Add an upgrading document
This outlines our baseline assumptions regarding upgrading. Change-Id: I2f63b3b3d643fc2aae4f35de8206d4081afa1494
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@ -5,6 +5,7 @@ Service Administration
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:maxdepth: 2
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installation
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upgrading
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components
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configuration
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drivers/index
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@ -0,0 +1,45 @@
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Upgrading
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=========
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Rolling Upgrades
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----------------
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If more than one of each Zuul component is present in a system, then
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Zuul may be upgrading without downtime by performing a rolling
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upgrade. During a rolling upgrade, components are stopped and started
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one at a time until all components are upgraded. If there is a
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behavior change during an upgrade, Zuul will typically wait until all
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components are upgraded before changing behavior, but in some cases
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when it is deemed safe, new behaviors may start to appear as soon as
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the first component is upgraded. Be sure not to begin using or rely
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on new behaviors until all components are upgraded.
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Unless specified in the release notes, there is no specific order for
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which components should be upgraded first, but the following order is
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likely to produce the least disruption and delay the use of new
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behaviors until closer to the end of the process:
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* Gracefully restart executors (one at a time, or as many as a
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system's over-allocation of resources will allow).
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* Gracefully restart mergers.
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* Restart schedulers.
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* Restart web and finger gateways.
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Skipping Versions
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-----------------
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Zuul versions are specified as `major.minor.micro`. In general,
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skipping minor or micro versions during upgrades is considered safe.
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Skipping major versions is not recommended, as backwards compatibility
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code for older systems may be removed during a major upgrade. This
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means that, for example, an upgrade from 5.x.y to 7.0.0 should include
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at least an upgrade to 6.4.0 (the latest 6.x release) before
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proceeding to 7.0.0.
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If skipping major versions is required, then a rolling upgrade is not
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possible, and Zuul should be completely stopped, and the ``zuul-admin
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delete-state`` command should be run before restarting on the new
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version.
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Some versions may have unique upgrade requirements. See release notes
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for additional information about specific version upgrades.
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