High availability

When you run pull queries, it’s often the case that you need your data to remain available for querying even if one server fails. Because ksqlDB supports clustering, it can remain highly available to support pull queries on replicas of your data, even in the face of partial cluster failures.

High availability is turned off by default, but you can enable it with the following server configuration parameters. These parameters must be turned on for all nodes in your ksqlDB cluster.

1. Set ksql.streams.num.standby.replicas to a value greater than 0.
2. Set ksql.query.pull.enable.standby.reads to true.
3. Set ksql.heartbeat.enable to true.
4. Set ksql.lag.reporting.enable to true.

In addition, make sure that all of your nodes identify as part of the same cluster by setting ksql.service.id to the same value.

Note

In Confluent Cloud, ksqlDB clusters with 8 or 12 CSUs are automatically configured for high availability. High availability can't be enabled for Confluent Cloud clusters with fewer than 8 CSUs.

Controlling consistency

Because ksqlDB replicates data between its servers asynchronously, you may want to bound the potential staleness that your query will tolerate. You can control this per pull query by using the ksql.query.pull.max.allowed.offset.lag parameter. For instance, a value of 10,000 means that results of pull queries forwarded to servers whose current offset is more than 10,000 positions behind the end offset of the changelog topic are rejected.

Recommended Resources

• Course: Inside ksqlDB High Availability
• Highly Available, Fault-Tolerant Pull Queries in ksqlDB

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Last update: 2022-06-06