When starting an application that uses Akka Cluster Sharding, if no cluster has been joined (e.g., because of forgotten seed node configuration) there is a cryptic error message:

Trying to register to coordinator at [None], but no acknowledgement.

This doesn't communicate the underlying issue, which is usually a misconfiguration.

Reproduction

1. Clone the akka-samples repo and cd to akka-sample-sharding-scala
2. Edit src/main/resources/application.conf and delete the seed-nodes setting
3. sbt "runMain sample.sharding.ShardingApp 2551"
4. It logs an INFO-level message saying "Cluster Node [akka.tcp://ShardingSystem@127.0.0.1:2551] - No seed-nodes configured, manual cluster join required", but because the application continues normally and the message is at INFO level, it is easy to miss this.
5. Eventually, you will begin to see "Trying to register to coordinator at [None], but no acknowledgement." warnings logged. It isn't clear that this is connected to the fact that the actor system hasn't joined a cluster.

References

• https://groups.google.com/d/topic/akka-user/VDsHEM2Sxx0/discussion
• https://groups.google.com/d/topic/akka-user/Qf5b9ii4-Z4/discussion
• https://groups.google.com/d/topic/akka-user/JOQLWAI8Zvc/discussion
• Service startup is incomplete if using JDBC Persistent Entities in standalone / production mode lagom/lagom#500