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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2012</year><year>2013</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>Communication in Erlang</title>
<prepared>Rickard Green</prepared>
<responsible></responsible>
<docno></docno>
<approved></approved>
<checked></checked>
<date>2012-12-03</date>
<rev>PA1</rev>
<file>communication.xml</file>
</header>
<p>Communication in Erlang is conceptually performed using
asynchronous signaling. All different executing entities
such as processes, and ports communicate via asynchronous
signals. The most commonly used signal is a message. Other
common signals are exit, link, unlink, monitor, demonitor
signals.</p>
<section>
<title>Passing of Signals</title>
<p>The amount of time that passes between a signal being sent
and the arrival of the signal at the destination is unspecified
but positive. If the receiver has terminated, the signal will
not arrive, but it is possible that it triggers another signal.
For example, a link signal sent to a non-existing process will
trigger an exit signal which will be sent back to where the link
signal originated from. When communicating over the distribution,
signals may be lost if the distribution channel goes down.</p>
<p>The only signal ordering guarantee given is the following. If
an entity sends multiple signals to the same destination entity,
the order will be preserved. That is, if <c>A</c> sends
a signal <c>S1</c> to <c>B</c>, and later sends
the signal <c>S2</c> to <c>B</c>, <c>S1</c> is guaranteed not to
arrive after <c>S2</c>.</p>
</section>
<section>
<title>Synchronous Communication</title>
<p>Some communication is synchronous. If broken down into pieces,
a synchronous communication operation, consists of two asynchronous
signals. One request signal and one reply signal. An example of
such a synchronous communication is a call to <c>process_info/2</c>
when the first argument is not <c>self()</c>. The caller will send
an asynchronous signal requesting information, and will then
wait for the reply signal containing the requested information. When
the request signal reaches its destination the destination process
replies with the requested information.</p>
</section>
<section>
<title>Implementation</title>
<p>The implementation of different asynchronous signals in the
VM may vary over time, but the behaviour will always respect this
concept of asynchronous signals being passed between entities
as described above.</p>
<p>By inspecting the implementation you might notice that some
specific signal actually gives a stricter guarantee than described
above. It is of vital importance that such knowledge about the
implementation is <em>not</em> used by Erlang code, since the
implementation might change at any time without prior notice.</p>
<p>Some example of major implementation changes:</p>
<list type="bulleted">
<item>As of ERTS version 5.5.2 exit signals to processes are truly
asynchronously delivered.</item>
<item>As of ERTS version 5.10 all signals from processes to ports
are truly asynchronously delivered.</item>
</list>
</section>
</chapter>
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