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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<TITLE>Included Applications</TITLE>
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<A NAME="8"><!-- Empty --></A>
<H2>8 Included Applications</H2>
<A NAME="8.1"><!-- Empty --></A>
<H3>8.1 Definition</H3>
<P>An application can <STRONG>include</STRONG> other applications.
An <STRONG>included application</STRONG> has its own application directory
and <CODE>.app</CODE> file, but it is started as part of the supervisor
tree of another application.
<P>An application can only be included by one other application.
<P>An included application can include other applications.
<P>An application which is not included by any other application is
called a <STRONG>primary application</STRONG>.
<P>
<CENTER>
<IMG ALT="inclappls" SRC="inclappls.gif"><BR>
<EM><A NAME="inclappls"><!-- Empty --></A>
Primary Application and Included Applications.
</EM>
</CENTER>
<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>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.<A NAME="8.2"><!-- Empty --></A>
<H3>8.2 Specifying Included Applications</H3>
<P>Which applications to include is defined by
the <CODE>included_applications</CODE> key in the <CODE>.app</CODE> file.
<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>
<A NAME="8.3"><!-- Empty --></A>
<H3>8.3 Synchronizing Processes During Startup</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>Start phases are defined by the <CODE>start_phases</CODE> key in
the <CODE>.app</CODE> file as a list of tuples <CODE>{Phase,PhaseArgs}</CODE>,
where <CODE>Phase</CODE> is an atom and <CODE>PhaseArgs</CODE> is a term.
Also, the value of the <CODE>mod</CODE> key of the including application
must be set to <CODE>{application_starter,[Module,StartArgs]}</CODE>,
where <CODE>Module</CODE> as usual is the application callback module
and <CODE>StartArgs</CODE> a term provided as argument to the callback
function <CODE>Module:start/2</CODE>.
<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>
<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
<CODE>Module:start(normal, StartArgs)</CODE> to start the top
supervisor.
<P>Then, for the primary application and each included application
in top-down, left-to-right order, the application master calls
<CODE>Module:start_phase(Phase, Type, PhaseArgs)</CODE> 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>The following requirements apply to the <CODE>.app</CODE> file for
an included application:
<P>
<UL>
<LI>
The <CODE>{mod, {Module,StartArgs}}</CODE> option must be
included. This option is used to find the callback module
<CODE>Module</CODE> of the application. <CODE>StartArgs</CODE> is ignored,
as <CODE>Module:start/2</CODE> is called only for the primary
application.
</LI>
<LI>
If the included application itself contains included
applications, instead the option
<CODE>{mod, {application_starter, [Module,StartArgs]}}</CODE> must be
included.
</LI>
<LI>
The <CODE>{start_phases, [{Phase,PhaseArgs}]}</CODE> 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 <CODE>prim_app</CODE> as defined above, the application
controller will call the following callback functions, before
<CODE>application:start(prim_app)</CODE> returns a value:
<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>
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