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High availability for pull queries

High Availability configuration reference

The following list shows the required configs for using high availability (HA).

High Availability for pull queries

ksqlDB supports pull queries, which you use to query materialized state that is stored while executing a persistent query. This works without issue when all nodes in your ksqlDB cluster are operating correctly, but what happens when a node storing that state goes down? First, you must start multiple nodes and make sure inter-node communication is configured so that query forwarding works correctly:

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listeners=http://0.0.0.0:8088
ksql.advertised.listener=http://host1.example.com:8088

The ksql.advertised.listener configuration specifies the URL that is propagated to other nodes for inter-node requests, so it must be reachable from other hosts/pods in the cluster. Inter-node requests are critical in a multi-node cluster. For more information, see configuring listeners of a ksqlDB cluster.

While waiting for a failed node to restart is one possibility, this approach may incur more downtime than you want, and it may not be possible if there is a more serious failure. The other possibility is to have replicas of the data, ready to go when they're needed. Fortunately, Kafka Streams provides a mechanism to do this:

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ksql.streams.num.standby.replicas=1
ksql.query.pull.enable.standby.reads=true

This first configuration tells Kafka Streams to use a separate task that operates independently of the active (writer) state store to build up a replica of the state. The second config indicates that reading is allowed from the replicas (or standbys) if reading fails from the active store.

This approach is sufficient to enable high availability for pull queries in ksqlDB, but it requires that every request must try the active first. A better approach is to use a heartbeating mechanism to detect failed nodes preemptively, before a pull query arrives, so the request can forward straight to a replica. Set the following configs to detect failed nodes preemptively.

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ksql.heartbeat.enable=true
ksql.lag.reporting.enable=true

The first configuration enables heartbeating, which should improve the speed of request handling significantly during failures, as described above. The second config allows for lag data of each of the standbys to be collected and sent to the other nodes to make routing decisions. In this case, the lag is defined by how many messages behind the active a given standby is. If ensuring freshness is a priority, you can provide a threshold in a pull query request to avoid the largest outliers:

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SET 'ksql.query.pull.max.allowed.offset.lag'='100';
SELECT * FROM QUERYABLE_TABLE WHERE ID = 456;

This configuration causes the request to consider only standbys that are within 100 messages of the active host.

With these configurations, you can introduce as much redundancy as you require and ensure that your pull queries succeed with controlled lag and low latency.

Note

Confluent Cloud is configured with HA enabled by default on clusters 8 CSUs or more.

Last update: 2021-12-05