could have many children,
it shuts them all down at same time. So, order in which they are stopped is
not defined. For the same reason, it could have an overhead with regards to
the <span class="code">Shutdown</span> strategy.</p>
<h3><a name="id72567">5.10
Stopping</a></h3>
<p>Since the supervisor is part of a supervision tree, it will
automatically be terminated by its supervisor. When asked to
shutdown, it will terminate all child processes in reversed start
order according to the respective shutdown specifications, and
then terminate itself.</p>
</div>
<div class="footer">
<hr>
<p>Copyright © 1997-2012 Ericsson AB. All Rights Reserved.</p>
</div>
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