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<chapter>
  <header>
    <copyright>
      <year>2014</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
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    <title>Upgrade when Erlang/OTP has Changed</title>
    <prepared></prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2014-02-19</date>
    <rev></rev>
    <file>upgrade.xml</file>
  </header>
  <marker id="upgrade section"></marker>

  <section>
    <title>Introduction</title>
    <marker id="upgrade"></marker>
    <p>As of Erlang/OTP 17, most applications deliver a valid
      application upgrade file (<c>appup</c>). In earlier releases, a
      majority of the applications in Erlang/OTP did not support
      upgrade. Many of the applications use the
      <c>restart_application</c> instruction. These are applications
      for which it is not crucial to support real soft upgrade, for
      example, tools and library applications. The
      <c>restart_application</c> instruction
      ensures that all modules in the application are reloaded and
      thereby running the new code.</p>
  </section>

  <section>
    <title>Upgrade of Core Applications</title>
    <p>The core applications ERTS, Kernel, STDLIB,
      and SASL never allow real soft upgrade, but require the
      Erlang emulator to be restarted. This is indicated to the
      <c>release_handler</c> by the upgrade instruction
      <c>restart_new_emulator</c>. This instruction is always the
      very first instruction executed, and it restarts the
      emulator with the new versions of the above mentioned core
      applications and the old versions of all other applications.
      When the node is back up, all other upgrade instructions are
      executed, making sure each application is finally running its
      new version.</p>
    <p>It might seem strange to do a two-step upgrade instead of
      just restarting the emulator with the new version of all
      applications. The reason for this design decision is to allow
      <c>code_change</c> functions to have side effects, for example,
      changing data on disk. It also guarantees that the upgrade
      mechanism for non-core applications does not differ depending
      on whether or not core applications are changed at the same time.</p>
    <p>If, however, the more brutal variant is preferred, the
      the release upgrade file can be handwritten using only the
      single upgrade instruction <c>restart_emulator</c>. This
      instruction, in contrast to <c>restart_new_emulator</c>,
      causes the emulator to restart with the new versions of
      <em>all</em> applications.</p>
    <p><em>Note:</em> If other instructions are included before
      <c>restart_emulator</c> in the handwritten <c>relup</c> file,
      they are executed in the old emulator. This is a big risk
      since there is no guarantee that new beam code can be loaded
      into the old emulator. Adding instructions after
      <c>restart_emulator</c> has no effect as the
      <c>release_handler</c> will not execute them.</p>
    <p>For information about the release upgrade file, see the
      <seefile marker="sasl:relup">relup(4)</seefile> manual page
      in SASL.
      For more information about upgrade instructions, see the
      <seefile marker="sasl:appup">appup(4)</seefile> manual page
      in SASL.</p>
  </section>

  <section>
    <title>Applications that Still do Not Allow Code Upgrade</title>
    <p>A few applications, such as HiPE, do not support upgrade.
      This is indicated by an application upgrade file containing only
      <c>{Vsn,[],[]}</c>. Any attempt at creating a release upgrade file
      with such input fails. The only way to force an upgrade involving
      applications like this is to
      handwrite the file <c>relup</c>, preferably as described above
      with only the <c>restart_emulator</c> instruction.</p>
  </section>
</chapter>
