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