How to Fix When all Instances of a ksqlDB Cluster Crash

A complete crash of a ksqlDB cluster can be disruptive, but the good news is that recovery is straightforward. The key is that a ksqlDB cluster's state is persisted in Kafka, along with the consumer group offsets. This means that restarting the ksqlDB servers is all that's required to get a cluster back up and running.

ksqlDB was designed as a fault-tolerant system that leverages Apache Kafka for data retention and distribution. The ksqlDB servers themselves are stateless, meaning all the state associated with queries, streams, and tables is stored externally in Kafka topics.

So when all the ksqlDB servers in a cluster crash, the data remains safe in Kafka. The only thing lost is the ephemeral state within the live ksqlDB servers. Once restarted, the servers repopulate their state by reading from the Kafka logs.

Recovering a fully crashed ksqlDB cluster only requires restarting the ksqlDB servers. Follow these steps:

The ksql.service.id property identifies a ksqlDB server instance. This should be unique per ksqlDB server.

Check this property in the server configuration files or the output of the SHOW PROPERTIES; command in the ksqlDB CLI.

Restart each of the ksqlDB servers, making sure to use the same ksql.service.id property as was originally configured.

For example, restart the servers using the same commands, Docker configs, or Kubernetes configs that were used to start them initially.

Once restarted, the ksqlDB servers will reconnect to the Kafka cluster and repopulate their state by consuming the query topics.

The servers should begin processing new data from input topics. Check the health of persistent queries and monitor for errors to confirm the cluster has recovered.

• ksqlDB leverages Kafka for data retention, so data is not lost when servers crash.
• Simply restarting the ksqlDB servers with the same ksql.service.id is all that's required.
• The servers will rebuild the state by consuming query topics from Kafka.

So while a total cluster outage can certainly disrupt operations, recovering a ksqlDB deployment is straightforward. The fault tolerance of Kafka combined with stateless servers makes ksqlDB resilient to even catastrophic failure scenarios.