Database Migration Flow
When Deployment boots up after the image upgrade, it will detect that the currently active version is lower than the version it can support. In that case, the API Server will work on the older version normally, but the new version API will become available in read-only mode. Deployment is responsible for asynchronous, background syncing of higher version database with current version database. Clients are expected to use older versions anyway, so they won’t necessarily see incomplete higher versions. Besides, it’s fine, because what matters is the current version pointed out by Deployment.
It is expected that all Deployments will get new images first before we start switching to the next versions. Each Deployment will be responsible for silent copying.
For the MultiRegion case, when multiple deployments of the same service are on version v1, but they run on images that can support version v2, they will be still synced with each other, but on both versions: v1 and v2. When images are being deployed region by region (Deployment by Deployment), they may experience Unimplemented error messages, but it should be till images are updated in all regions. We may improve this and try to detect “available” versions first, before making cross-region watches.
Anyway, it will be required that new images are deployed to all regions before the upgrade procedure is triggered on any Regional deployment.
Upgrade then can be done one Deployment by one, using the procedure described in the migration section of the developer guide.
When one Deployment is officially upgraded to the new version, but still uses primarily the old version, then all deployments still watch each other for both versions, for the sake of multi-region syncing. However, Deployment using a newer version may already opt-out from pulling older API resources from other Deployments at this point.
Meta owner references are owned by Deployment they point to. It means that they are upgraded asynchronously after deployment switch the version to the newer one.