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new a2d875b8dfc KAFKA-20376: Update consumer rebalance docs (#21930)
a2d875b8dfc is described below
commit a2d875b8dfc1a3ccf363e8509521237b0ae25240
Author: Lianet Magrans <[email protected]>
AuthorDate: Tue Apr 7 11:47:59 2026 -0400
KAFKA-20376: Update consumer rebalance docs (#21930)
Adding more info about assignors, evolution timeline. Updating existing
info with latest state.
Reviewers: David Jacot <[email protected]>, Andrew Schofield
<[email protected]>
---
docs/operations/consumer-rebalance-protocol.md | 49 ++++++++++++++++++--------
1 file changed, 35 insertions(+), 14 deletions(-)
diff --git a/docs/operations/consumer-rebalance-protocol.md
b/docs/operations/consumer-rebalance-protocol.md
index 97ac44605ad..2ee2b7c20d2 100644
--- a/docs/operations/consumer-rebalance-protocol.md
+++ b/docs/operations/consumer-rebalance-protocol.md
@@ -28,13 +28,14 @@ type: docs
## Overview
-Starting from Apache Kafka 4.0, the Next Generation of the Consumer Rebalance
Protocol ([KIP-848](https://cwiki.apache.org/confluence/x/HhD1D)) is Generally
Available (GA). It improves the scalability of consumer groups while
simplifying consumers. It also decreases rebalance times, thanks to its fully
incremental design, which no longer relies on a global synchronization barrier.
+Starting from Apache Kafka 4.0, the Next Generation of the Consumer Rebalance
Protocol ([KIP-848](https://cwiki.apache.org/confluence/x/HhD1D)) is Generally
Available (GA), ready for production workloads.
+It improves the scalability of consumer groups while simplifying consumers. It
also decreases rebalance times, thanks to its fully incremental design, which
no longer relies on a global synchronization barrier.
-Consumer Groups using the new protocol are now referred to as `Consumer`
groups, while groups using the old protocol are referred to as `Classic`
groups. Note that Classic groups can still be used to form consumer groups
using the old protocol.
+There are now two types of consumer groups, named depending upon whether the
consumers are using the new "consumer" group protocol or the earlier "classic"
group protocol.
## Server
-The new consumer protocol is automatically enabled on the server since Apache
Kafka 4.0. Enabling and disabling the protocol is controlled by the
`group.version` feature flag.
+The new consumer protocol is automatically enabled on the server since Apache
Kafka 4.0. Enabling and disabling the protocol on the server side is controlled
by the `group.version` feature flag.
The consumer heartbeat interval and the session timeout are controlled by the
server now with the following configs:
@@ -43,15 +44,28 @@ The consumer heartbeat interval and the session timeout are
controlled by the se
-The assignment strategy is also controlled by the server. The
`group.consumer.assignors` configuration can be used to specify the list of
available assignors for `Consumer` groups. By default, the `uniform` assignor
and the `range` assignor are configured. The first assignor in the list is used
by default unless the Consumer selects a different one. It is also possible to
implement custom assignment strategies on the server side by implementing the
`ConsumerGroupPartitionAssignor` interf [...]
+The assignment strategy is also controlled by the server. The
`group.consumer.assignors` configuration can be used to specify the list of
available assignors for `Consumer` groups.
+* `uniform` and `range` assignors are provided by default
+* `uniform` is the default one (first assignor in the list of
`group.consumer.assignors`) unless the Consumer selects a different one (via
the client config `group.remote.assignor`)
+* it is possible to implement custom assignment strategies on the server side,
by implementing the `ConsumerGroupPartitionAssignor` interface and specifying
the full class name in the `group.consumer.assignors` configuration.
-## Consumer
+### Migrating from Client-side Assignors
+
+The following table shows the mapping from client-side assignors to the new
server-side assignors:
+
+| Client-side Assignor | Server-side Assignor |
+|---------------------------|----------------------|
+| RangeAssignor | range |
+| CooperativeStickyAssignor | uniform |
+| StickyAssignor | uniform |
+| RoundRobinAssignor | uniform |
-Since Apache Kafka 4.0, the Consumer supports the new consumer rebalance
protocol. However, the protocol is not enabled by default. The `group.protocol`
configuration must be set to `consumer` to enable it. When enabled, the new
consumer protocol is used alongside an improved threading model.
-The `group.remote.assignor` configuration is introduced as an optional
configuration to overwrite the default assignment strategy configured on the
server side.
+## Consumer
+
+Since Apache Kafka 4.0, the Consumer fully supports the new Consumer rebalance
protocol. However, the protocol is not enabled by default. The `group.protocol`
configuration must be set to `consumer` to enable it. When enabled, the new
consumer protocol is used alongside an improved threading model.
-The `subscribe(SubscriptionPattern)` and `subscribe(SubscriptionPattern,
ConsumerRebalanceListener)` methods have been added to subscribe to a regular
expression with the new consumer rebalance protocol. With these methods, the
regular expression uses the RE2J format and is now evaluated on the server side.
+The `subscribe(SubscriptionPattern)` and `subscribe(SubscriptionPattern,
ConsumerRebalanceListener)` methods have been added to subscribe to a regular
expression with the new Consumer rebalance protocol. With these methods, the
regular expression uses the RE2J format and is now evaluated on the server side.
New metrics have been added to the Consumer when using the new rebalance
protocol, mainly providing visibility over the improved threading model. See
[New Consumer Metrics](https://cwiki.apache.org/confluence/x/lQ_TEg).
@@ -72,15 +86,22 @@ Consumer groups are automatically converted from `Classic`
to `Consumer` and vic
### Online
-Consumer groups can be upgraded without downtime by rolling out the consumer
with the `group.protocol=consumer` configuration. When the first consumer using
the new consumer rebalance protocol joins the group, the group is converted
from `Classic` to `Consumer`, and the classic rebalance protocol is
interoperated to work with the new consumer rebalance protocol. This is only
possible when the classic group uses an assignor that does not embed custom
metadata.
+Consumer groups can be upgraded without downtime by rolling out the consumer
with the `group.protocol=consumer` configuration. When the first consumer using
the new Consumer rebalance protocol joins the group, the group is converted
from `Classic` to `Consumer`, and the Classic rebalance protocol is
interoperated to work with the new Consumer rebalance protocol. This is only
possible when the classic group uses an assignor that does not embed custom
metadata.
-Consumer groups can be downgraded using the opposite process. In this case,
the group is converted from `Consumer` to `Classic` when the last consumer
using the new consumer rebalance protocol leaves the group.
+Consumer groups can be downgraded using the opposite process. In this case,
the group is converted from `Consumer` to `Classic` when the last consumer
using the new Consumer rebalance protocol leaves the group.
-## Limitations
+## Evolution Timeline
+
+The evolution timeline of the Consumer rebalance protocol is described in
[KIP-1274](https://cwiki.apache.org/confluence/display/KAFKA/KIP-1274%3A+Deprecate+and+remove+support+for+Classic+rebalance+protocol+in+KafkaConsumer),
and expected to be as follows:
-While the new consumer rebalance protocol works for most use cases, it is
still important to be aware of the following limitations:
+ * **Apache Kafka 3.7**: Early Access
+ * **Apache Kafka 4.0**: GA (production-ready).
+ * **Apache Kafka 5.0**: `KafkaConsumer` defaults to `Consumer` protocol,
while still supporting `Classic`
+ * **Apache Kafka 6.0**: `KafkaConsumer` only supports `Consumer` as
rebalance protocol, while the broker still supports `Classic` for backward
compatibility.
+
+## Limitations
- * Client-side assignors are not supported. (see
[KAFKA-18327](https://issues.apache.org/jira/browse/KAFKA-18327))
- * Rack-aware assignment strategies are not fully supported. (see
[KAFKA-17747](https://issues.apache.org/jira/browse/KAFKA-17747))
+ * Client-side assignors are not supported and not in scope at the moment.
Use [KAFKA-18327](https://issues.apache.org/jira/browse/KAFKA-18327) to provide
feedback if you have custom assignment strategies that may not be covered.
+ * Rack-aware assignment strategies are not fully supported yet (work is in
progress, see [KAFKA-19387](https://issues.apache.org/jira/browse/KAFKA-19387)).
