Zero-downtime upgrade of TeamCity installation

Symptoms

1. A major TeamCity upgrade requires updating the database and data directory formats.
2. Upgrading the database requires an exclusive lock, meaning no other processes can access it during the upgrade.
3. This means that even in a multi-node setup, following the recommended upgrade path, there will be downtime.

Cause

Even for multi-node TeamCity installations, some downtime is inevitable due to the data format upgrade, as it requires an exclusive database lock.

Resolution

The zero-downtime upgrade is implemented as an experimental feature for the Kubernetes TeamCity deployment using the teamcity-operator.