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<chapter>
  <header>
    <copyright>
      <year>2012</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>Communication in Erlang</title>
    <prepared>Rickard Green</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved>uy</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 through asynchronous
     signals. The most commonly used signal is a message. Other
     common signals are exit, link, unlink, monitor, and demonitor
     signals.</p>

  <section>
    <title>Passing of Signals</title>
    <p>The amount of time that passes between a signal is sent
    and the arrival of the signal at the destination is unspecified
    but positive. If the receiver has terminated, the signal does
    not arrive, but it can trigger another signal.
    For example, a link signal sent to a non-existing process
    triggers an exit signal, which is sent back to where the link
    signal originated from. When communicating over the distribution,
    signals can 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 is preserved; that is, if <c>A</c> sends
    a signal <c>S1</c> to <c>B</c>, and later sends
    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
    <seealso marker="erlang:process_info/2">
    <c>erlang:process_info/2</c></seealso>
    when the first argument is not <c>self()</c>. The caller sends
    an asynchronous signal requesting information, and then
    waits 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 virtual
    machine can vary over time, but the behavior always respects 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 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, as the
    implementation can change at any time without prior notice.</p>

    <p>Examples of major implementation changes:</p>

    <list type="bulleted">
      <item>As from ERTS 5.5.2 exit signals to processes are truly
      asynchronously delivered.</item>
      <item>As from ERTS 5.10 all signals from processes to ports
      are truly asynchronously delivered.</item>
    </list>
  </section>
</chapter>

