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new 2bca261e6 Add 4.2 blog post (#786)
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commit 2bca261e6ca6ee160956e1918db9e68ba157a1bb
Author: Christo Lolov <[email protected]>
AuthorDate: Mon Feb 16 16:58:27 2026 +0000
Add 4.2 blog post (#786)
* Add 4.2 blog post
Reviewers: Matthias J. Sax <[email protected]>, Mickael Maison
<[email protected]>
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+---
+date: 2026-01-14
+title: Apache Kafka 4.2.0 Release Announcement
+linkTitle: AK 4.2.0
+author: Christo Lolov
+---
+
+<!--
+ Licensed to the Apache Software Foundation (ASF) under one or more
+ contributor license agreements. See the NOTICE file distributed with
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+ The ASF licenses this file to You under the Apache License, Version 2.0
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+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+-->
+
+
+
+
+We are proud to announce the release of Apache Kafka® 4.2. This release
contains many new features and improvements. This blog post will highlight some
of the more prominent ones. For a full list of changes, be sure to check the
[release notes](https://downloads.apache.org/kafka/4.2.0/RELEASE_NOTES.html).
+
+Kafka Queues (Share Groups) is now production-ready with new features like the
RENEW acknowledgement type for extended processing times, adaptive batching for
share coordinators, soft and strict enforcements of quantity of fetched
records, and comprehensive lag metrics.
+
+Kafka Streams brings the server-side rebalance protocol to GA with a limited
feature set, adds dead letter queue support in exception handlers, introduces
anchored wall-clock punctuation for deterministic scheduling, and gives users
full control over whether to send a leave group request on closing.
+
+This release also delivers significant improvements to consistency and
observability: CLI tools now feature standardized arguments like
--bootstrap-server across all tools, metric naming has been corrected to follow
the kafka.COMPONENT convention, and new idle ratio metrics provide better
visibility into controller and MetadataLoader performance.
+
+Security is enhanced with a new allowlist connector client configuration
override policy, while thread-safety improvements to RecordHeader eliminate
concurrency risks.
+
+Additional highlights include external schema support in JsonConverter for
reduced message sizes, dynamic configuration for remote log manager thread
pools, adaptive batching in group coordinators, and rack ID exposure in the
Admin API for consumer and share group members.
+
+See the [Upgrading to
4.2](https://kafka.apache.org/documentation.html#upgrade_4_2_0) section in the
documentation for the list of notable changes and detailed upgrade steps.
+
+## Deprecation Notices
+* [KIP-1136: Make ConsumerGroupMetadata an
interface](https://cwiki.apache.org/confluence/x/r45EF)
+ Deprecates the constructors of ConsumerGroupMetadata. Marked for removal in
Apache Kafka 5.0.
+
+* [KIP-1193: Deprecate MX4j
support](https://cwiki.apache.org/confluence/x/dAxJFg)
+ Adds various deprecation warnings for MX4j. Marked for removal in Apache
Kafka 5.0.
+
+* [KIP-1195: deprecate and remove
org.apache.kafka.streams.errors.BrokerNotFoundException](https://cwiki.apache.org/confluence/x/8AxJFg)
+ Deprecates BrokerNotFoundException. Marked for removal in Apache Kafka 5.0.
+
+## Kafka Broker, Controller, Producer, Consumer and Admin Client
+
+* [KIP-932: Queues for Kafka](https://cwiki.apache.org/confluence/x/4hA0Dw)
+ Introduces share groups, a new cooperative consumption model where multiple
consumers can concurrently process records from the same partitions with
individual acknowledgment and delivery counting - enabling queue-like use cases
without strict partition-to-consumer assignment.
+
+* [KIP-1052: Enable warmup in producer performance
test](https://cwiki.apache.org/confluence/x/iIxyEg)
+ Adds an optional `--warmup-records` argument to the producer performance
test, separating startup measurements from steady-state statistics for cleaner
performance analysis.
+
+* [KIP-1100: Rename org.apache.kafka.server:type=AssignmentsManager and
org.apache.kafka.storage.internals.log.RemoteStorageThreadPool
metrics](https://cwiki.apache.org/confluence/x/6oqMEw)
+ Fixes inconsistent metric naming by deprecating metrics accidentally changed
to `org.apache.kafka.COMPONENT` format and introducing new metrics using the
correct `kafka.COMPONENT` convention.
+
+* [KIP-1147: Improve consistency of command-line
arguments](https://cwiki.apache.org/confluence/x/DguWF)
+ Standardizes command-line tool arguments by introducing consistent options
like `--bootstrap-server` and `--command-config` across all tools, deprecating
inconsistent legacy options.
+
+* [KIP-1157: Enforce KafkaPrincipalSerde Implementation for
KafkaPrincipalBuilder](https://cwiki.apache.org/confluence/x/1gq9F)
+ Makes `KafkaPrincipalBuilder` extend `KafkaPrincipalSerde` to enforce
serialization/deserialization support at compile-time rather than failing at
runtime during KRaft broker-controller communication.
+
+* [KIP-1160: Enable returning supported features from a specific
broker](https://cwiki.apache.org/confluence/x/5gnXF)
+ Adds an optional `--node-id` argument to `describeFeatures`, allowing users
to query supported features from a specific node to address version
inconsistencies across nodes.
+
+* [KIP-1161: Unifying LIST-Type Configuration Validation and Default
Values](https://cwiki.apache.org/confluence/x/HArXF)
+ Standardizes validation for comma-separated list configurations by rejecting
null/empty values, ignoring duplicates, converting string configs to proper
LIST types, and throwing ConfigExceptions during parsing rather than at runtime.
+
+* [KIP-1172: Improve EndToEndLatency Tool with argument parser and message
key/header support](https://cwiki.apache.org/confluence/x/Awu9F)
+ Improves the `EndToEndLatency` tool with robust named-argument parsing, new
optional parameters for message keys and headers, and renamed arguments aligned
with Kafka conventions.
+
+* [KIP-1175: Fix the typo `PARTITIONER_ADPATIVE_PARTITIONING_ENABLE` in
ProducerConfig](https://cwiki.apache.org/confluence/x/KYogFQ)
+ Fixes a misspelling by introducing the correctly spelled
`PARTITIONER_ADAPTIVE_PARTITIONING_ENABLE_CONFIG` constant while deprecating
the misspelled version.
+
+* [KIP-1179: Introduce remote.log.manager.follower.thread.pool.size
config](https://cwiki.apache.org/confluence/x/xAlWFQ)
+ Introduces a new dynamic configuration
`remote.log.manager.follower.thread.pool.size` with proper naming conventions
and dynamic configuration support for the follower partition thread pool.
+
+* [KIP-1180: Add generic feature level
metrics](https://cwiki.apache.org/confluence/x/Cwo-FQ)
+ Adds new metrics displaying finalized, minimum supported, and maximum
supported feature levels for each production feature, improving visibility into
upgrade/downgrade scenarios.
+
+* [KIP-1186: Update AddRaftVoterRequest RPC to support
auto-join](https://cwiki.apache.org/confluence/x/1QkBFg)
+ Adds a boolean `AckWhenCommitted` flag to `AddRaftVoterRequest` that allows
immediate response after locally writing the new voter set, preventing
availability issues during auto-joining controller operations.
+
+* [KIP-1190: Add a metric for controller thread
idleness](https://cwiki.apache.org/confluence/x/TApJFg)
+ Adds a new `AvgIdleRatio` metric measuring the proportion of time the
controller thread spends idle versus actively processing events, improving
performance visibility.
+
+* [KIP-1192: Add include argument to ConsumerPerformance
tool](https://cwiki.apache.org/confluence/x/PAxJFg)
+ Adds an `--include` argument to ConsumerPerformance for regex-based topic
filtering, enabling multi-topic performance testing.
+
+* [KIP-1197: Introduce new method to improve the
TopicBasedRemoteLogMetadataManager's
initialization](https://cwiki.apache.org/confluence/x/Hg9JFg)
+ Fixes TopicBasedRemoteLogMetadataManager initialization failures by
introducing a `BrokerReadyCallback` interface that delays initialization until
the broker is fully ready.
+
+* [KIP-1205: Improve RecordHeader to be
Thread-Safe](https://cwiki.apache.org/confluence/x/nYmhFg)
+ Addresses thread-safety issues in `RecordHeader` by implementing
double-checked locking with volatile fields, eliminating `NullPointerException`
risks during concurrent access with negligible overhead.
+
+* [KIP-1206: Strict max fetch records in share
fetch](https://cwiki.apache.org/confluence/x/p4mhFg)
+ Introduces a new `ShareAcquireMode` configuration for shared fetch
operations, offering "batch_optimized" (soft limit) and "record_limit" (strict
enforcement) modes for different processing scenarios.
+
+* [KIP-1207: Fix anomaly of JMX metrics RequestHandlerAvgIdlePercent in kraft
combined mode](https://cwiki.apache.org/confluence/x/E4qhFg)
+ Fixes the `RequestHandlerAvgIdlePercent` metric in KRaft combined mode by
normalizing by combined thread count and introducing separate broker and
controller metrics for per-pool visibility.
+
+* [KIP-1217: Include push interval in ClientTelemetryReceiver
context](https://cwiki.apache.org/confluence/x/6QnxFg)
+ Addresses stale client telemetry metrics by introducing new interfaces that
include push interval information, enabling proper metric lifecycle management.
+
+* [KIP-1222: Acquisition lock timeout renewal in share consumer explicit
mode](https://cwiki.apache.org/confluence/x/pwkuFw)
+ Adds a new `RENEW` acknowledgement type for share consumers, allowing
applications to extend acquisition lock timeouts on records requiring longer
processing times.
+
+* [KIP-1224: Adaptive append.linger.ms for the group coordinator and share
coordinator](https://cwiki.apache.org/confluence/x/NopKFw)
+ Introduces adaptive batching mode for group and share coordinators that
automatically adjusts batch linger times based on workload, eliminating the 5ms
latency floor without manual tuning.
+
+* [KIP-1226: Introducing Share Partition Lag Persistence and
Retrieval](https://cwiki.apache.org/confluence/x/DItKFw)
+ Adds share partition lag metrics for share groups, enabling operators to
monitor consumption progress, detect imbalances, and support future autoscaling
capabilities.
+
+* [KIP-1227: Expose Rack ID in MemberDescription and
ShareMemberDescription](https://cwiki.apache.org/confluence/x/nwpeFw)
+ Exposes rack ID information for consumer and share group members in the
Admin API by adding a `rackId` field to member description classes.
+
+* [KIP-1228: Add Transaction Version to
WriteTxnMarkersRequest](https://cwiki.apache.org/confluence/x/3gpeFw)
+ Adds a TransactionVersion field to WriteTxnMarkersRequest, enabling stricter
epoch validation for Transaction Version 2 markers and strengthening
exactly-once semantics guarantees.
+
+* [KIP-1229: Add Idle Thread Ratio Metric to
MetadataLoader](https://cwiki.apache.org/confluence/x/ZQteFw)
+ Adds an `AvgIdleRatio` metric to the MetadataLoader component in KRaft
clusters, improving visibility into event queue processing efficiency.
+
+## Kafka Streams
+
+* [KIP-1034: Dead letter queue in Kafka
Streams](https://cwiki.apache.org/confluence/x/HwviEQ)
+ Adds dead letter queue (DLQ) support to Kafka Streams exception handlers by
introducing a new `Response` class with DLQ records, new `handleError()`
methods, and raw source record bytes in error contexts.
+
+* [KIP-1071: Streams Rebalance
Protocol](https://cwiki.apache.org/confluence/x/2BCTEg)
+ Introduces a new server-side group management protocol for Kafka Streams,
enabling broker-side task assignment, centralized topology metadata storage,
and improved observability through dedicated RPCs and admin tools.
+
+* [KIP-1146: Anchored
punctuation](https://cwiki.apache.org/confluence/x/9QqWF)
+ Introduces anchored wall-clock punctuation for Kafka Streams by adding an
optional `startTime` parameter to `schedule()`, enabling callbacks at fixed,
deterministic times (e.g., exactly at the start of every hour).
+
+* [KIP-1153: Refactor Kafka Streams CloseOptions to Fluent API
Style](https://cwiki.apache.org/confluence/x/QAq9F)
+ Gives users explicit control over whether KafkaStreams sends a leave-group
request on shutdown via a new `GroupMembershipOperation` enum, wrapped in a
fluent-style `CloseOptions` class that replaces the deprecated boolean-based
API.
+
+* [KIP-1216: Add rebalance listener metrics for Kafka
Streams](https://cwiki.apache.org/confluence/x/ywnxFg)
+ Adds thread-level latency metrics for tasks-revoked, tasks-assigned, and
tasks-lost rebalance callbacks in Kafka Streams, restoring observability after
the move to the dedicated streams rebalance protocol.
+
+* [KIP-1221: Add application-id tag to Kafka Streams state
metric](https://cwiki.apache.org/confluence/x/jQobFw)
+ Adds an `application-id` tag to the Kafka Streams `client-state` JMX metric,
enabling operators to group multiple instances belonging to the same logical
application.
+
+* [KIP-1230: Add config for file system
permissions](https://cwiki.apache.org/confluence/x/jgl3Fw)
+ Adds an opt-in `allow.os.group.write.access` configuration for Kafka
Streams, allowing users to grant write access to their OS user-group for the
local state directory.
+
+## Kafka Connect
+
+* [KIP-1054: Support external schemas in
JSONConverter](https://cwiki.apache.org/confluence/x/q4tyEg)
+ Adds an optional `schema.content` configuration to JsonConverter, allowing
schemas to be specified externally rather than embedded in every JSON message -
reducing message size and simplifying plain JSON integration.
+
+* [KIP-1120: AppInfo metrics don't contain the
client-id](https://cwiki.apache.org/confluence/x/3gn0Ew)
+ Adds a `client-id` tag to AppInfo metrics for Kafka Worker and MirrorMaker 2
clients, improving monitoring and debugging consistency with other Kafka
clients.
+
+* [KIP-1188: New ConnectorClientConfigOverridePolicy with allowlist of
configurations](https://cwiki.apache.org/confluence/x/2IkvFg)
+ Introduces a new "Allowlist" connector client configuration override policy
to address security vulnerabilities, allowing administrators to explicitly
specify which client configurations connectors can override.
+
+## Summary
+
+Ready to get started with Apache Kafka 4.2.0? Check out all the details in the
[upgrade notes](https://kafka.apache.org/documentation.html#upgrade_4_2_0) and
the [release
notes](https://downloads.apache.org/kafka/4.2.0/RELEASE_NOTES.html), and
[download](https://kafka.apache.org/downloads) Apache Kafka 4.2.0.
+
+This was a community effort, so thank you to everyone who contributed to this
release, including all our users and our 155 contributors:
+Abhi Tiwari, Abhijeet Kumar, Abhinav Dixit, Abhiram98, Alex, Alieh Saeedi,
ally heev, Alyssa Huang, Andrew J Schofield, Anton Vasanth, Apoorv Mittal,
Arpit Goyal, Artem Livshits, Bill Bejeck, Bolin Lin, Bruno Cadonna, Calvin Liu,
Chang-Chi Hsu, Chang-Yu Huang, Chia-Ping Tsai, Chih-Yuan Chien, Chirag Wadhwa,
Chris Egerton, Christo Lolov, Chuckame, Clemens Hutter, Colin Patrick McCabe,
d00791190, Dave Troiano, David Arthur, David Jacot, Deep Golani, Dejan
Stojadinović, devtrace404, Dmitry [...]
