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// ======================================================================================

//= Overview
[partintro]
--
This guide describes the Apache Kafka implementation of the Spring Cloud Stream Binder.
It contains information about its design, usage, and configuration options, as well as information on how the Stream Cloud Stream concepts map onto Apache Kafka specific constructs.
In addition, this guide explains the Kafka Streams binding capabilities of Spring Cloud Stream.
--

== Apache Kafka Binder

=== Usage

To use Apache Kafka binder, you need to add `spring-cloud-stream-binder-kafka` as a dependency to your Spring Cloud Stream application, as shown in the following example for Maven:

[source,xml]
----
<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-stream-binder-kafka</artifactId>
</dependency>
----

Alternatively, you can also use the Spring Cloud Stream Kafka Starter, as shown inn the following example for Maven:

[source,xml]
----
<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-starter-stream-kafka</artifactId>
</dependency>
----

=== Overview

The following image shows a simplified diagram of how the Apache Kafka binder operates:

.Kafka Binder
image::{github-raw}/docs/src/main/asciidoc/images/kafka-binder.png[width=300,scaledwidth="50%"]

The Apache Kafka Binder implementation maps each destination to an Apache Kafka topic.
The consumer group maps directly to the same Apache Kafka concept.
Partitioning also maps directly to Apache Kafka partitions as well.

The binder currently uses the Apache Kafka `kafka-clients` 1.0.0 jar and is designed to be used with a broker of at least that version.
This client can communicate with older brokers (see the Kafka documentation), but certain features may not be available.
For example, with versions earlier than 0.11.x.x, native headers are not supported.
Also, 0.11.x.x does not support the `autoAddPartitions` property.

=== Configuration Options

This section contains the configuration options used by the Apache Kafka binder.

For common configuration options and properties pertaining to binder, see the <<binding-properties,core documentation>>.

==== Kafka Binder Properties

spring.cloud.stream.kafka.binder.brokers::
A list of brokers to which the Kafka binder connects.
+
Default: `localhost`.
spring.cloud.stream.kafka.binder.defaultBrokerPort::
`brokers` allows hosts specified with or without port information (for example, `host1,host2:port2`).
This sets the default port when no port is configured in the broker list.
+
Default: `9092`.
spring.cloud.stream.kafka.binder.configuration::
Key/Value map of client properties (both producers and consumer) passed to all clients created by the binder.
Due to the fact that these properties are used by both producers and consumers, usage should be restricted to common properties -- for example, security settings.
Unknown Kafka producer or consumer properties provided through this configuration are filtered out and not allowed to propagate.
Properties here supersede any properties set in boot.
+
Default: Empty map.
spring.cloud.stream.kafka.binder.consumerProperties::
Key/Value map of arbitrary Kafka client consumer properties.
In addition to support known Kafka consumer properties, unknown consumer properties are allowed here as well.
Properties here supersede any properties set in boot and in the `configuration` property above.
+
Default: Empty map.
spring.cloud.stream.kafka.binder.headers::
The list of custom headers that are transported by the binder.
Only required when communicating with older applications (<= 1.3.x) with a `kafka-clients` version < 0.11.0.0. Newer versions support headers natively.
+
Default: empty.
spring.cloud.stream.kafka.binder.healthTimeout::
The time to wait to get partition information, in seconds.
Health reports as down if this timer expires.
+
Default: 10.
spring.cloud.stream.kafka.binder.requiredAcks::
The number of required acks on the broker.
See the Kafka documentation for the producer `acks` property.
+
Default: `1`.
spring.cloud.stream.kafka.binder.minPartitionCount::
Effective only if `autoCreateTopics` or `autoAddPartitions` is set.
The global minimum number of partitions that the binder configures on topics on which it produces or consumes data.
It can be superseded by the `partitionCount` setting of the producer or by the value of `instanceCount * concurrency` settings of the producer (if either is larger).
+
Default: `1`.
spring.cloud.stream.kafka.binder.producerProperties::
Key/Value map of arbitrary Kafka client producer properties.
In addition to support known Kafka producer properties, unknown producer properties are allowed here as well.
Properties here supersede any properties set in boot and in the `configuration` property above.
+
Default: Empty map.
spring.cloud.stream.kafka.binder.replicationFactor::
The replication factor of auto-created topics if `autoCreateTopics` is active.
Can be overridden on each binding.
+
Default: `1`.
spring.cloud.stream.kafka.binder.autoCreateTopics::
If set to `true`, the binder creates new topics automatically.
If set to `false`, the binder relies on the topics being already configured.
In the latter case, if the topics do not exist, the binder fails to start.
+
NOTE: This setting is independent of the `auto.create.topics.enable` setting of the broker and does not influence it.
If the server is set to auto-create topics, they may be created as part of the metadata retrieval request, with default broker settings.
+
Default: `true`.
spring.cloud.stream.kafka.binder.autoAddPartitions::
If set to `true`, the binder creates new partitions if required.
If set to `false`, the binder relies on the partition size of the topic being already configured.
If the partition count of the target topic is smaller than the expected value, the binder fails to start.
+
Default: `false`.
spring.cloud.stream.kafka.binder.transaction.transactionIdPrefix::
Enables transactions in the binder. See `transaction.id` in the Kafka documentation and https://docs.spring.io/spring-kafka/reference/html/_reference.html#transactions[Transactions] in the `spring-kafka` documentation.
When transactions are enabled, individual `producer` properties are ignored and all producers use the `spring.cloud.stream.kafka.binder.transaction.producer.*` properties.
+
Default `null` (no transactions)
spring.cloud.stream.kafka.binder.transaction.producer.*::
Global producer properties for producers in a transactional binder.
See `spring.cloud.stream.kafka.binder.transaction.transactionIdPrefix` and <<kafka-producer-properties>> and the general producer properties supported by all binders.
+
Default: See individual producer properties.

spring.cloud.stream.kafka.binder.headerMapperBeanName::
The bean name of a `KafkaHeaderMapper` used for mapping `spring-messaging` headers to and from Kafka headers.
Use this, for example, if you wish to customize the trusted packages in a `DefaultKafkaHeaderMapper` that uses JSON deserialization for the headers.
+
Default: none.

[[kafka-consumer-properties]]
==== Kafka Consumer Properties

NOTE: To avoid repetition, Spring Cloud Stream supports setting values for all channels, in the format of `spring.cloud.stream.default.<property>=<value>`.


The following properties are available for Kafka consumers only and
must be prefixed with `spring.cloud.stream.kafka.bindings.<channelName>.consumer.`.

admin.configuration::
Since version 2.1.1, this property is deprecated in favor of `topic.properties`, and support for it will be removed in a future version.

admin.replicas-assignment::
Since version 2.1.1, this property is deprecated in favor of `topic.replicas-assignment`, and support for it will be removed in a future version.

admin.replication-factor::
Since version 2.1.1, this property is deprecated in favor of `topic.replication-factor`, and support for it will be removed in a future version.

autoRebalanceEnabled::
When `true`, topic partitions is automatically rebalanced between the members of a consumer group.
When `false`, each consumer is assigned a fixed set of partitions based on `spring.cloud.stream.instanceCount` and `spring.cloud.stream.instanceIndex`.
This requires both the `spring.cloud.stream.instanceCount` and `spring.cloud.stream.instanceIndex` properties to be set appropriately on each launched instance.
The value of the `spring.cloud.stream.instanceCount` property must typically be greater than 1 in this case.
+
Default: `true`.
ackEachRecord::
When `autoCommitOffset` is `true`, this setting dictates whether to commit the offset after each record is processed.
By default, offsets are committed after all records in the batch of records returned by `consumer.poll()` have been processed.
The number of records returned by a poll can be controlled with the `max.poll.records` Kafka property, which is set through the consumer `configuration` property.
Setting this to `true` may cause a degradation in performance, but doing so reduces the likelihood of redelivered records when a failure occurs.
Also, see the binder `requiredAcks` property, which also affects the performance of committing offsets.
+
Default: `false`.
autoCommitOffset::
Whether to autocommit offsets when a message has been processed.
If set to `false`, a header with the key `kafka_acknowledgment` of the type `org.springframework.kafka.support.Acknowledgment` header is present in the inbound message.
Applications may use this header for acknowledging messages.
See the examples section for details.
When this property is set to `false`, Kafka binder sets the ack mode to `org.springframework.kafka.listener.AbstractMessageListenerContainer.AckMode.MANUAL` and the application is responsible for acknowledging records.
Also see `ackEachRecord`.
+
Default: `true`.
autoCommitOnError::
Effective only if `autoCommitOffset` is set to `true`.
If set to `false`, it suppresses auto-commits for messages that result in errors and commits only for successful messages. It allows a stream to automatically replay from the last successfully processed message, in case of persistent failures.
If set to `true`, it always auto-commits (if auto-commit is enabled).
If not set (the default), it effectively has the same value as `enableDlq`, auto-committing erroneous messages if they are sent to a DLQ and not committing them otherwise.
+
Default: not set.
resetOffsets::
Whether to reset offsets on the consumer to the value provided by startOffset.
Must be false if a `KafkaRebalanceListener` is provided; see <<rebalance-listener>>.
+
Default: `false`.
startOffset::
The starting offset for new groups.
Allowed values: `earliest` and `latest`.
If the consumer group is set explicitly for the consumer 'binding' (through `spring.cloud.stream.bindings.<channelName>.group`), 'startOffset' is set to `earliest`. Otherwise, it is set to `latest` for the `anonymous` consumer group.
Also see `resetOffsets` (earlier in this list).
+
Default: null (equivalent to `earliest`).
enableDlq::
When set to true, it enables DLQ behavior for the consumer.
By default, messages that result in errors are forwarded to a topic named `error.<destination>.<group>`.
The DLQ topic name can be configurable by setting the `dlqName` property.
This provides an alternative option to the more common Kafka replay scenario for the case when the number of errors is relatively small and replaying the entire original topic may be too cumbersome.
See <<kafka-dlq-processing>> processing for more information.
Starting with version 2.0, messages sent to the DLQ topic are enhanced with the following headers: `x-original-topic`, `x-exception-message`, and `x-exception-stacktrace` as `byte[]`.
**Not allowed when `destinationIsPattern` is `true`.**
+
Default: `false`.
configuration::
Map with a key/value pair containing generic Kafka consumer properties.
In addition to having Kafka consumer properties, other configuration properties can be passed here.
For example some properties needed by the application such as `spring.cloud.stream.kafka.bindings.input.consumer.configuration.foo=bar`.
+
Default: Empty map.
dlqName::
The name of the DLQ topic to receive the error messages.
+
Default: null (If not specified, messages that result in errors are forwarded to a topic named `error.<destination>.<group>`).
dlqProducerProperties::
Using this, DLQ-specific producer properties can be set.
All the properties available through kafka producer properties can be set through this property.
+
Default: Default Kafka producer properties.
standardHeaders::
Indicates which standard headers are populated by the inbound channel adapter.
Allowed values: `none`, `id`, `timestamp`, or `both`.
Useful if using native deserialization and the first component to receive a message needs an `id` (such as an aggregator that is configured to use a JDBC message store).
+
Default: `none`
converterBeanName::
The name of a bean that implements `RecordMessageConverter`. Used in the inbound channel adapter to replace the default `MessagingMessageConverter`.
+
Default: `null`
idleEventInterval::
The interval, in milliseconds, between events indicating that no messages have recently been received.
Use an `ApplicationListener<ListenerContainerIdleEvent>` to receive these events.
See <<pause-resume>> for a usage example.
+
Default: `30000`
destinationIsPattern::
When true, the destination is treated as a regular expression `Pattern` used to match topic names by the broker.
When true, topics are not provisioned, and `enableDlq` is not allowed, because the binder does not know the topic names during the provisioning phase.
Note, the time taken to detect new topics that match the pattern is controlled by the consumer property `metadata.max.age.ms`, which (at the time of writing) defaults to 300,000ms (5 minutes).
This can be configured using the `configuration` property above.
+
Default: `false`
topic.properties::
A `Map` of Kafka topic properties used when provisioning new topics -- for example, `spring.cloud.stream.kafka.bindings.input.consumer.topic.properties.message.format.version=0.9.0.0`
+
Default: none.
topic.replicas-assignment::
A Map<Integer, List<Integer>> of replica assignments, with the key being the partition and the value being the assignments.
Used when provisioning new topics.
See the `NewTopic` Javadocs in the `kafka-clients` jar.
+
Default: none.
topic.replication-factor::
The replication factor to use when provisioning topics. Overrides the binder-wide setting.
Ignored if `replicas-assignments` is present.
+
Default: none (the binder-wide default of 1 is used).

[[kafka-producer-properties]]
==== Kafka Producer Properties

NOTE: To avoid repetition, Spring Cloud Stream supports setting values for all channels, in the format of `spring.cloud.stream.default.<property>=<value>`.


The following properties are available for Kafka producers only and
must be prefixed with `spring.cloud.stream.kafka.bindings.<channelName>.producer.`.

admin.configuration::
Since version 2.1.1, this property is deprecated in favor of `topic.properties`, and support for it will be removed in a future version.

admin.replicas-assignment::
Since version 2.1.1, this property is deprecated in favor of `topic.replicas-assignment`, and support for it will be removed in a future version.

admin.replication-factor::
Since version 2.1.1, this property is deprecated in favor of `topic.replication-factor`, and support for it will be removed in a future version.

bufferSize::
Upper limit, in bytes, of how much data the Kafka producer attempts to batch before sending.
+
Default: `16384`.
sync::
Whether the producer is synchronous.
+
Default: `false`.
batchTimeout::
How long the producer waits to allow more messages to accumulate in the same batch before sending the messages.
(Normally, the producer does not wait at all and simply sends all the messages that accumulated while the previous send was in progress.) A non-zero value may increase throughput at the expense of latency.
+
Default: `0`.
messageKeyExpression::
A SpEL expression evaluated against the outgoing message used to populate the key of the produced Kafka message -- for example, `headers['myKey']`.
With versions before 3.0, the payload could not be used unless native encoding was being used because, by the time this expression was evaluated, the payload was already in the form of a `byte[]`.
Now, the expression is evaluated before the payload is converted.
+
Default: `none`.
headerPatterns::
A comma-delimited list of simple patterns to match Spring messaging headers to be mapped to the Kafka `Headers` in the `ProducerRecord`.
Patterns can begin or end with the wildcard character (asterisk).
Patterns can be negated by prefixing with `!`.
Matching stops after the first match (positive or negative).
For example `!ask,as*` will pass `ash` but not `ask`.
`id` and `timestamp` are never mapped.
+
Default: `*` (all headers - except the `id` and `timestamp`)
configuration::
Map with a key/value pair containing generic Kafka producer properties.
+
Default: Empty map.
topic.properties::
A `Map` of Kafka topic properties used when provisioning new topics -- for example, `spring.cloud.stream.kafka.bindings.output.producer.topic.properties.message.format.version=0.9.0.0`
+
topic.replicas-assignment::
A Map<Integer, List<Integer>> of replica assignments, with the key being the partition and the value being the assignments.
Used when provisioning new topics.
See the `NewTopic` Javadocs in the `kafka-clients` jar.
+
Default: none.
topic.replication-factor::
The replication factor to use when provisioning topics. Overrides the binder-wide setting.
Ignored if `replicas-assignments` is present.
+
Default: none (the binder-wide default of 1 is used).
useTopicHeader::
Set to `true` to override the default binding destination (topic name) with the value of the `KafkaHeaders.TOPIC` message header in the outbound message.
If the header is not present, the default binding destination is used.
Default: `false`.
+
recordMetadataChannel::
The bean name of a `MessageChannel` to which successful send results should be sent; the bean must exist in the application context.
The message sent to the channel is the sent message (after conversion, if any) with an additional header `KafkaHeaders.RECORD_METADATA`.
The header contains a `RecordMetadata` object provided by the Kafka client; it includes the partition and offset where the record was written in the topic.

`ResultMetadata meta = sendResultMsg.getHeaders().get(KafkaHeaders.RECORD_METADATA, RecordMetadata.class)`

Failed sends go the producer error channel (if configured); see <<kafka-error-channels>>.
Default: null
+

NOTE: The Kafka binder uses the `partitionCount` setting of the producer as a hint to create a topic with the given partition count (in conjunction with the `minPartitionCount`, the maximum of the two being the value being used).
Exercise caution when configuring both `minPartitionCount` for a binder and `partitionCount` for an application, as the larger value is used.
If a topic already exists with a smaller partition count and `autoAddPartitions` is disabled (the default), the binder fails to start.
If a topic already exists with a smaller partition count and `autoAddPartitions` is enabled, new partitions are added.
If a topic already exists with a larger number of partitions than the maximum of (`minPartitionCount` or `partitionCount`), the existing partition count is used.

compression::
Set the `compression.type` producer property.
Supported values are `none`, `gzip`, `snappy` and `lz4`.
If you override the `kafka-clients` jar to 2.1.0 (or later), as discussed in the https://docs.spring.io/spring-kafka/docs/2.2.x/reference/html/deps-for-21x.html[Spring for Apache Kafka documentation], and wish to use `zstd` compression, use `spring.cloud.stream.kafka.bindings.<binding-name>.producer.configuration.compression.type=zstd`.
+
Default: `none`.

==== Usage examples

In this section, we show the use of the preceding properties for specific scenarios.

===== Example: Setting `autoCommitOffset` to `false` and Relying on Manual Acking

This example illustrates how one may manually acknowledge offsets in a consumer application.

This example requires that `spring.cloud.stream.kafka.bindings.input.consumer.autoCommitOffset` be set to `false`.
Use the corresponding input channel name for your example.

[source]
----
@SpringBootApplication
@EnableBinding(Sink.class)
public class ManuallyAcknowdledgingConsumer {

 public static void main(String[] args) {
     SpringApplication.run(ManuallyAcknowdledgingConsumer.class, args);
 }

 @StreamListener(Sink.INPUT)
 public void process(Message<?> message) {
     Acknowledgment acknowledgment = message.getHeaders().get(KafkaHeaders.ACKNOWLEDGMENT, Acknowledgment.class);
     if (acknowledgment != null) {
         System.out.println("Acknowledgment provided");
         acknowledgment.acknowledge();
     }
 }
}
----

===== Example: Security Configuration

Apache Kafka 0.9 supports secure connections between client and brokers.
To take advantage of this feature, follow the guidelines in the https://kafka.apache.org/090/documentation.html#security_configclients[Apache Kafka Documentation] as well as the Kafka 0.9 https://docs.confluent.io/2.0.0/kafka/security.html[security guidelines from the Confluent documentation].
Use the `spring.cloud.stream.kafka.binder.configuration` option to set security properties for all clients created by the binder.

For example, to set `security.protocol` to `SASL_SSL`, set the following property:

[source]
----
spring.cloud.stream.kafka.binder.configuration.security.protocol=SASL_SSL
----

All the other security properties can be set in a similar manner.

When using Kerberos, follow the instructions in the https://kafka.apache.org/090/documentation.html#security_sasl_clientconfig[reference documentation] for creating and referencing the JAAS configuration.

Spring Cloud Stream supports passing JAAS configuration information to the application by using a JAAS configuration file and using Spring Boot properties.

====== Using JAAS Configuration Files

The JAAS and (optionally) krb5 file locations can be set for Spring Cloud Stream applications by using system properties.
The following example shows how to launch a Spring Cloud Stream application with SASL and Kerberos by using a JAAS configuration file:

[source,bash]
----
 java -Djava.security.auth.login.config=/path.to/kafka_client_jaas.conf -jar log.jar \
   --spring.cloud.stream.kafka.binder.brokers=secure.server:9092 \
   --spring.cloud.stream.bindings.input.destination=stream.ticktock \
   --spring.cloud.stream.kafka.binder.configuration.security.protocol=SASL_PLAINTEXT
----

====== Using Spring Boot Properties

As an alternative to having a JAAS configuration file, Spring Cloud Stream provides a mechanism for setting up the JAAS configuration for Spring Cloud Stream applications by using Spring Boot properties.

The following properties can be used to configure the login context of the Kafka client:

spring.cloud.stream.kafka.binder.jaas.loginModule::
The login module name. Not necessary to be set in normal cases.
+
Default: `com.sun.security.auth.module.Krb5LoginModule`.
spring.cloud.stream.kafka.binder.jaas.controlFlag::
The control flag of the login module.
+
Default: `required`.
spring.cloud.stream.kafka.binder.jaas.options::
Map with a key/value pair containing the login module options.
+
Default: Empty map.

The following example shows how to launch a Spring Cloud Stream application with SASL and Kerberos by using Spring Boot configuration properties:

[source,bash]
----
 java --spring.cloud.stream.kafka.binder.brokers=secure.server:9092 \
   --spring.cloud.stream.bindings.input.destination=stream.ticktock \
   --spring.cloud.stream.kafka.binder.autoCreateTopics=false \
   --spring.cloud.stream.kafka.binder.configuration.security.protocol=SASL_PLAINTEXT \
   --spring.cloud.stream.kafka.binder.jaas.options.useKeyTab=true \
   --spring.cloud.stream.kafka.binder.jaas.options.storeKey=true \
   --spring.cloud.stream.kafka.binder.jaas.options.keyTab=/etc/security/keytabs/kafka_client.keytab \
   --spring.cloud.stream.kafka.binder.jaas.options.principal=kafka-client-1@EXAMPLE.COM
----

The preceding example represents the equivalent of the following JAAS file:

[source]
----
KafkaClient {
    com.sun.security.auth.module.Krb5LoginModule required
    useKeyTab=true
    storeKey=true
    keyTab="/etc/security/keytabs/kafka_client.keytab"
    principal="kafka-client-1@EXAMPLE.COM";
};
----

If the topics required already exist on the broker or will be created by an administrator, autocreation can be turned off and only client JAAS properties need to be sent.

NOTE: Do not mix JAAS configuration files and Spring Boot properties in the same application.
If the `-Djava.security.auth.login.config` system property is already present, Spring Cloud Stream ignores the Spring Boot properties.

NOTE: Be careful when using the `autoCreateTopics` and `autoAddPartitions` with Kerberos.
Usually, applications may use principals that do not have administrative rights in Kafka and Zookeeper.
Consequently, relying on Spring Cloud Stream to create/modify topics may fail.
In secure environments, we strongly recommend creating topics and managing ACLs administratively by using Kafka tooling.

[[pause-resume]]
===== Example: Pausing and Resuming the Consumer

If you wish to suspend consumption but not cause a partition rebalance, you can pause and resume the consumer.
This is facilitated by adding the `Consumer` as a parameter to your `@StreamListener`.
To resume, you need an `ApplicationListener` for `ListenerContainerIdleEvent` instances.
The frequency at which events are published is controlled by the `idleEventInterval` property.
Since the consumer is not thread-safe, you must call these methods on the calling thread.

The following simple application shows how to pause and resume:

[source, java]
----
@SpringBootApplication
@EnableBinding(Sink.class)
public class Application {

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}

	@StreamListener(Sink.INPUT)
	public void in(String in, @Header(KafkaHeaders.CONSUMER) Consumer<?, ?> consumer) {
		System.out.println(in);
		consumer.pause(Collections.singleton(new TopicPartition("myTopic", 0)));
	}

	@Bean
	public ApplicationListener<ListenerContainerIdleEvent> idleListener() {
		return event -> {
			System.out.println(event);
			if (event.getConsumer().paused().size() > 0) {
				event.getConsumer().resume(event.getConsumer().paused());
			}
		};
	}

}
----

[[kafka-transactional-binder]]
=== Transactional Binder

Enable transactions by setting `spring.cloud.stream.kafka.binder.transaction.transactionIdPrefix` to a non-empty value, e.g. `tx-`.
When used in a processor application, the consumer starts the transaction; any records sent on the consumer thread participate in the same transaction.
When the listener exits normally, the listener container will send the offset to the transaction and commit it.
A common producer factory is used for all producer bindings configured using `spring.cloud.stream.kafka.binder.transaction.producer.*` properties; individual binding Kafka producer properties are ignored.

If you wish to use transactions in a source application, or from some arbitrary thread for producer-only transaction (e.g. `@Scheduled` method), you must get a reference to the transactional producer factory and define a `KafkaTransactionManager` bean using it.

====
[source, java]
----
@Bean
public PlatformTransactionManager transactionManager(BinderFactory binders) {
    ProducerFactory<byte[], byte[]> pf = ((KafkaMessageChannelBinder) binders.getBinder(null,
            MessageChannel.class)).getTransactionalProducerFactory();
    return new KafkaTransactionManager<>(pf);
}
----
====

Notice that we get a reference to the binder using the `BinderFactory`; use `null` in the first argument when there is only one binder configured.
If more than one binder is configured, use the binder name to get the reference.
Once we have a reference to the binder, we can obtain a reference to the `ProducerFactory` and create a transaction manager.

Then you would use normal Spring transaction support, e.g. `TransactionTemplate` or `@Transactional`, for example:

====
[source, java]
----
public static class Sender {

    @Transactional
    public void doInTransaction(MessageChannel output, List<String> stuffToSend) {
        stuffToSend.forEach(stuff -> output.send(new GenericMessage<>(stuff)));
    }

}
----
====

If you wish to synchronize producer-only transactions with those from some other transaction manager, use a `ChainedTransactionManager`.

[[kafka-error-channels]]
=== Error Channels

Starting with version 1.3, the binder unconditionally sends exceptions to an error channel for each consumer destination and can also be configured to send async producer send failures to an error channel.
See <<spring-cloud-stream-overview-error-handling>> for more information.

The payload of the `ErrorMessage` for a send failure is a `KafkaSendFailureException` with properties:

* `failedMessage`: The Spring Messaging `Message<?>` that failed to be sent.
* `record`: The raw `ProducerRecord` that was created from the `failedMessage`

There is no automatic handling of producer exceptions (such as sending to a <<kafka-dlq-processing, Dead-Letter queue>>).
You can consume these exceptions with your own Spring Integration flow.

[[kafka-metrics]]
=== Kafka Metrics

Kafka binder module exposes the following metrics:

`spring.cloud.stream.binder.kafka.offset`: This metric indicates how many messages have not been yet consumed from a given binder's topic by a given consumer group.
The metrics provided are based on the Mircometer metrics library. The metric contains the consumer group information, topic and the actual lag in committed offset from the latest offset on the topic.
This metric is particularly useful for providing auto-scaling feedback to a PaaS platform.

[[kafka-tombstones]]
=== Tombstone Records (null record values)

When using compacted topics, a record with a `null` value (also called a tombstone record) represents the deletion of a key.
To receive such messages in a `@StreamListener` method, the parameter must be marked as not required to receive a `null` value argument.

====
[source, java]
----
@StreamListener(Sink.INPUT)
public void in(@Header(KafkaHeaders.RECEIVED_MESSAGE_KEY) byte[] key,
               @Payload(required = false) Customer customer) {
    // customer is null if a tombstone record
    ...
}
----
====

[[rebalance-listener]]
=== Using a KafkaRebalanceListener

Applications may wish to seek topics/partitions to arbitrary offsets when the partitions are initially assigned, or perform other operations on the consumer.
Starting with version 2.1, if you provide a single `KafkaRebalanceListener` bean in the application context, it will be wired into all Kafka consumer bindings.

====
[source, java]
----
public interface KafkaBindingRebalanceListener {

	/**
	 * Invoked by the container before any pending offsets are committed.
	 * @param bindingName the name of the binding.
	 * @param consumer the consumer.
	 * @param partitions the partitions.
	 */
	default void onPartitionsRevokedBeforeCommit(String bindingName, Consumer<?, ?> consumer,
			Collection<TopicPartition> partitions) {

	}

	/**
	 * Invoked by the container after any pending offsets are committed.
	 * @param bindingName the name of the binding.
	 * @param consumer the consumer.
	 * @param partitions the partitions.
	 */
	default void onPartitionsRevokedAfterCommit(String bindingName, Consumer<?, ?> consumer, Collection<TopicPartition> partitions) {

	}

	/**
	 * Invoked when partitions are initially assigned or after a rebalance.
	 * Applications might only want to perform seek operations on an initial assignment.
	 * @param bindingName the name of the binding.
	 * @param consumer the consumer.
	 * @param partitions the partitions.
	 * @param initial true if this is the initial assignment.
	 */
	default void onPartitionsAssigned(String bindingName, Consumer<?, ?> consumer, Collection<TopicPartition> partitions,
			boolean initial) {

	}

}
----
====

You cannot set the `resetOffsets` consumer property to `true` when you provide a rebalance listener.

= Appendices
[appendix]
[[building]]
== Building

:jdkversion: 1.7

=== Basic Compile and Test

To build the source you will need to install JDK {jdkversion}.

The build uses the Maven wrapper so you don't have to install a specific
version of Maven. To enable the tests, you should have Kafka server 0.9 or above running
before building. See below for more information on running the servers.

The main build command is

----
$ ./mvnw clean install
----

You can also add '-DskipTests' if you like, to avoid running the tests.

NOTE: You can also install Maven (>=3.3.3) yourself and run the `mvn` command
in place of `./mvnw` in the examples below. If you do that you also
might need to add `-P spring` if your local Maven settings do not
contain repository declarations for spring pre-release artifacts.

NOTE: Be aware that you might need to increase the amount of memory
available to Maven by setting a `MAVEN_OPTS` environment variable with
a value like `-Xmx512m -XX:MaxPermSize=128m`. We try to cover this in
the `.mvn` configuration, so if you find you have to do it to make a
build succeed, please raise a ticket to get the settings added to
source control.


The projects that require middleware generally include a
`docker-compose.yml`, so consider using
https://compose.docker.io/[Docker Compose] to run the middeware servers
in Docker containers.

=== Documentation

There is a "full" profile that will generate documentation.

=== Working with the code
If you don't have an IDE preference we would recommend that you use
https://www.springsource.com/developer/sts[Spring Tools Suite] or
https://eclipse.org[Eclipse] when working with the code. We use the
https://eclipse.org/m2e/[m2eclipe] eclipse plugin for maven support. Other IDEs and tools
should also work without issue.

==== Importing into eclipse with m2eclipse
We recommend the https://eclipse.org/m2e/[m2eclipe] eclipse plugin when working with
eclipse. If you don't already have m2eclipse installed it is available from the "eclipse
marketplace".

Unfortunately m2e does not yet support Maven 3.3, so once the projects
are imported into Eclipse you will also need to tell m2eclipse to use
the `.settings.xml` file for the projects.  If you do not do this you
may see many different errors related to the POMs in the
projects.  Open your Eclipse preferences, expand the Maven
preferences, and select User Settings.  In the User Settings field
click Browse and navigate to the Spring Cloud project you imported
selecting the `.settings.xml` file in that project.  Click Apply and
then OK to save the preference changes.

NOTE: Alternatively you can copy the repository settings from https://github.com/spring-cloud/spring-cloud-build/blob/master/.settings.xml[`.settings.xml`] into your own `~/.m2/settings.xml`.

==== Importing into eclipse without m2eclipse
If you prefer not to use m2eclipse you can generate eclipse project metadata using the
following command:

[indent=0]
----
	$ ./mvnw eclipse:eclipse
----

The generated eclipse projects can be imported by selecting `import existing projects`
from the `file` menu.
[[contributing]
== Contributing

Spring Cloud is released under the non-restrictive Apache 2.0 license,
and follows a very standard Github development process, using Github
tracker for issues and merging pull requests into master. If you want
to contribute even something trivial please do not hesitate, but
follow the guidelines below.

=== Sign the Contributor License Agreement
Before we accept a non-trivial patch or pull request we will need you to sign the
https://support.springsource.com/spring_committer_signup[contributor's agreement].
Signing the contributor's agreement does not grant anyone commit rights to the main
repository, but it does mean that we can accept your contributions, and you will get an
author credit if we do.  Active contributors might be asked to join the core team, and
given the ability to merge pull requests.

=== Code Conventions and Housekeeping
None of these is essential for a pull request, but they will all help.  They can also be
added after the original pull request but before a merge.

* Use the Spring Framework code format conventions. If you use Eclipse
  you can import formatter settings using the
  `eclipse-code-formatter.xml` file from the
  https://github.com/spring-cloud/build/tree/master/eclipse-coding-conventions.xml[Spring
  Cloud Build] project. If using IntelliJ, you can use the
  https://plugins.jetbrains.com/plugin/6546[Eclipse Code Formatter
  Plugin] to import the same file.
* Make sure all new `.java` files to have a simple Javadoc class comment with at least an
  `@author` tag identifying you, and preferably at least a paragraph on what the class is
  for.
* Add the ASF license header comment to all new `.java` files (copy from existing files
  in the project)
* Add yourself as an `@author` to the .java files that you modify substantially (more
  than cosmetic changes).
* Add some Javadocs and, if you change the namespace, some XSD doc elements.
* A few unit tests would help a lot as well -- someone has to do it.
* If no-one else is using your branch, please rebase it against the current master (or
  other target branch in the main project).
* When writing a commit message please follow https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html[these conventions],
  if you are fixing an existing issue please add `Fixes gh-XXXX` at the end of the commit
  message (where XXXX is the issue number).

// ======================================================================================