Release upgrade file
The release upgrade file describes how a release is upgraded in a running system.
This file is automatically generated by systools:make_relup/3,4, using a release resource file (.rel), application resource files (.app) and application upgrade files (.appup) as input.
In a target system, the release upgrade file should be located in the OTP_ROOT/erts-EVsn/Vsn directory.
The relup file contains one single Erlang term, which defines the instructions used to upgrade the release. The file has the following syntax:
{Vsn,
[{UpFromVsn, Descr, Instructions}, ...],
[{DownToVsn, Descr, Instructions}, ...]}.
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Vsn = string() is the current version of the release.
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UpFromVsn = string() is an earlier version of the release to upgrade from.
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Descr = term() is a user defined parameter passed from the systools:make_relup/3,4 function. It will be used in the return value of release_handler:install_release/1,2.
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Instructions is a list of low-level release upgrade instructions, see appup(5).
It consists of the release upgrade instructions from the respective application upgrade files (high-level instructions are translated to low-level instructions), in the same order as in the start script.
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DownToVsn = string() is an earlier version of the release to downgrade to.
When upgrading from UpFromVsn with release_handler:install_release/1,2, there does not have to be an exact match of versions, but UpFromVsn can be a sub-string of the current release version.