Apache Kafka Connector

This connector provides access to event streams served by Apache Kafka.

Flink provides special Kafka Connectors for reading and writing data from/to Kafka topics. The Flink Kafka Consumer integrates with Flink’s checkpointing mechanism to provide exactly-once processing semantics. To achieve that, Flink does not purely rely on Kafka’s consumer group offset tracking, but tracks and checkpoints these offsets internally as well.

Please pick a package (maven artifact id) and class name for your use-case and environment. For most users, the FlinkKafkaConsumer08 (part of flink-connector-kafka) is appropriate.

Maven Dependency Supported since Consumer and
Producer Class name
Kafka version Notes
flink-connector-kafka-0.8_2.10 1.0.0 FlinkKafkaConsumer08
FlinkKafkaProducer08
0.8.x Uses the SimpleConsumer API of Kafka internally. Offsets are committed to ZK by Flink.
flink-connector-kafka-0.9_2.10 1.0.0 FlinkKafkaConsumer09
FlinkKafkaProducer09
0.9.x Uses the new Consumer API Kafka.
flink-connector-kafka-0.10_2.10 1.2.0 FlinkKafkaConsumer010
FlinkKafkaProducer010
0.10.x This connector supports Kafka messages with timestamps both for producing and consuming.

Then, import the connector in your maven project:

<dependency>
  <groupId>org.apache.flink</groupId>
  <artifactId>flink-connector-kafka-0.8_2.10</artifactId>
  <version>1.3.0</version>
</dependency>

Note that the streaming connectors are currently not part of the binary distribution. See how to link with them for cluster execution here.

Installing Apache Kafka

  • Follow the instructions from Kafka’s quickstart to download the code and launch a server (launching a Zookeeper and a Kafka server is required every time before starting the application).
  • If the Kafka and Zookeeper servers are running on a remote machine, then the advertised.host.name setting in the config/server.properties file must be set to the machine’s IP address.

Kafka Consumer

Flink’s Kafka consumer is called FlinkKafkaConsumer08 (or 09 for Kafka 0.9.0.x versions, etc.). It provides access to one or more Kafka topics.

The constructor accepts the following arguments:

  1. The topic name / list of topic names
  2. A DeserializationSchema / KeyedDeserializationSchema for deserializing the data from Kafka
  3. Properties for the Kafka consumer. The following properties are required:
    • “bootstrap.servers” (comma separated list of Kafka brokers)
    • “zookeeper.connect” (comma separated list of Zookeeper servers) (only required for Kafka 0.8)
    • “group.id” the id of the consumer group

Example:

Properties properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");
// only required for Kafka 0.8
properties.setProperty("zookeeper.connect", "localhost:2181");
properties.setProperty("group.id", "test");
DataStream<String> stream = env
	.addSource(new FlinkKafkaConsumer08<>("topic", new SimpleStringSchema(), properties));
val properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");
// only required for Kafka 0.8
properties.setProperty("zookeeper.connect", "localhost:2181");
properties.setProperty("group.id", "test");
stream = env
    .addSource(new FlinkKafkaConsumer08[String]("topic", new SimpleStringSchema(), properties))
    .print

The DeserializationSchema

The Flink Kafka Consumer needs to know how to turn the binary data in Kafka into Java/Scala objects. The DeserializationSchema allows users to specify such a schema. The T deserialize(byte[] message) method gets called for each Kafka message, passing the value from Kafka.

It is usually helpful to start from the AbstractDeserializationSchema, which takes care of describing the produced Java/Scala type to Flink’s type system. Users that implement a vanilla DeserializationSchema need to implement the getProducedType(...) method themselves.

For accessing both the key and value of the Kafka message, the KeyedDeserializationSchema has the following deserialize method ` T deserialize(byte[] messageKey, byte[] message, String topic, int partition, long offset)`.

For convenience, Flink provides the following schemas:

  1. TypeInformationSerializationSchema (and TypeInformationKeyValueSerializationSchema) which creates a schema based on a Flink’s TypeInformation. This is useful if the data is both written and read by Flink. This schema is a performant Flink-specific alternative to other generic serialization approaches.

  2. JsonDeserializationSchema (and JSONKeyValueDeserializationSchema) which turns the serialized JSON into an ObjectNode object, from which fields can be accessed using objectNode.get(“field”).as(Int/String/…)(). The KeyValue objectNode contains a “key” and “value” field which contain all fields, as well as an optional “metadata” field that exposes the offset/partition/topic for this message.

When encountering a corrupted message that cannot be deserialized for any reason, there are two options - either throwing an exception from the deserialize(...) method which will cause the job to fail and be restarted, or returning null to allow the Flink Kafka consumer to silently skip the corrupted message. Note that due to the consumer’s fault tolerance (see below sections for more details), failing the job on the corrupted message will let the consumer attempt to deserialize the message again. Therefore, if deserialization still fails, the consumer will fall into a non-stop restart and fail loop on that corrupted message.

Kafka Consumers Start Position Configuration

The Flink Kafka Consumer allows configuring how the start position for Kafka partitions are determined.

Example:

final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

FlinkKafkaConsumer08<String> myConsumer = new FlinkKafkaConsumer08<>(...);
myConsumer.setStartFromEarliest();     // start from the earliest record possible
myConsumer.setStartFromLatest();       // start from the latest record
myConsumer.setStartFromGroupOffsets(); // the default behaviour

DataStream<String> stream = env.addSource(myConsumer);
...
val env = StreamExecutionEnvironment.getExecutionEnvironment()

val myConsumer = new FlinkKafkaConsumer08[String](...)
myConsumer.setStartFromEarliest()      // start from the earliest record possible
myConsumer.setStartFromLatest()        // start from the latest record
myConsumer.setStartFromGroupOffsets()  // the default behaviour

val stream = env.addSource(myConsumer)
...

All versions of the Flink Kafka Consumer have the above explicit configuration methods for start position.

  • setStartFromGroupOffsets (default behaviour): Start reading partitions from the consumer group’s (group.id setting in the consumer properties) committed offsets in Kafka brokers (or Zookeeper for Kafka 0.8). If offsets could not be found for a partition, the auto.offset.reset setting in the properties will be used.
  • setStartFromEarliest() / setStartFromLatest(): Start from the earliest / latest record. Under these modes, committed offsets in Kafka will be ignored and not used as starting positions.

You can also specify the exact offsets the consumer should start from for each partition:

Map<KafkaTopicPartition, Long> specificStartOffsets = new HashMap<>();
specificStartOffsets.put(new KafkaTopicPartition("myTopic", 0), 23L);
specificStartOffsets.put(new KafkaTopicPartition("myTopic", 1), 31L);
specificStartOffsets.put(new KafkaTopicPartition("myTopic", 2), 43L);

myConsumer.setStartFromSpecificOffsets(specificStartOffsets);
val specificStartOffsets = new java.util.HashMap[KafkaTopicPartition, java.lang.Long]()
specificStartOffsets.put(new KafkaTopicPartition("myTopic", 0), 23L)
specificStartOffsets.put(new KafkaTopicPartition("myTopic", 1), 31L)
specificStartOffsets.put(new KafkaTopicPartition("myTopic", 2), 43L)

myConsumer.setStartFromSpecificOffsets(specificStartOffsets)

The above example configures the consumer to start from the specified offsets for partitions 0, 1, and 2 of topic myTopic. The offset values should be the next record that the consumer should read for each partition. Note that if the consumer needs to read a partition which does not have a specified offset within the provided offsets map, it will fallback to the default group offsets behaviour (i.e. setStartFromGroupOffsets()) for that particular partition.

Note that these start position configuration methods do not affect the start position when the job is automatically restored from a failure or manually restored using a savepoint. On restore, the start position of each Kafka partition is determined by the offsets stored in the savepoint or checkpoint (please see the next section for information about checkpointing to enable fault tolerance for the consumer).

Kafka Consumers and Fault Tolerance

With Flink’s checkpointing enabled, the Flink Kafka Consumer will consume records from a topic and periodically checkpoint all its Kafka offsets, together with the state of other operations, in a consistent manner. In case of a job failure, Flink will restore the streaming program to the state of the latest checkpoint and re-consume the records from Kafka, starting from the offsets that were stored in the checkpoint.

The interval of drawing checkpoints therefore defines how much the program may have to go back at most, in case of a failure.

To use fault tolerant Kafka Consumers, checkpointing of the topology needs to be enabled at the execution environment:

final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
env.enableCheckpointing(5000); // checkpoint every 5000 msecs
val env = StreamExecutionEnvironment.getExecutionEnvironment()
env.enableCheckpointing(5000) // checkpoint every 5000 msecs

Also note that Flink can only restart the topology if enough processing slots are available to restart the topology. So if the topology fails due to loss of a TaskManager, there must still be enough slots available afterwards. Flink on YARN supports automatic restart of lost YARN containers.

If checkpointing is not enabled, the Kafka consumer will periodically commit the offsets to Zookeeper.

Kafka Consumers Offset Committing Behaviour Configuration

The Flink Kafka Consumer allows configuring the behaviour of how offsets are committed back to Kafka brokers (or Zookeeper in 0.8). Note that the Flink Kafka Consumer does not rely on the committed offsets for fault tolerance guarantees. The committed offsets are only a means to expose the consumer’s progress for monitoring purposes.

The way to configure offset commit behaviour is different, depending on whether or not checkpointing is enabled for the job.

  • Checkpointing disabled: if checkpointing is disabled, the Flink Kafka Consumer relies on the automatic periodic offset committing capability of the internally used Kafka clients. Therefore, to disable or enable offset committing, simply set the enable.auto.commit (or auto.commit.enable for Kafka 0.8) / auto.commit.interval.ms keys to appropriate values in the provided Properties configuration.

  • Checkpointing enabled: if checkpointing is enabled, the Flink Kafka Consumer will commit the offsets stored in the checkpointed states when the checkpoints are completed. This ensures that the committed offsets in Kafka brokers is consistent with the offsets in the checkpointed states. Users can choose to disable or enable offset committing by calling the setCommitOffsetsOnCheckpoints(boolean) method on the consumer (by default, the behaviour is true). Note that in this scenario, the automatic periodic offset committing settings in Properties is completely ignored.

Kafka Consumers and Timestamp Extraction/Watermark Emission

In many scenarios, the timestamp of a record is embedded (explicitly or implicitly) in the record itself. In addition, the user may want to emit watermarks either periodically, or in an irregular fashion, e.g. based on special records in the Kafka stream that contain the current event-time watermark. For these cases, the Flink Kafka Consumer allows the specification of an AssignerWithPeriodicWatermarks or an AssignerWithPunctuatedWatermarks.

You can specify your custom timestamp extractor/watermark emitter as described here, or use one from the predefined ones. After doing so, you can pass it to your consumer in the following way:

Properties properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");
// only required for Kafka 0.8
properties.setProperty("zookeeper.connect", "localhost:2181");
properties.setProperty("group.id", "test");

FlinkKafkaConsumer08<String> myConsumer =
    new FlinkKafkaConsumer08<>("topic", new SimpleStringSchema(), properties);
myConsumer.assignTimestampsAndWatermarks(new CustomWatermarkEmitter());

DataStream<String> stream = env
	.addSource(myConsumer)
	.print();
val properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");
// only required for Kafka 0.8
properties.setProperty("zookeeper.connect", "localhost:2181");
properties.setProperty("group.id", "test");

val myConsumer = new FlinkKafkaConsumer08[String]("topic", new SimpleStringSchema(), properties);
myConsumer.assignTimestampsAndWatermarks(new CustomWatermarkEmitter());
stream = env
    .addSource(myConsumer)
    .print

Internally, an instance of the assigner is executed per Kafka partition. When such an assigner is specified, for each record read from Kafka, the extractTimestamp(T element, long previousElementTimestamp) is called to assign a timestamp to the record and the Watermark getCurrentWatermark() (for periodic) or the Watermark checkAndGetNextWatermark(T lastElement, long extractedTimestamp) (for punctuated) is called to determine if a new watermark should be emitted and with which timestamp.

Kafka Producer

Flink’s Kafka Producer is called FlinkKafkaProducer08 (or 09 for Kafka 0.9.0.x versions, etc.). It allows writing a stream of records to one or more Kafka topics.

Example:

DataStream<String> stream = ...;

FlinkKafkaProducer08<String> myProducer = new FlinkKafkaProducer08<String>(
        "localhost:9092",            // broker list
        "my-topic",                  // target topic
        new SimpleStringSchema());   // serialization schema

// the following is necessary for at-least-once delivery guarantee
myProducer.setLogFailuresOnly(false);   // "false" by default
myProducer.setFlushOnCheckpoint(true);  // "false" by default

stream.addSink(myProducer);
DataStream<String> stream = ...;

FlinkKafkaProducer010Configuration myProducerConfig = FlinkKafkaProducer010.writeToKafkaWithTimestamps(
        stream,                     // input stream
        "my-topic",                 // target topic
        new SimpleStringSchema(),   // serialization schema
        properties);                // custom configuration for KafkaProducer (including broker list)

// the following is necessary for at-least-once delivery guarantee
myProducerConfig.setLogFailuresOnly(false);   // "false" by default
myProducerConfig.setFlushOnCheckpoint(true);  // "false" by default
val stream: DataStream[String] = ...

val myProducer = new FlinkKafkaProducer08[String](
        "localhost:9092",         // broker list
        "my-topic",               // target topic
        new SimpleStringSchema)   // serialization schema

// the following is necessary for at-least-once delivery guarantee
myProducer.setLogFailuresOnly(false)   // "false" by default
myProducer.setFlushOnCheckpoint(true)  // "false" by default

stream.addSink(myProducer)
val stream: DataStream[String] = ...

val myProducerConfig = FlinkKafkaProducer010.writeToKafkaWithTimestamps(
        stream,                   // input stream
        "my-topic",               // target topic
        new SimpleStringSchema,   // serialization schema
        properties)               // custom configuration for KafkaProducer (including broker list)

// the following is necessary for at-least-once delivery guarantee
myProducerConfig.setLogFailuresOnly(false)   // "false" by default
myProducerConfig.setFlushOnCheckpoint(true)  // "false" by default

The above examples demonstrate the basic usage of creating a Flink Kafka Producer to write streams to a single Kafka target topic. For more advanced usages, there are other constructor variants that allow providing the following:

  • Providing custom properties: The producer allows providing a custom properties configuration for the internal KafkaProducer. Please refer to the Apache Kafka documentation for details on how to configure Kafka Producers.
  • Custom partitioner: To assign records to specific partitions, you can provide an implementation of a FlinkKafkaPartitioner to the constructor. This partitioner will be called for each record in the stream to determine which exact partition of the target topic the record should be sent to.
  • Advanced serialization schema: Similar to the consumer, the producer also allows using an advanced serialization schema called KeyedSerializationSchema, which allows serializing the key and value separately. It also allows to override the target topic, so that one producer instance can send data to multiple topics.

Kafka Producers and Fault Tolerance

With Flink’s checkpointing enabled, the Flink Kafka Producer can provide at-least-once delivery guarantees.

Besides enabling Flink’s checkpointing, you should also configure the setter methods setLogFailuresOnly(boolean) and setFlushOnCheckpoint(boolean) appropriately, as shown in the above examples in the previous section:

  • setLogFailuresOnly(boolean): enabling this will let the producer log failures only instead of catching and rethrowing them. This essentially accounts the record to have succeeded, even if it was never written to the target Kafka topic. This must be disabled for at-least-once.
  • setFlushOnCheckpoint(boolean): with this enabled, Flink’s checkpoints will wait for any on-the-fly records at the time of the checkpoint to be acknowledged by Kafka before succeeding the checkpoint. This ensures that all records before the checkpoint have been written to Kafka. This must be enabled for at-least-once.

Note: By default, the number of retries is set to “0”. This means that when setLogFailuresOnly is set to false, the producer fails immediately on errors, including leader changes. The value is set to “0” by default to avoid duplicate messages in the target topic that are caused by retries. For most production environments with frequent broker changes, we recommend setting the number of retries to a higher value.

Note: There is currently no transactional producer for Kafka, so Flink can not guarantee exactly-once delivery into a Kafka topic.

Since Apache Kafka 0.10+, Kafka’s messages can carry timestamps, indicating the time the event has occurred (see “event time” in Apache Flink) or the time when the message has been written to the Kafka broker.

The FlinkKafkaConsumer010 will emit records with the timestamp attached, if the time characteristic in Flink is set to TimeCharacteristic.EventTime (StreamExecutionEnvironment.setStreamTimeCharacteristic(TimeCharacteristic.EventTime)).

The Kafka consumer does not emit watermarks. To emit watermarks, the same mechanisms as described above in “Kafka Consumers and Timestamp Extraction/Watermark Emission” using the assignTimestampsAndWatermarks method are applicable.

There is no need to define a timestamp extractor when using the timestamps from Kafka. The previousElementTimestamp argument of the extractTimestamp() method contains the timestamp carried by the Kafka message.

A timestamp extractor for a Kafka consumer would look like this:

public long extractTimestamp(Long element, long previousElementTimestamp) {
    return previousElementTimestamp;
}

The FlinkKafkaProducer010 only emits the record timestamp, if setWriteTimestampToKafka(true) is set.

FlinkKafkaProducer010.FlinkKafkaProducer010Configuration config = FlinkKafkaProducer010.writeToKafkaWithTimestamps(streamWithTimestamps, topic, new SimpleStringSchema(), standardProps);
config.setWriteTimestampToKafka(true);

Kafka Connector metrics

Flink’s Kafka connectors provide some metrics through Flink’s metrics system to analyze the behavior of the connector. The producers export Kafka’s internal metrics through Flink’s metric system for all supported versions. The consumers export all metrics starting from Kafka version 0.9. The Kafka documentation lists all exported metrics in its documentation.

In addition to these metrics, all consumers expose the current-offsets and committed-offsets for each topic partition. The current-offsets refers to the current offset in the partition. This refers to the offset of the last element that we retrieved and emitted successfully. The committed-offsets is the last committed offset.

The Kafka Consumers in Flink commit the offsets back to Zookeeper (Kafka 0.8) or the Kafka brokers (Kafka 0.9+). If checkpointing is disabled, offsets are committed periodically. With checkpointing, the commit happens once all operators in the streaming topology have confirmed that they’ve created a checkpoint of their state. This provides users with at-least-once semantics for the offsets committed to Zookeer or the broker. For offsets checkpointed to Flink, the system provides exactly once guarantees.

The offsets committed to ZK or the broker can also be used to track the read progress of the Kafka consumer. The difference between the committed offset and the most recent offset in each partition is called the consumer lag. If the Flink topology is consuming the data slower from the topic than new data is added, the lag will increase and the consumer will fall behind. For large production deployments we recommend monitoring that metric to avoid increasing latency.

Enabling Kerberos Authentication (for versions 0.9+ and above only)

Flink provides first-class support through the Kafka connector to authenticate to a Kafka installation configured for Kerberos. Simply configure Flink in flink-conf.yaml to enable Kerberos authentication for Kafka like so:

  1. Configure Kerberos credentials by setting the following -
    • security.kerberos.login.use-ticket-cache: By default, this is true and Flink will attempt to use Kerberos credentials in ticket caches managed by kinit. Note that when using the Kafka connector in Flink jobs deployed on YARN, Kerberos authorization using ticket caches will not work. This is also the case when deploying using Mesos, as authorization using ticket cache is not supported for Mesos deployments.
    • security.kerberos.login.keytab and security.kerberos.login.principal: To use Kerberos keytabs instead, set values for both of these properties.
  2. Append KafkaClient to security.kerberos.login.contexts: This tells Flink to provide the configured Kerberos credentials to the Kafka login context to be used for Kafka authentication.

Once Kerberos-based Flink security is enabled, you can authenticate to Kafka with either the Flink Kafka Consumer or Producer by simply including the following two settings in the provided properties configuration that is passed to the internal Kafka client:

  • Set security.protocol to SASL_PLAINTEXT (default NONE): The protocol used to communicate to Kafka brokers. When using standalone Flink deployment, you can also use SASL_SSL; please see how to configure the Kafka client for SSL here.
  • Set sasl.kerberos.service.name to kafka (default kafka): The value for this should match the sasl.kerberos.service.name used for Kafka broker configurations. A mismatch in service name between client and server configuration will cause the authentication to fail.

For more information on Flink configuration for Kerberos security, please see here. You can also find here further details on how Flink internally setups Kerberos-based security.