docs(tuning): consumer config: updates content on rebalances

documentation/modules/managing/con-consumer-config-properties.adoc

@@ -250,6 +250,19 @@ A shorter interval between consecutive heartbeats allows for quicker detection o
 The heartbeat interval must be lower, usually by a third, than the session timeout. 
 Decreasing the heartbeat interval reduces the chance of accidental rebalancing, but more frequent heartbeats increases the overhead on broker resources.
 
+[source,env]
+----
+# ...
+max.poll.records=100 # <1>
+session.timeout.ms=60000 # <2>
+heartbeat.interval.ms=10000 # <3>
+# ...
+----
+<1> Set the number records returned to the consumer when calling the `poll()` method.
+<2> Set the timeout for detecting client failure. 
+If the broker configuration has a `group.min.session.timeout.ms` and `group.max.session.timeout.ms`, the session timeout value must be within that range.
+<3> Adjust the heartbeat interval lower according to anticipated rebalances.
+
 == Managing offset policy
 
 Use the `auto.offset.reset` property to control how a consumer behaves when no offsets have been committed,
@@ -267,49 +280,88 @@ If a consumer group or standalone consumer is inactive and commits no offsets du
 [source,env]
 ----
 # ...
-heartbeat.interval.ms=3000 <1>
-session.timeout.ms=45000 <2>
-auto.offset.reset=earliest <3>
+auto.offset.reset=earliest # <1>
 # ...
 ----
-<1> Adjust the heartbeat interval lower according to anticipated rebalances.
-<2> If no heartbeats are received by the Kafka broker before the timeout duration expires, the consumer is removed from the consumer group and a rebalance is initiated.
-If the broker configuration has a `group.min.session.timeout.ms` and `group.max.session.timeout.ms`, the session timeout value must be within that range.
-<3> Set to `earliest` to return to the start of a partition and avoid data loss if offsets were not committed.
+<1> Set to `earliest` to return to the start of a partition and avoid data loss if offsets were not committed.
 
 If the amount of data returned in a single fetch request is large,
 a timeout might occur before the consumer has processed it.
 In this case, you can lower `max.partition.fetch.bytes` or increase `session.timeout.ms`.
 
-== Minimizing the impact of rebalances
+== Minimizing the impact of rebalances 
 
+Rebalances in Kafka consumer groups can introduce latency and reduce throughput, impacting overall service performance. 
 The rebalancing of a partition between active consumers in a group is the time it takes for the following to take place:
 
 * Consumers to commit their offsets
 * The new consumer group to be formed
 * The group leader to assign partitions to group members
 * The consumers in the group to receive their assignments and start fetching
 
-The rebalancing process can increase the downtime of a service, particularly if it happens repeatedly during a rolling restart of a consumer group cluster.
+Rebalances are triggered by changes in consumer health, network issues, configuration updates, and scaling events.
+This process can increase service downtime, especially if it occurs frequently, such as during rolling restarts of consumers in a group.
 
-In this situation, you can introduce _static membership_ by assigning a unique identifier (`group.instance.id`) to each consumer instance within the group.
-Static membership uses persistence so that a consumer instance is recognized during a restart after a session timeout.
-Consequently, the consumer maintains its assignment of topic partitions, reducing unnecessary rebalancing when it rejoins the group after a failure or restart.
-
-Additionally, adjusting the `max.poll.interval.ms` configuration can prevent rebalances caused by prolonged processing tasks, allowing you to specify the maximum interval between polls for new messages.
-Use the `max.poll.records` property to cap the number of records returned from the consumer buffer during each poll. 
-Reducing the number of records allows the consumer to process fewer messages more efficiently.  
-In cases where lengthy message processing is unavoidable, consider offloading such tasks to a pool of worker threads. 
-This parallel processing approach prevents delays and potential rebalances caused by overwhelming the consumer with a large volume of records.
+To minimize the impact of rebalances, consider the following strategies and configurations:
 
-[source,shell,subs="+quotes"]
+Assess throughput and parallelism:: Assess the expected throughput (bytes and records per second) and parallelism (number of partitions) of the input topics against the number of consumers.
++
+If adjustments are needed, start by setting up static membership, adopting a partition assignment strategy, and setting a limit on the number of records returned using the `max.poll.records` property. 
+Add further configurations for timeouts and intervals, if required and with care, as these can introduce issues related to the handling of failures.
+
+Use static membership:: Assign a unique identifier (`group.instance.id`) to each consumer instance. 
+Static membership introduces persistence so static consumers retain partition assignments across restarts, reducing unnecessary rebalances. 
+
+Adopt partition assignment strategies:: 
+* Use appropriate partition assignment strategies to reduce the number of partitions that need to be reassigned during a rebalance, minimizing the impact on active consumers.
+* The `org.apache.kafka.clients.consumer.CooperativeStickyAssignor` strategy is particularly effective, as it ensures minimal partition movement and better stability during rebalances.
+
+Adjust record limits and poll intervals::
+* Use the `max.poll.records` property to limit the number of records returned during each poll. 
+Processing fewer messages more efficiently can prevent delays. 
+* Use the `max.poll.interval.ms` property to prevent rebalances caused by prolonged processing tasks by setting the maximum interval between calls to the `poll()` method.
+* Alternatively, consider pausing partitions to retrieve fewer records at a time.
+
+Adjust session timeout and heartbeat intervals::
+* Use the `session.timeout.ms` property to set a longer timeout to reduce rebalances caused by temporary network glitches or minor processing delays.
+* Adjust the `heartbeat.interval.ms` property to balance failure detection checks with minimizing unnecessary rebalances.
+
+Monitor consumer health:: Instability in consumer applications, such as frequent crashes, can trigger rebalances. 
+Use Kafka consumer metrics to monitor such things as rebalance rates, session timouts, and failed fetch requests.
+
+.Example configuration to minimize the impact of rebalances
+[source,shell]
 ----
 # ...
-group.instance.id=_UNIQUE-ID_ <1>
-max.poll.interval.ms=300000 <2>
-max.poll.records=500 <3>
+group.instance.id=<unique_id>
+max.poll.interval.ms=300000
+max.poll.records=500
+session.timeout.ms=45000
+heartbeat.interval.ms=3000
+partition.assignment.strategy=org.apache.kafka.clients.consumer.CooperativeStickyAssignor
 # ...
 ----
-<1> The unique instance id ensures that a new consumer instance receives the same assignment of topic partitions.
-<2> Set the interval to check the consumer is continuing to process messages.
-<3> Sets the number of processed records returned from the consumer.
+
+.Scaling strategies
+To minimize the impact of rebalances during scaling of consumer groups, consider the following approaches:
+
+Set a rebalance delay:: Use the `group.initial.rebalance.delay.ms` property in the Kafka configuration to delay the time it takes for consumers to join a new consumer group before performing a rebalance.
+Introducing a delay helps avoid triggering several rebalances when starting multiple consumers near the same time. 
+The appropriate delay depends on the orchestration used and might not be suitable in some circumstances. 
+
+Avoid frequent scaling:: 
+* Keep the number of consumers stable, scaling only when necessary and in controlled increments.
+* Monitor system performance and adjust your scaling strategy as needed.
+** Lag per partition should be constant and low.
+** Records processed per second by consumers should match the records per second in the input topics.
+* Use the Kafka Exporter to check for consumer lag and determine if scaling is required.
+
+Implement dynamic scaling policies::
+* If using dynamic or event-driven tools for scaling of consumer applications, set lag thresholds based on the backlog of messages.
+* Define maximum and minimum replica counts for consumer groups.
+* Set periods between scaling events to prevent rapid scaling.
+
+NOTE: In cases where lengthy message processing is unavoidable, consider pausing and resuming partitions as needed.
+If you pause all partitions, `poll()` returns no records, allowing you to keep calling it without overwhelming the consumers. 
+Alternatively, you can offload the processing tasks to a pool of worker threads. 
+This helps prevents delays and potential rebalances.