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<li title="Definition"><a href="included_applications.html#id74617">Definition</a></li>
<li title="Specifying Included Applications"><a href="included_applications.html#id74682">Specifying Included Applications</a></li>
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<h1>8 Included Applications</h1>
<h3><a name="id74617">8.1
Definition</a></h3>
<p>An application can <strong>include</strong> other applications.
An <strong>included application</strong> has its own application directory
and <span class="code">.app</span> file, but it is started as part of the supervisor
tree of another application.</p>
<p>An application can only be included by one other application.</p>
<p>An included application can include other applications.</p>
<p>An application which is not included by any other application is
called a <strong>primary application</strong>.</p>
<a name="inclappls"></a>
<img alt="IMAGE MISSING" src="../design_principles/inclappls.gif"><br>
<em>Figure
8.1:
Primary Application and Included Applications.</em>
<p>The application controller will automatically load any included
applications when loading a primary application, but not start
them. Instead, the top supervisor of the included application
must be started by a supervisor in the including application.</p>
<p>This means that when running, an included application is in fact
part of the primary application and a process in an included
application will consider itself belonging to the primary
application.</p>
<h3><a name="id74682">8.2
Specifying Included Applications</a></h3>
<p>Which applications to include is defined by
the <span class="code">included_applications</span> key in the <span class="code">.app</span> file.</p>
<div class="example"><pre>
{application, prim_app,
[{description, "Tree application"},
{vsn, "1"},
{modules, [prim_app_cb, prim_app_sup, prim_app_server]},
{registered, [prim_app_server]},
{included_applications, [incl_app]},
{applications, [kernel, stdlib, sasl]},
{mod, {prim_app_cb,[]}},
{env, [{file, "/usr/local/log"}]}
]}.</pre></div>
<h3><a name="id74708">8.3
Synchronizing Processes During Startup</a></h3>
<p>The supervisor tree of an included application is started as
part of the supervisor tree of the including application.
If there is a need for synchronization between processes in
the including and included applications, this can be achieved
by using <strong>start phases</strong>.</p>
<p>Start phases are defined by the <span class="code">start_phases</span> key in
the <span class="code">.app</span> file as a list of tuples <span class="code">{Phase,PhaseArgs}</span>,
where <span class="code">Phase</span> is an atom and <span class="code">PhaseArgs</span> is a term.
Also, the value of the <span class="code">mod</span> key of the including application
must be set to <span class="code">{application_starter,[Module,StartArgs]}</span>,
where <span class="code">Module</span> as usual is the application callback module
and <span class="code">StartArgs</span> a term provided as argument to the callback
function <span class="code">Module:start/2</span>.</p>
<div class="example"><pre>
{application, prim_app,
[{description, "Tree application"},
{vsn, "1"},
{modules, [prim_app_cb, prim_app_sup, prim_app_server]},
{registered, [prim_app_server]},
{included_applications, [incl_app]},
{start_phases, [{init,[]}, {go,[]}]},
{applications, [kernel, stdlib, sasl]},
{mod, {application_starter,[prim_app_cb,[]]}},
{env, [{file, "/usr/local/log"}]}
]}.
{application, incl_app,
[{description, "Included application"},
{vsn, "1"},
{modules, [incl_app_cb, incl_app_sup, incl_app_server]},
{registered, []},
{start_phases, [{go,[]}]},
{applications, [kernel, stdlib, sasl]},
{mod, {incl_app_cb,[]}}
]}.</pre></div>
<p>When starting a primary application with included applications,
the primary application is started the normal way:
The application controller creates an application master for
the application, and the application master calls
<span class="code">Module:start(normal, StartArgs)</span> to start the top
supervisor.</p>
<p>Then, for the primary application and each included application
in top-down, left-to-right order, the application master calls
<span class="code">Module:start_phase(Phase, Type, PhaseArgs)</span> for each phase
defined for for the primary application, in that order.
Note that if a phase is not defined for an included application,
the function is not called for this phase and application.</p>
<p>The following requirements apply to the <span class="code">.app</span> file for
an included application:</p>
<ul>
<li>The <span class="code">{mod, {Module,StartArgs}}</span> option must be
included. This option is used to find the callback module
<span class="code">Module</span> of the application. <span class="code">StartArgs</span> is ignored,
as <span class="code">Module:start/2</span> is called only for the primary
application.</li>
<li>If the included application itself contains included
applications, instead the option
<span class="code">{mod, {application_starter, [Module,StartArgs]}}</span> must be
included.</li>
<li>The <span class="code">{start_phases, [{Phase,PhaseArgs}]}</span> option must
be included, and the set of specified phases must be a subset
of the set of phases specified for the primary application.</li>
</ul>
<p>When starting <span class="code">prim_app</span> as defined above, the application
controller will call the following callback functions, before
<span class="code">application:start(prim_app)</span> returns a value:</p>
<div class="example"><pre>
application:start(prim_app)
=> prim_app_cb:start(normal, [])
=> prim_app_cb:start_phase(init, normal, [])
=> prim_app_cb:start_phase(go, normal, [])
=> incl_app_cb:start_phase(go, normal, [])
ok</pre></div>
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