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<header>
<copyright>
<year>1997</year><year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
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Licensed under the Apache License, Version 2.0 (the "License");
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<title>app</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<file>app</file>
<filesummary>Application resource file.</filesummary>
<description>
<p>The <em>application resource file</em> specifies the resources an
application uses, and how the application is started. There must
always be one application resource file called
<c>Application.app</c> for each application <c>Application</c> in
the system.</p>
<p>The file is read by the application controller when an
application is loaded/started. It is also used by the functions in
<c>systools</c>, for example when generating start scripts.</p>
</description>
<section>
<title>File Syntax</title>
<p>The application resource file is to be called
<c>Application.app</c>, where <c>Application</c> is the
application name. The file is to be located in directory <c>ebin</c>
for the application.</p>
<p>The file must contain a single Erlang term, which is called an
<em>application specification</em>:</p>
<code type="none">
{application, Application,
[{description, Description},
{id, Id},
{vsn, Vsn},
{modules, Modules},
{maxP, MaxP},
{maxT, MaxT},
{registered, Names},
{included_applications, Apps},
{applications, Apps},
{env, Env},
{mod, Start},
{start_phases, Phases},
{runtime_dependencies, RTDeps}]}.
Value Default
----- -------
Application atom() -
Description string() ""
Id string() ""
Vsn string() ""
Modules [Module] []
MaxP int() infinity
MaxT int() infinity
Names [Name] []
Apps [App] []
Env [{Par,Val}] []
Start {Module,StartArgs} []
Phases [{Phase,PhaseArgs}] undefined
RTDeps [ApplicationVersion] []
Module = Name = App = Par = Phase = atom()
Val = StartArgs = PhaseArgs = term()
ApplicationVersion = string()</code>
<taglist>
<tag><c>Application</c></tag>
<item>Application name.</item>
</taglist>
<p>For the application controller, all keys are optional.
The respective default values are used for any omitted keys.</p>
<p>The functions in <c>systools</c> require more information. If
they are used, the following keys are mandatory:</p>
<list type="bulleted">
<item><c>description</c></item>
<item><c>vsn</c></item>
<item><c>modules</c></item>
<item><c>registered</c></item>
<item><c>applications</c></item>
</list>
<p>The other keys are ignored by <c>systools</c>.</p>
<taglist>
<tag><c>description</c></tag>
<item>
<p>A one-line description of the application.</p>
</item>
<tag><c>id</c></tag>
<item>
<p>Product identification, or similar.</p>
</item>
<tag><c>vsn</c></tag>
<item>
<p>Version of the application.</p>
</item>
<tag><c>modules</c></tag>
<item>
<p>All modules introduced by this application. <c>systools</c>
uses this list when generating start scripts and tar files. A
module can only be defined in one application.</p>
</item>
<tag><c>maxP</c></tag>
<item>
<p><em>Deprecated - is ignored</em></p>
<p>Maximum number of processes allowed in the application.</p>
</item>
<tag><c>maxT</c></tag>
<item>
<p>Maximum time, in milliseconds, that the application is
allowed to run. After the specified time, the application
terminates automatically.</p>
</item>
<tag><c>registered</c></tag>
<item>
<p>All names of registered processes started in this
application. <c>systools</c> uses this list to detect name
clashes between different applications.</p>
</item>
<tag><c>included_applications</c></tag>
<item>
<p>All applications included by this application.
When this application is started, all included applications
are loaded automatically, but not started, by
the application controller. It is assumed that the top-most
supervisor of the included application is started by a
supervisor of this application.</p>
</item>
<tag><c>applications</c></tag>
<item>
<p>All applications that must be started before this
application is allowed to be started. <c>systools</c> uses
this list to generate correct start scripts. Defaults to
the empty list, but notice that all applications have
dependencies to (at least) <c>Kernel</c> and <c>STDLIB</c>.</p>
</item>
<tag><c>env</c></tag>
<item>
<p>Configuration parameters used by the application. The value
of a configuration parameter is retrieved by calling
<c>application:get_env/1,2</c>. The values in the application
resource file can be overridden by values in a configuration
file (see <seealso marker="config"><c>config(4)</c></seealso>)
or by command-line flags (see
<seealso marker="erts:erl"><c>erts:erl(1)</c></seealso>).</p>
</item>
<tag><c>mod</c></tag>
<item>
<p>Specifies the application callback module and a start argument, see
<seealso marker="application"><c>application(3)</c></seealso>.</p>
<p>Key <c>mod</c> is necessary for an application
implemented as a supervision tree, otherwise the application
controller does not know how to start it. <c>mod</c>
can be omitted for applications without processes, typically
code libraries, for example, <c>STDLIB</c>.</p>
</item>
<tag><c>start_phases</c></tag>
<item>
<p>A list of start phases and corresponding start arguments for
the application. If this key is present, the application
master, in addition to the usual call to
<c>Module:start/2</c>, also calls
<c>Module:start_phase(Phase,Type,PhaseArgs)</c> for each
start phase defined by key <c>start_phases</c>. Only
after this extended start procedure,
<c>application:start(Application)</c> returns.</p>
<p>Start phases can be used to synchronize startup of an
application and its included applications. In this case,
key <c>mod</c> must be specified as follows:</p>
<code type="none">
{mod, {application_starter,[Module,StartArgs]}}</code>
<p>The application master then calls <c>Module:start/2</c>
for the primary application, followed by calls to
<c>Module:start_phase/3</c> for each start phase (as defined
for the primary application), both for the primary application
and for each of its included applications, for which the start
phase is defined.</p>
<p>This implies that for an included application, the set of
start phases must be a subset of the set of phases defined
for the primary application. For more information, see
<seealso marker="doc/design_principles:applications">OTP Design Principles</seealso>.
</p>
</item>
<tag>
<marker id="runtime_dependencies"></marker>
<c>runtime_dependencies</c></tag>
<item>
<p>A list of application versions that the application
depends on. An example of such an application version is
<c>"kernel-3.0"</c>. Application versions specified as runtime
dependencies are minimum requirements. That is, a larger
application version than the one specified in the
dependency satisfies the requirement. For information about
how to compare application versions, see section
<seealso marker="doc/system_principles:versions">Versions</seealso>
in the System Principles User's Guide.</p>
<p>Notice that the application version
specifies a source code version. One more, indirect,
requirement is that the installed binary application of
the specified version is built so that it is
compatible with the rest of the system.</p>
<p>Some dependencies can only be required in specific runtime
scenarios. When such optional dependencies exist, these are
specified and documented in the corresponding "App" documentation
of the specific application.</p>
<warning><p>The <c>runtime_dependencies</c> key was introduced in
OTP 17.0. The type of its value might be subject to changes during
the OTP 17 release.</p></warning>
<warning><p>All runtime dependencies specified in OTP applications
during the OTP 17 release may not be completely correct. This
is actively being worked on. Declared runtime dependencies in OTP
applications are expected to be correct in OTP 18.</p></warning>
</item>
</taglist>
</section>
<section>
<title>See Also</title>
<p><seealso marker="application"><c>application(3)</c></seealso>,
<seealso marker="sasl:systools"><c>sasl:systools(3)</c></seealso></p>
</section>
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