Akka Cluster Rolling Upgrade and Persistent Actor Event Journal Compatibility

Answer to the Core Question

1. Deployment Ordering – Deploy the updated application—including all new event adapters and serializer classes—to every node in the cluster before any node writes a new event type. The typical safe sequence is:

1. Start a new node with the updated code and let it join the cluster.
2. Verify that the new node can deserialise existing events (e.g., by replaying a small batch of old events).
3. Once the new node is healthy, decommission the oldest node (the one currently running the old code).
4. Repeat until all nodes run the updated code.

2. Detection & Isolation of Incompatible Nodes – Akka Persistence exposes PersistenceFailure events on the event stream when deserialization fails. By subscribing to this event stream you can:

• Log the failure and the offending event type.
• Trigger a custom SupervisorStrategy that stops the actor and initiates a Cluster.leave on that node.
• Optionally, mark the node as unreachable in the cluster view so that shards with rememberEntities are re‑allocated.

3. Health‑Check Patterns – Implement a lightweight cluster‑wide health monitor that aggregates deserialization failures per node. A typical threshold strategy is:

• Maintain a sliding window of the last N recovery attempts.
• If failures exceed k in that window, flag the node for manual intervention before any further restarts.
• Optionally expose this metric via Prometheus or Akka's ClusterMetrics API for operator visibility.

Recommended Steps for Your Upgrade Scenario

1. Pre‑Upgrade Checklist
   • Ensure that all new event adapters are part of the deployment artifact.
   • Run a unit test that replays a subset of old events with the new code.
   • Verify that the ClusterEvent stream shows no deserialization failures when the new node starts.
2. Staged Rolling Upgrade
   • Deploy the new node, let it join, then confirm it can recover existing persistent actors.
   • Once confirmed, decommission the old node. Repeat.
3. Runtime Monitoring
   • Subscribe to PersistenceFailure events and log them with a correlation ID.
   • Configure a SupervisorStrategy that, on failure, triggers Cluster.leave for the offending node.
   • Use the health‑check aggregator to surface a “deserialization‑failure” metric.
4. Post‑Upgrade Validation
   • Run a full cluster replay on a test node to ensure all events are now deserialisable.
   • Confirm that shards with rememberEntities have been rebalanced.

Key Take‑aways

• Never let a node persist new event types until every cluster member can deserialise them.
• Akka Persistence does not automatically roll back journal state; manual snapshot restoration may be required if incompatible events already exist.
• Health‑check thresholds must be tuned per workload to avoid false positives.