<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2003</year><year>2013</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>Distributed Applications</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>distributed_applications.xml</file>
</header>
<marker id="distributed appl"></marker>
<section>
<title>Introduction</title>
<p>In a distributed system with several Erlang nodes, it can be
necessary to control applications in a distributed manner. If
the node, where a certain application is running, goes down,
the application is to be restarted at another node.</p>
<p>Such an application is called a <em>distributed application</em>.
Notice that it is the control of the application that is distributed.
All applications can be distributed in the sense that they,
for example, use services on other nodes.</p>
<p>Since a distributed application can move between nodes, some
addressing mechanism is required to ensure that it can be
addressed by other applications, regardless on which node it
currently executes. This issue is not addressed here, but the
<c>global</c> or <c>pg2</c> modules in Kernel
can be used for this purpose.</p>
</section>
<section>
<title>Specifying Distributed Applications</title>
<p>Distributed applications are controlled by both the application
controller and a distributed application controller process,
<c>dist_ac</c>. Both these processes are part of the Kernel
application. Distributed applications are thus specified by
configuring the Kernel application, using the following
configuration parameter (see also <c>kernel(6)</c>):</p>
<p><c>distributed = [{Application, [Timeout,] NodeDesc}]</c></p>
<list type="bulleted">
<item>Specifies where the application <c>Application = atom()</c>
can execute.</item>
<item>><c>NodeDesc = [Node | {Node,...,Node}]</c> is a list of
node names in priority order. The order between nodes in a tuple
is undefined.</item>
<item><c>Timeout = integer()</c> specifies how many milliseconds
to wait before restarting the application at another node. It
defaults to 0.</item>
</list>
<p>For distribution of application control to work properly,
the nodes where a distributed application can run must contact
each other and negotiate where to start the application. This is
done using the following configuration parameters in
Kernel:</p>
<list type="bulleted">
<item><c>sync_nodes_mandatory = [Node]</c> - Specifies which
other nodes must be started (within the time-out specified by
<c>sync_nodes_timeout</c>).</item>
<item><c>sync_nodes_optional = [Node]</c> - Specifies which
other nodes can be started (within the time-out specified by
<c>sync_nodes_timeout</c>).</item>
<item><c>sync_nodes_timeout = integer() | infinity</c> -
Specifies how many milliseconds to wait for the other nodes to
start.</item>
</list>
<p>When started, the node waits for all nodes specified by
<c>sync_nodes_mandatory</c> and <c>sync_nodes_optional</c> to
come up. When all nodes are up, or when all mandatory nodes
are up and the time specified by <c>sync_nodes_timeout</c>
has elapsed, all applications start. If not all
mandatory nodes are up, the node terminates.</p>
<p><em>Example:</em></p>
<p>An application <c>myapp</c> is to run at the node
<c>cp1@cave</c>. If this node goes down, <c>myapp</c> is to
be restarted at <c>cp2@cave</c> or <c>cp3@cave</c>. A system
configuration file <c>cp1.config</c> for <c>cp1@cave</c> can
look as follows:</p>
<code type="none">
[{kernel,
[{distributed, [{myapp, 5000, [cp1@cave, {cp2@cave, cp3@cave}]}]},
{sync_nodes_mandatory, [cp2@cave, cp3@cave]},
{sync_nodes_timeout, 5000}
]
}
].</code>
<p>The system configuration files for <c>cp2@cave</c> and
<c>cp3@cave</c> are identical, except for the list of mandatory
nodes, which is to be <c>[cp1@cave, cp3@cave]</c> for
<c>cp2@cave</c> and <c>[cp1@cave, cp2@cave]</c> for
<c>cp3@cave</c>.</p>
<note>
<p>All involved nodes must have the same value for
<c>distributed</c> and <c>sync_nodes_timeout</c>.
Otherwise the system behaviour is undefined.</p>
</note>
</section>
<section>
<title>Starting and Stopping Distributed Applications</title>
<p>When all involved (mandatory) nodes have been started,
the distributed application can be started by calling
<c>application:start(Application)</c> at <em>all of these
nodes.</em></p>
<p>A boot script (see
<seealso marker="release_structure">Releases</seealso>)
can be used that automatically starts the application.</p>
<p>The application is started at the first operational node that
is listed in the list of nodes in the <c>distributed</c>
configuration parameter. The application is started as usual.
That is, an application master is created and calls the
application callback function:</p>
<code type="none">
Module:start(normal, StartArgs)</code>
<p>Example:</p>
<p>Continuing the example from the previous section, the three nodes
are started, specifying the system configuration file:</p>
<pre>
> <input>erl -sname cp1 -config cp1</input>
> <input>erl -sname cp2 -config cp2</input>
> <input>erl -sname cp3 -config cp3</input></pre>
<p>When all nodes are operational, <c>myapp</c> can be started.
This is achieved by calling <c>application:start(myapp)</c> at
all three nodes. It is then started at <c>cp1</c>, as shown in
the following figure:</p>
<marker id="dist1"></marker>
<image file="../design_principles/dist1.gif">
<icaption>Application myapp - Situation 1</icaption>
</image>
<p>Similarly, the application must be stopped by calling
<c>application:stop(Application)</c> at all involved nodes.</p>
</section>
<section>
<title>Failover</title>
<p>If the node where the application is running goes down,
the application is restarted (after the specified time-out) at
the first operational node that is listed in the list of nodes
in the <c>distributed</c> configuration parameter. This is called a
<em>failover</em>.</p>
<p>The application is started the normal way at the new node,
that is, by the application master calling:</p>
<code type="none">
Module:start(normal, StartArgs)</code>
<p>An exception is if the application has the <c>start_phases</c>
key defined
(see <seealso marker="included_applications">Included Applications</seealso>).
The application is then instead started by calling:</p>
<code type="none">
Module:start({failover, Node}, StartArgs)</code>
<p>Here <c>Node</c> is the terminated node.</p>
<p><em>Example:</em></p>
<p> If <c>cp1</c> goes down, the system checks which one of
the other nodes, <c>cp2</c> or <c>cp3</c>, has the least number of
running applications, but waits for 5 seconds for <c>cp1</c> to
restart. If <c>cp1</c> does not restart and <c>cp2</c> runs fewer
applications than <c>cp3</c>, <c>myapp</c> is restarted on
<c>cp2</c>.</p>
<marker id="dist2"></marker>
<image file="../design_principles/dist2.gif">
<icaption>Application myapp - Situation 2</icaption>
</image>
<p>Suppose now that <c>cp2</c> goes also down and does not
restart within 5 seconds. <c>myapp</c> is now restarted on
<c>cp3</c>.</p>
<marker id="dist3"></marker>
<image file="../design_principles/dist3.gif">
<icaption>Application myapp - Situation 3</icaption>
</image>
</section>
<section>
<title>Takeover</title>
<p>If a node is started, which has higher priority according
to <c>distributed</c> than the node where a distributed
application is running, the application is restarted at the
new node and stopped at the old node. This is
called a <em>takeover</em>.</p>
<p>The application is started by the application master calling:</p>
<code type="none">
Module:start({takeover, Node}, StartArgs)</code>
<p>Here <c>Node</c> is the old node.</p>
<p><em>Example: </em></p>
<p>If <c>myapp</c> is running at <c>cp3</c>, and if
<c>cp2</c> now restarts, it does not restart <c>myapp</c>,
as the order between the <c>cp2</c> and <c>cp3</c> nodes is
undefined.</p>
<marker id="dist4"></marker>
<image file="../design_principles/dist4.gif">
<icaption>Application myapp - Situation 4</icaption>
</image>
<p>However, if <c>cp1</c> also restarts, the function
<c>application:takeover/2</c> moves <c>myapp</c> to <c>cp1</c>,
as <c>cp1</c> has a higher priority than <c>cp3</c> for this
application. In this case,
<c>Module:start({takeover, cp3@cave}, StartArgs)</c> is
executed at <c>cp1</c> to start the application.</p>
<marker id="dist5"></marker>
<image file="../design_principles/dist5.gif">
<icaption>Application myapp - Situation 5</icaption>
</image>
</section>
</chapter>
