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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2003</year><year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
</legalnotice>
<title>Included Applications</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>included_applications.xml</file>
</header>
<marker id="included appl"></marker>
<section>
<title>Introduction</title>
<p>An application can <em>include</em> other applications.
An <em>included application</em> has its own application directory
and <c>.app</c> 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 that is not included by any other application is
called a <em>primary application</em>.</p>
<marker id="inclappls"></marker>
<image file="../design_principles/inclappls.gif">
<icaption>Primary Application and Included Applications</icaption>
</image>
<p>The application controller automatically loads any included
applications when loading a primary application, but does 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 considers itself belonging to the primary
application.</p>
</section>
<section>
<title>Specifying Included Applications</title>
<p>Which applications to include is defined by
the <c>included_applications</c> key in the <c>.app</c> file:</p>
<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>
</section>
<section>
<title>Synchronizing Processes during Startup</title>
<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 <em>start phases</em>.</p>
<p>Start phases are defined by the <c>start_phases</c> key in
the <c>.app</c> file as a list of tuples <c>{Phase,PhaseArgs}</c>,
where <c>Phase</c> is an atom and <c>PhaseArgs</c> is a term.</p>
<p>The value of the <c>mod</c> key of the including application
must be set to <c>{application_starter,[Module,StartArgs]}</c>,
where <c>Module</c> as usual is the application callback module.
<c>StartArgs</c> is a term provided as argument to the callback
function <c>Module:start/2</c>:</p>
<code type="none">
{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,[]}}
]}.</code>
<p>When starting a primary application with included applications,
the primary application is started the normal way, that is:</p>
<list type="bulleted">
<item>The application controller creates an application master for
the application</item>
<item>The application master calls
<c>Module:start(normal, StartArgs)</c> to start the top
supervisor.</item>
</list>
<p>Then, for the primary application and each included application
in top-down, left-to-right order, the application master calls
<c>Module:start_phase(Phase, Type, PhaseArgs)</c> for each phase
defined for the primary application, in that order. 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 <c>.app</c> file for
an included application:</p>
<list type="bulleted">
<item>The <c>{mod, {Module,StartArgs}}</c> option must be included.
This option is used to find the callback module <c>Module</c> of the
application. <c>StartArgs</c> is ignored, as <c>Module:start/2</c>
is called only for the primary application.</item>
<item>If the included application itself contains included
applications, instead the
<c>{mod, {application_starter, [Module,StartArgs]}}</c> option
must be included.</item>
<item>The <c>{start_phases, [{Phase,PhaseArgs}]}</c> option must
be included, and the set of specified phases must be a subset
of the set of phases specified for the primary application.</item>
</list>
<p>When starting <c>prim_app</c> as defined above, the application
controller calls the following callback functions before
<c>application:start(prim_app)</c> returns a value:</p>
<code type="none">
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</code>
</section>
</chapter>
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