The release notes should always be read carefully when upgrading the ironic-inspector service. Starting with the Mitaka series, specific upgrade steps and considerations are well-documented in the release notes.
Upgrades are only supported one series at a time, or within a series. Only offline (with downtime) upgrades are currently supported.
When upgrading ironic-inspector, the following steps should always be taken:
Update ironic-inspector code, without restarting the service yet.
Stop the ironic-inspector service.
Run database migrations:
ironic-inspector-dbsync --config-file <PATH-TO-INSPECTOR.CONF> upgrade
Start the ironic-inspector service.
Upgrade the ironic-python-agent image used for introspection.
There is no implicit upgrade order between ironic and ironic-inspector, unless the release notes say otherwise.