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<h1>7 Applications</h1>
<a name="appl"></a>
<p>This chapter should be read in conjunction with <span class="code">app(4)</span> and
<span class="code">application(3)</span>.</p>
<h3><a name="id73539">7.1
Application Concept</a></h3>
<p>When we have written code implementing some specific
functionality, we might want to make the code into an
<strong>application</strong>, that is a component that can be started and
stopped as a unit, and which can be re-used in other systems as
well.</p>
<p>To do this, we create an
<span class="bold_code"><a href="#callback_module">application callback module</a></span>, where we describe how the application should
be started and stopped.</p>
<p>Then, an <strong>application specification</strong> is needed, which is
put in an <span class="bold_code"><a href="#appl_res_file">application resource file</a></span>. Among other things, we specify which
modules the application consists of and the name of the callback
module.</p>
<p>If we use <span class="code">systools</span>, the Erlang/OTP tools for packaging code
(see <span class="bold_code"><a href="release_structure.html">Releases</a></span>),
the code for each application is placed in a separate directory
following a pre-defined <span class="bold_code"><a href="#app_dir">directory structure</a></span>.</p>
<h3><a name="id73606">7.2
Application Callback Module</a></h3>
<a name="callback_module"></a>
<p>How to start and stop the code for the application, i.e.
the supervision tree, is described by two callback functions:</p>
<div class="example"><pre>
start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State}
stop(State)</pre></div>
<p><span class="code">start</span> is called when starting the application and should
create the supervision tree by starting the top supervisor.
It is expected to return the pid of the top supervisor and an
optional term <span class="code">State</span>, which defaults to []. This term is
passed as-is to <span class="code">stop</span>.</p>
<p><span class="code">StartType</span> is usually the atom <span class="code">normal</span>. It has other
values only in the case of a takeover or failover, see
<span class="bold_code"><a href="distributed_applications.html">Distributed Applications</a></span>. <span class="code">StartArgs</span> is defined by the key
<span class="code">mod</span> in the <span class="bold_code"><a href="#appl_res_file">application resource file</a></span> file.</p>
<p><span class="code">stop/1</span> is called <strong>after</strong> the application has been
stopped and should do any necessary cleaning up. Note that
the actual stopping of the application, that is the shutdown of
the supervision tree, is handled automatically as described in
<span class="bold_code"><a href="#stopping">Starting and Stopping Applications</a></span>.</p>
<a name="ch_app"></a>
<p>Example of an application callback module for packaging
the supervision tree from
the <span class="bold_code"><a href="sup_princ.html#ex">Supervisor</a></span> chapter:</p>
<div class="example"><pre>
-module(ch_app).
-behaviour(application).
-export([start/2, stop/1]).
start(_Type, _Args) ->
ch_sup:start_link().
stop(_State) ->
ok.</pre></div>
<p>A library application, which can not be started or stopped,
does not need any application callback module.</p>
<h3><a name="id73720">7.3
Application Resource File</a></h3>
<a name="appl_res_file"></a>
<p>To define an application, we create an <strong>application specification</strong> which is put in an <strong>application resource file</strong>, or in short <span class="code">.app</span> file:</p>
<div class="example"><pre>
{application, Application, [Opt1,...,OptN]}.</pre></div>
<p><span class="code">Application</span>, an atom, is the name of the application.
The file must be named <span class="code">Application.app</span>.</p>
<p>Each <span class="code">Opt</span> is a tuple <span class="code">{Key, Value}</span> which define a
certain property of the application. All keys are optional.
Default values are used for any omitted keys.</p>
<p>The contents of a minimal <span class="code">.app</span> file for a library
application <span class="code">libapp</span> looks like this:</p>
<div class="example"><pre>
{application, libapp, []}.</pre></div>
<p>The contents of a minimal <span class="code">.app</span> file <span class="code">ch_app.app</span> for
a supervision tree application like <span class="code">ch_app</span> looks like this:</p>
<div class="example"><pre>
{application, ch_app,
[{mod, {ch_app,[]}}]}.</pre></div>
<p>The key <span class="code">mod</span> defines the callback module and start
argument of the application, in this case <span class="code">ch_app</span> and
[], respectively. This means that</p>
<div class="example"><pre>
ch_app:start(normal, [])</pre></div>
<p>will be called when the application should be started and</p>
<div class="example"><pre>
ch_app:stop([])</pre></div>
<p>will be called when the application has been stopped.</p>
<p>When using <span class="code">systools</span>, the Erlang/OTP tools for packaging
code (see <span class="bold_code"><a href="release_structure.html">Releases</a></span>),
the keys <span class="code">description</span>, <span class="code">vsn</span>, <span class="code">modules</span>,
<span class="code">registered</span> and <span class="code">applications</span> should also be
specified:</p>
<div class="example"><pre>
{application, ch_app,
[{description, "Channel allocator"},
{vsn, "1"},
{modules, [ch_app, ch_sup, ch3]},
{registered, [ch3]},
{applications, [kernel, stdlib, sasl]},
{mod, {ch_app,[]}}
]}.</pre></div>
<dl>
<dt><strong><span class="code">description</span></strong></dt>
<dd>A short description, a string. Defaults to "".</dd>
<dt><strong><span class="code">vsn</span></strong></dt>
<dd>Version number, a string. Defaults to "".</dd>
<dt><strong><span class="code">modules</span></strong></dt>
<dd>All modules <strong>introduced</strong> by this application.
<span class="code">systools</span> uses this list when generating boot scripts and
tar files. A module must be defined in one and only one
application. Defaults to [].</dd>
<dt><strong><span class="code">registered</span></strong></dt>
<dd>All names of registered processes in the application.
<span class="code">systools</span> uses this list to detect name clashes
between applications. Defaults to [].</dd>
<dt><strong><span class="code">applications</span></strong></dt>
<dd>All applications which must be started before this
application is started. <span class="code">systools</span> uses this list to
generate correct boot scripts. Defaults to [], but note that
all applications have dependencies to at least <span class="code">kernel</span>
and <span class="code">stdlib</span>.</dd>
</dl>
<p>The syntax and contents of of the application resource file
are described in detail in <span class="code">app(4)</span>.</p>
<h3><a name="id73969">7.4
Directory Structure</a></h3>
<a name="app_dir"></a>
<p>When packaging code using <span class="code">systools</span>, the code for each
application is placed in a separate directory
<span class="code">lib/Application-Vsn</span>, where <span class="code">Vsn</span> is the version number.</p>
<p>This may be useful to know, even if <span class="code">systools</span> is not used,
since Erlang/OTP itself is packaged according to the OTP principles
and thus comes with this directory structure. The code server
(see <span class="code">code(3)</span>) will automatically use code from
the directory with the highest version number, if there are
more than one version of an application present.</p>
<p>The application directory structure can of course be used in
the development environment as well. The version number may then
be omitted from the name.</p>
<p>The application directory have the following sub-directories:</p>
<ul>
<li><span class="code">src</span></li>
<li><span class="code">ebin</span></li>
<li><span class="code">priv</span></li>
<li><span class="code">include</span></li>
</ul>
<dl>
<dt><strong><span class="code">src</span></strong></dt>
<dd>Contains the Erlang source code.</dd>
<dt><strong><span class="code">ebin</span></strong></dt>
<dd>Contains the Erlang object code, the <span class="code">beam</span> files.
The <span class="code">.app</span> file is also placed here.</dd>
<dt><strong><span class="code">priv</span></strong></dt>
<dd>Used for application specific files. For example, C
executables are placed here. The function <span class="code">code:priv_dir/1</span>
should be used to access this directory.</dd>
<dt><strong><span class="code">include</span></strong></dt>
<dd>Used for include files.</dd>
</dl>
<h3><a name="id74095">7.5
Application Controller</a></h3>
<a name="application_controller"></a>
<p>When an Erlang runtime system is started, a number of processes
are started as part of the Kernel application. One of these
processes is the <strong>application controller</strong> process,
registered as <span class="code">application_controller</span>.</p>
<p>All operations on applications are coordinated by the application
controller. It is interfaced through the functions in
the module <span class="code">application</span>, see <span class="code">application(3)</span>.
In particular, applications can be loaded, unloaded, started and
stopped.</p>
<h3><a name="id74128">7.6
Loading and Unloading Applications</a></h3>
<p>Before an application can be started, it must be <strong>loaded</strong>.
The application controller reads and stores the information from
the <span class="code">.app</span> file.</p>
<div class="example"><pre>
1> <span class="bold_code">application:load(ch_app).</span>
ok
2> <span class="bold_code">application:loaded_applications().</span>
[{kernel,"ERTS CXC 138 10","2.8.1.3"},
{stdlib,"ERTS CXC 138 10","1.11.4.3"},
{ch_app,"Channel allocator","1"}]</pre></div>
<p>An application that has been stopped, or has never been started,
can be unloaded. The information about the application is
erased from the internal database of the application controller.</p>
<div class="example"><pre>
3> <span class="bold_code">application:unload(ch_app).</span>
ok
4> <span class="bold_code">application:loaded_applications().</span>
[{kernel,"ERTS CXC 138 10","2.8.1.3"},
{stdlib,"ERTS CXC 138 10","1.11.4.3"}]</pre></div>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>Loading/unloading an application does not load/unload the code
used by the application. Code loading is done the usual way.</p>
</p></div>
</div>
<h3><a name="id74190">7.7
Starting and Stopping Applications</a></h3>
<a name="stopping"></a>
<p>An application is started by calling:</p>
<div class="example"><pre>
5> <span class="bold_code">application:start(ch_app).</span>
ok
6> <span class="bold_code">application:which_applications().</span>
[{kernel,"ERTS CXC 138 10","2.8.1.3"},
{stdlib,"ERTS CXC 138 10","1.11.4.3"},
{ch_app,"Channel allocator","1"}]</pre></div>
<p>If the application is not already loaded, the application
controller will first load it using <span class="code">application:load/1</span>. It
will check the value of the <span class="code">applications</span> key, to ensure
that all applications that should be started before this
application are running.</p>
<a name="application_master"></a>
<p>The application controller then creates an <strong>application master</strong> for the application. The application master is
the group leader of all the processes in the application.
The application master starts the application by calling
the application callback function <span class="code">start/2</span> in the module,
and with the start argument, defined by the <span class="code">mod</span> key in
the <span class="code">.app</span> file.</p>
<p>An application is stopped, but not unloaded, by calling:</p>
<div class="example"><pre>
7> <span class="bold_code">application:stop(ch_app).</span>
ok</pre></div>
<p>The application master stops the application by telling the top
supervisor to shutdown. The top supervisor tells all its child
processes to shutdown etc. and the entire tree is terminated in
reversed start order. The application master then calls
the application callback function <span class="code">stop/1</span> in the module
defined by the <span class="code">mod</span> key.</p>
<h3><a name="id74278">7.8
Configuring an Application</a></h3>
<p>An application can be configured using <strong>configuration parameters</strong>. These are a list of <span class="code">{Par, Val}</span> tuples
specified by a key <span class="code">env</span> in the <span class="code">.app</span> file.</p>
<div class="example"><pre>
{application, ch_app,
[{description, "Channel allocator"},
{vsn, "1"},
{modules, [ch_app, ch_sup, ch3]},
{registered, [ch3]},
{applications, [kernel, stdlib, sasl]},
{mod, {ch_app,[]}},
{env, [{file, "/usr/local/log"}]}
]}.</pre></div>
<p><span class="code">Par</span> should be an atom, <span class="code">Val</span> is any term.
The application can retrieve the value of a configuration
parameter by calling <span class="code">application:get_env(App, Par)</span> or a
number of similar functions, see <span class="code">application(3)</span>.</p>
<p>Example:</p>
<div class="example"><pre>
% <span class="bold_code">erl</span>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]
Eshell V5.2.3.6 (abort with ^G)
1> <span class="bold_code">application:start(ch_app).</span>
ok
2> <span class="bold_code">application:get_env(ch_app, file).</span>
{ok,"/usr/local/log"}</pre></div>
<p>The values in the <span class="code">.app</span> file can be overridden by values
in a <strong>system configuration file</strong>. This is a file which
contains configuration parameters for relevant applications:</p>
<div class="example"><pre>
[{Application1, [{Par11,Val11},...]},
...,
{ApplicationN, [{ParN1,ValN1},...]}].</pre></div>
<p>The system configuration should be called <span class="code">Name.config</span> and
Erlang should be started with the command line argument
<span class="code">-config Name</span>. See <span class="code">config(4)</span> for more information.</p>
<p>Example: A file <span class="code">test.config</span> is created with the following
contents:</p>
<div class="example"><pre>
[{ch_app, [{file, "testlog"}]}].</pre></div>
<p>The value of <span class="code">file</span> will override the value of <span class="code">file</span>
as defined in the <span class="code">.app</span> file:</p>
<div class="example"><pre>
% <span class="bold_code">erl -config test</span>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]
Eshell V5.2.3.6 (abort with ^G)
1> <span class="bold_code">application:start(ch_app).</span>
ok
2> <span class="bold_code">application:get_env(ch_app, file).</span>
{ok,"testlog"}</pre></div>
<p>If
<span class="bold_code"><a href="release_handling.html#sys">release handling</a></span>
is used, exactly one system configuration file should be used and
that file should be called <span class="code">sys.config</span></p>
<p>The values in the <span class="code">.app</span> file, as well as the values in a
system configuration file, can be overridden directly from
the command line:</p>
<div class="example"><pre>
% <span class="bold_code">erl -ApplName Par1 Val1 ... ParN ValN</span></pre></div>
<p>Example:</p>
<div class="example"><pre>
% <span class="bold_code">erl -ch_app file '"testlog"'</span>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]
Eshell V5.2.3.6 (abort with ^G)
1> <span class="bold_code">application:start(ch_app).</span>
ok
2> <span class="bold_code">application:get_env(ch_app, file).</span>
{ok,"testlog"}</pre></div>
<h3><a name="id74477">7.9
Application Start Types</a></h3>
<p>A <strong>start type</strong> is defined when starting the application:</p>
<div class="example"><pre>
application:start(Application, Type)</pre></div>
<p><span class="code">application:start(Application)</span> is the same as calling
<span class="code">application:start(Application, temporary)</span>. The type can
also be <span class="code">permanent</span> or <span class="code">transient</span>:</p>
<ul>
<li>If a permanent application terminates, all other
applications and the runtime system are also terminated.</li>
<li>If a transient application terminates with reason
<span class="code">normal</span>, this is reported but no other applications are
terminated. If a transient application terminates abnormally,
that is with any other reason than <span class="code">normal</span>, all other
applications and the runtime system are also terminated.</li>
<li>If a temporary application terminates, this is reported but
no other applications are terminated.</li>
</ul>
<p>It is always possible to stop an application explicitly by
calling <span class="code">application:stop/1</span>. Regardless of the mode, no
other applications will be affected.</p>
<p>Note that transient mode is of little practical use, since when
a supervision tree terminates, the reason is set to
<span class="code">shutdown</span>, not <span class="code">normal</span>.</p>
</div>
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