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<chapter>
<header>
<copyright>
<year>2014</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>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
<seealso marker="sasl:relup">relup(4)</seealso> manual page
in SASL.
For more information about upgrade instructions, see the
<seealso marker="sasl:appup">appup(4)</seealso> 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>
