This documentation is for an out-of-date version of Apache Flink. We recommend you use the latest stable version.

Apache Kafka Connector #

Flink provides an Apache Kafka connector for reading data from and writing data to Kafka topics with exactly-once guarantees.

Dependency #

Apache Flink ships with a universal Kafka connector which attempts to track the latest version of the Kafka client. The version of the client it uses may change between Flink releases. Modern Kafka clients are backwards compatible with broker versions 0.10.0 or later. For details on Kafka compatibility, please refer to the official Kafka documentation.


if you are using Kafka source, flink-connector-base is also required as dependency:


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

Kafka Source #

This part describes the Kafka source based on the new data source API.

Usage #

Kafka source provides a builder class for constructing instance of KafkaSource. The code snippet below shows how to build a KafkaSource to consume messages from the earliest offset of topic “input-topic”, with consumer group “my-group” and deserialize only the value of message as string.

KafkaSource<String> source = KafkaSource.<String>builder()
    .setValueOnlyDeserializer(new SimpleStringSchema())

env.fromSource(source, WatermarkStrategy.noWatermarks(), "Kafka Source");

The following properties are required for building a KafkaSource:

  • Bootstrap servers, configured by setBootstrapServers(String)
  • Topics / partitions to subscribe, see the following Topic-partition subscription for more details.
  • Deserializer to parse Kafka messages, see the following Deserializer for more details.

Topic-partition Subscription #

Kafka source provide 3 ways of topic-partition subscription:

  • Topic list, subscribing messages from all partitions in a list of topics. For example:
    KafkaSource.builder().setTopics("topic-a", "topic-b")
  • Topic pattern, subscribing messages from all topics whose name matches the provided regular expression. For example:
  • Partition set, subscribing partitions in the provided partition set. For example:
    final HashSet<TopicPartition> partitionSet = new HashSet<>(Arrays.asList(
            new TopicPartition("topic-a", 0),    // Partition 0 of topic "topic-a"
            new TopicPartition("topic-b", 5)));  // Partition 5 of topic "topic-b"

Deserializer #

A deserializer is required for parsing Kafka messages. Deserializer (Deserialization schema) can be configured by setDeserializer(KakfaRecordDeserializationSchema), where KafkaRecordDeserializationSchema defines how to deserialize a Kafka ConsumerRecord.

If only the value of Kafka ConsumerRecord is needed, you can use setValueOnlyDeserializer(DeserializationSchema) in the builder, where DeserializationSchema defines how to deserialize binaries of Kafka message value.

You can also use a Kafka Deserializer for deserializing Kafka message value. For example using StringDeserializer for deserializing Kafka message value as string:

import org.apache.kafka.common.serialization.StringDeserializer;


Starting Offset #

Kafka source is able to consume messages starting from different offsets by specifying OffsetsInitializer. Built-in initializers include:

    // Start from committed offset of the consuming group, without reset strategy
    // Start from committed offset, also use EARLIEST as reset strategy if committed offset doesn't exist
    // Start from the first record whose timestamp is greater than or equals a timestamp
    // Start from earliest offset
    // Start from latest offset

You can also implement a custom offsets initializer if built-in initializers above cannot fulfill your requirement.

If offsets initializer is not specified, OffsetsInitializer.earliest() will be used by default.

Boundedness #

Kafka source is designed to support both streaming and batch running mode. By default, the KafkaSource is set to run in streaming manner, thus never stops until Flink job fails or is cancelled. You can use setBounded(OffsetsInitializer) to specify stopping offsets and set the source running in batch mode. When all partitions have reached their stoping offsets, the source will exit.

You can also set KafkaSource running in streaming mode, but still stop at the stopping offset by using setUnbounded(OffsetsInitializer). The source will exit when all partitions reach their specified stopping offset.

Additional Properties #

In addition to properties described above, you can set arbitrary properties for KafkaSource and KafkaConsumer by using setProperties(Properties) and setProperty(String, String). KafkaSource has following options for configuration:

  • client.id.prefix defines the prefix to use for Kafka consumer’s client ID
  • partition.discovery.interval.ms defines the interval im milliseconds for Kafka source to discover new partitions. See Dynamic Partition Discovery below for more details.
  • register.consumer.metrics specifies whether to register metrics of KafkaConsumer in Flink metric group
  • commit.offsets.on.checkpoint specifies whether to commit consuming offsets to Kafka brokers on checkpoint

For configurations of KafkaConsumer, you can refer to Apache Kafka documentation for more details.

Please note that the following keys will be overridden by the builder even if it is configured:

  • key.deserializer is always set to ByteArrayDeserializer
  • value.deserializer is always set to ByteArrayDeserializer
  • auto.offset.reset.strategy is overridden by OffsetsInitializer#getAutoOffsetResetStrategy() for the starting offsets
  • partition.discovery.interval.ms is overridden to -1 when setBounded(OffsetsInitializer) has been invoked

The code snippet below shows configuring KafkaConsumer to use “PLAIN” as SASL mechanism and provide JAAS configuration:

    .setProperty("sasl.mechanism", "PLAIN")
    .setProperty("sasl.jaas.config", "org.apache.kafka.common.security.plain.PlainLoginModule required username=\"username\" password=\"password\";")

Dynamic Partition Discovery #

In order to handle scenarios like topic scaling-out or topic creation without restarting the Flink job, Kafka source can be configured to periodically discover new partitions under provided topic-partition subscribing pattern. To enable partition discovery, set a non-negative value for property partition.discovery.interval.ms:

    .setProperty("partition.discovery.interval.ms", "10000") // discover new partitions per 10 seconds
Partition discovery is disabled by default. You need to explicitly set the partition discovery interval to enable this feature.

Event Time and Watermarks #

By default, the record will use the timestamp embedded in Kafka ConsumerRecord as the event time. You can define your own WatermarkStrategy for extract event time from the record itself, and emit watermark downstream:

env.fromSource(kafkaSource, new CustomWatermarkStrategy(), "Kafka Source With Custom Watermark Strategy")

This documentation describes details about how to define a WatermarkStrategy.

Idleness #

The Kafka Source does not go automatically in an idle state if the parallelism is higher than the number of partitions. You will either need to lower the parallelism or add an idle timeout to the watermark strategy. If no records flow in a partition of a stream for that amount of time, then that partition is considered “idle” and will not hold back the progress of watermarks in downstream operators.

This documentation describes details about how to define a WatermarkStrategy#withIdleness.

Consumer Offset Committing #

Kafka source commits the current consuming offset when checkpoints are completed, for ensuring the consistency between Flink’s checkpoint state and committed offsets on Kafka brokers.

If checkpointing is not enabled, Kafka source relies on Kafka consumer’s internal automatic periodic offset committing logic, configured by enable.auto.commit and auto.commit.interval.ms in the properties of Kafka consumer.

Note that Kafka source does NOT rely on committed offsets for fault tolerance. Committing offset is only for exposing the progress of consumer and consuming group for monitoring.

Monitoring #

Kafka source exposes metrics in Flink’s metric group for monitoring and diagnosing.

Scope of Metric #

All metrics of Kafka source reader are registered under group KafkaSourceReader, which is a child group of operator metric group. Metrics related to a specific topic partition will be registered in the group KafkaSourceReader.topic.<topic_name>.partition.<partition_id>.

For example, current consuming offset of topic “my-topic” and partition 1 will be reported in metric: <some_parent_groups>.operator.KafkaSourceReader.topic.my-topic.partition.1.currentOffset ,

and number of successful commits will be reported in metric: <some_parent_groups>.operator.KafkaSourceReader.commitsSucceeded .

List of Metrics #

Metric Name Description Scope
currentOffset Current consuming offset of the topic partition TopicPartition
committedOffset Committed offset of the topic partition TopicPartition
commitsSucceeded Number of successful commits KafkaSourceReader
commitsFailed Number of failed commits KafkaSourceReader

Kafka Consumer Metrics #

All metrics of Kafka consumer are also registered under group KafkaSourceReader.KafkaConsumer. For example, Kafka consumer metric “records-consumed-total” will be reported in metric: <some_parent_groups>.operator.KafkaSourceReader.KafkaConsumer.records-consumed-total .

You can configure whether to register Kafka consumer’s metric by configuring option register.consumer.metrics. This option will be set as true by default.

For metrics of Kafka consumer, you can refer to Apache Kafka Documentation for more details.

Behind the Scene #

If you are interested in how Kafka source works under the design of new data source API, you may want to read this part as a reference. For details about the new data source API, documentation of data source and FLIP-27 provide more descriptive discussions.

Under the abstraction of the new data source API, Kafka source consists of the following components:

Source Split #

A source split in Kafka source represents a partition of Kafka topic. A Kafka source split consists of:

  • TopicPartition the split representing
  • Starting offset of the partition
  • Stopping offset of the partition, only available when the source is running in bounded mode

The state of Kafka source split also stores current consuming offset of the partition, and the state will be converted to immutable split when Kafka source reader is snapshot, assigning current offset to the starting offset of the immutable split.

You can check class KafkaPartitionSplit and KafkaPartitionSplitState for more details.

Split Enumerator #

The split enumerator of Kafka is responsible for discovering new splits (partitions) under the provided topic partition subscription pattern, and assigning splits to readers, uniformly distributed across subtasks, in round-robin style. Note that the split enumerator of Kafka source pushes splits eagerly to source readers, so it won’t need to handle split requests from source reader.

Source Reader #

The source reader of Kafka source extends the provided SourceReaderBase, and use single-thread-multiplexed thread model, which read multiple assigned splits (partitions) with one KafkaConsumer driven by one SplitReader. Messages are deserialized right after they are fetched from Kafka in SplitReader. The state of split, or current progress of message consuming is updated by KafkaRecordEmitter , which is also responsible for assigning event time when the record is emitted downstream.

Kafka SourceFunction #

This part describes Kafka source based on the legacy SourceFunction API.

Flink’s Kafka consumer - FlinkKafkaConsumer provides access to read from one or more Kafka topics.

The constructor accepts the following arguments:

  1. The topic name / list of topic names
  2. A DeserializationSchema / KafkaDeserializationSchema 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)
  • “group.id” the id of the consumer group
Properties properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");
properties.setProperty("group.id", "test");
DataStream<String> stream = env
	.addSource(new FlinkKafkaConsumer<>("topic", new SimpleStringSchema(), properties));
val properties = new Properties()
properties.setProperty("bootstrap.servers", "localhost:9092")
properties.setProperty("group.id", "test")
val stream = env
    .addSource(new FlinkKafkaConsumer[String]("topic", new SimpleStringSchema(), properties)

The DeserializationSchema #

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

For convenience, Flink provides the following schemas out of the box:

  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.

  3. AvroDeserializationSchema which reads data serialized with Avro format using a statically provided schema. It can infer the schema from Avro generated classes (AvroDeserializationSchema.forSpecific(...)) or it can work with GenericRecords with a manually provided schema (with AvroDeserializationSchema.forGeneric(...)). This deserialization schema expects that the serialized records DO NOT contain embedded schema.

    • There is also a version of this schema available that can lookup the writer’s schema (schema which was used to write the record) in Confluent Schema Registry. Using these deserialization schema record will be read with the schema that was retrieved from Schema Registry and transformed to a statically provided( either through ConfluentRegistryAvroDeserializationSchema.forGeneric(...) or ConfluentRegistryAvroDeserializationSchema.forSpecific(...)).

    To use this deserialization schema one has to add the following additional dependency:


When encountering a corrupted message that cannot be deserialized for any reason the deserialization schema should return null which will result in the record being skipped. 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 positions for Kafka partitions are determined.

final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

FlinkKafkaConsumer<String> myConsumer = new FlinkKafkaConsumer<>(...);
myConsumer.setStartFromEarliest();     // start from the earliest record possible
myConsumer.setStartFromLatest();       // start from the latest record
myConsumer.setStartFromTimestamp(...); // start from specified epoch timestamp (milliseconds)
myConsumer.setStartFromGroupOffsets(); // the default behaviour

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

val myConsumer = new FlinkKafkaConsumer[String](...)
myConsumer.setStartFromEarliest()      // start from the earliest record possible
myConsumer.setStartFromLatest()        // start from the latest record
myConsumer.setStartFromTimestamp(...)  // start from specified epoch timestamp (milliseconds)
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. 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. If offsets become out of range for a partition, the auto.offset.reset setting in the properties will be used.
  • setStartFromTimestamp(long): Start from the specified timestamp. For each partition, the record whose timestamp is larger than or equal to the specified timestamp will be used as the start position. If a partition’s latest record is earlier than the timestamp, the partition will simply be read from the latest record. Under this mode, 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);

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)


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 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 in the job.

If checkpointing is disabled, the Kafka consumer will periodically commit the offsets to Zookeeper.

Kafka Consumers Topic and Partition Discovery #

Partition discovery #

The Flink Kafka Consumer supports discovering dynamically created Kafka partitions, and consumes them with exactly-once guarantees. All partitions discovered after the initial retrieval of partition metadata (i.e., when the job starts running) will be consumed from the earliest possible offset.

By default, partition discovery is disabled. To enable it, set a non-negative value for flink.partition-discovery.interval-millis in the provided properties config, representing the discovery interval in milliseconds.

Topic discovery #

The Kafka Consumer is also capable of discovering topics by matching topic names using regular expressions.

final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

Properties properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");
properties.setProperty("group.id", "test");

FlinkKafkaConsumer<String> myConsumer = new FlinkKafkaConsumer<>(
    new SimpleStringSchema(),

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

val properties = new Properties()
properties.setProperty("bootstrap.servers", "localhost:9092")
properties.setProperty("group.id", "test")

val myConsumer = new FlinkKafkaConsumer[String](
  new SimpleStringSchema,

val stream = env.addSource(myConsumer)

In the above example, all topics with names that match the specified regular expression (starting with test-topic- and ending with a single digit) will be subscribed by the consumer when the job starts running.

To allow the consumer to discover dynamically created topics after the job started running, set a non-negative value for flink.partition-discovery.interval-millis. This allows the consumer to discover partitions of new topics with names that also match the specified pattern.

Kafka Consumers Offset Committing Behaviour Configuration #

The Flink Kafka Consumer allows configuring the behaviour of how offsets are committed back to Kafka brokers. 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 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 / 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 in the record itself, or the metadata of the ConsumerRecord. In addition, users may want to emit watermarks either periodically, or irregularly, 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 a watermark strategy.

You can specify your custom strategy as described here, or use one from the predefined ones.

Properties properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");
properties.setProperty("group.id", "test");

FlinkKafkaConsumer<String> myConsumer =
    new FlinkKafkaConsumer<>("topic", new SimpleStringSchema(), properties);

DataStream<String> stream = env.addSource(myConsumer);
val properties = new Properties()
properties.setProperty("bootstrap.servers", "localhost:9092")
properties.setProperty("group.id", "test")

val myConsumer =
    new FlinkKafkaConsumer("topic", new SimpleStringSchema(), properties)

val stream = env.addSource(myConsumer)

Note: If a watermark assigner depends on records read from Kafka to advance its watermarks (which is commonly the case), all topics and partitions need to have a continuous stream of records. Otherwise, the watermarks of the whole application cannot advance and all time-based operations, such as time windows or functions with timers, cannot make progress. A single idle Kafka partition causes this behavior. Consider setting appropriate idelness timeouts to mitigate this issue.

Kafka Producer #

Flink’s Kafka Producer - FlinkKafkaProducer allows writing a stream of records to one or more Kafka topics.

The constructor accepts the following arguments:

  1. A default output topic where events should be written
  2. A SerializationSchema / KafkaSerializationSchema for serializing data into Kafka
  3. Properties for the Kafka client. The following properties are required:
    • “bootstrap.servers” (comma separated list of Kafka brokers)
  4. A fault-tolerance semantic
DataStream<String> stream = ...

Properties properties = new Properties();
properties.setProperty("bootstrap.servers", "localhost:9092");

KafkaSerializationSchema<String> serializationSchema = new KafkaSerializationSchema<String>() {
        public ProducerRecord<byte[], byte[]> serialize(String element, @Nullable Long timestamp) {
            return new ProducerRecord<>(
                    "my-topic", // target topic
                    element.getBytes(StandardCharsets.UTF_8)); // record contents

FlinkKafkaProducer<String> myProducer = new FlinkKafkaProducer<>(
        "my-topic",             // target topic
        serializationSchema,    // serialization schema
        properties,             // producer config
        FlinkKafkaProducer.Semantic.EXACTLY_ONCE); // fault-tolerance

val stream: DataStream[String] = ...

val properties = new Properties
properties.setProperty("bootstrap.servers", "localhost:9092")

val serializationSchema = new KafkaSerializationSchema[String] {
        override def serialize(element: String,
                               timestamp: lang.Long): ProducerRecord[Array[Byte], Array[Byte]] =
            new ProducerRecord[Array[Byte], Array[Byte]](
                    "my-topic",      // target topic
                    element.getBytes(StandardCharsets.UTF_8)) // record contents

val myProducer = new FlinkKafkaProducer[String](
        "my-topic",                  // target topic
        serializationSchema,         // serialization schema
        properties,                  // producer config
        FlinkKafkaProducer.Semantic.EXACTLY_ONCE) // fault-tolerance


The SerializationSchema #

The Flink Kafka Producer needs to know how to turn Java/Scala objects into binary data. The KafkaSerializationSchema allows users to specify such a schema. The ProducerRecord<byte[], byte[]> serialize(T element, @Nullable Long timestamp) method gets called for each record, generating a ProducerRecord that is written to Kafka.

This gives users fine-grained control over how data is written out to Kafka. Through the producer record you can:

  • Set header values
  • Define keys for each record
  • Specify custom partitioning of data

Kafka Producers and Fault Tolerance #

With Flink’s checkpointing enabled, the FlinkKafkaProducer can provide exactly-once delivery guarantees.

Besides enabling Flink’s checkpointing, you can also choose three different modes of operating chosen by passing appropriate semantic parameter to the FlinkKafkaProducer:

  • Semantic.NONE: Flink will not guarantee anything. Produced records can be lost or they can be duplicated.
  • Semantic.AT_LEAST_ONCE (default setting): This guarantees that no records will be lost (although they can be duplicated).
  • Semantic.EXACTLY_ONCE: Kafka transactions will be used to provide exactly-once semantic. Whenever you write to Kafka using transactions, do not forget about setting desired isolation.level (read_committed or read_uncommitted - the latter one is the default value) for any application consuming records from Kafka.
Caveats #

Semantic.EXACTLY_ONCE mode relies on the ability to commit transactions that were started before taking a checkpoint, after recovering from the said checkpoint. If the time between Flink application crash and completed restart is larger than Kafka’s transaction timeout there will be data loss (Kafka will automatically abort transactions that exceeded timeout time). Having this in mind, please configure your transaction timeout appropriately to your expected down times.

Kafka brokers by default have transaction.max.timeout.ms set to 15 minutes. This property will not allow to set transaction timeouts for the producers larger than it’s value. FlinkKafkaProducer by default sets the transaction.timeout.ms property in producer config to 1 hour, thus transaction.max.timeout.ms should be increased before using the Semantic.EXACTLY_ONCE mode.

In read_committed mode of KafkaConsumer, any transactions that were not finished (neither aborted nor completed) will block all reads from the given Kafka topic past any un-finished transaction. In other words after following sequence of events:

  1. User started transaction1 and written some records using it
  2. User started transaction2 and written some further records using it
  3. User committed transaction2

Even if records from transaction2 are already committed, they will not be visible to the consumers until transaction1 is committed or aborted. This has two implications:

  • First of all, during normal working of Flink applications, user can expect a delay in visibility of the records produced into Kafka topics, equal to average time between completed checkpoints.
  • Secondly in case of Flink application failure, topics into which this application was writing, will be blocked for the readers until the application restarts or the configured transaction timeout time will pass. This remark only applies for the cases when there are multiple agents/applications writing to the same Kafka topic.

Note: Semantic.EXACTLY_ONCE mode uses a fixed size pool of KafkaProducers per each FlinkKafkaProducer instance. One of each of those producers is used per one checkpoint. If the number of concurrent checkpoints exceeds the pool size, FlinkKafkaProducer will throw an exception and will fail the whole application. Please configure max pool size and max number of concurrent checkpoints accordingly.

Note: Semantic.EXACTLY_ONCE takes all possible measures to not leave any lingering transactions that would block the consumers from reading from Kafka topic more then it is necessary. However in the event of failure of Flink application before first checkpoint, after restarting such application there is no information in the system about previous pool sizes. Thus it is unsafe to scale down Flink application before first checkpoint completes, by factor larger than FlinkKafkaProducer.SAFE_SCALE_DOWN_FACTOR.

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 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 the Kafka brokers. 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 Zookeeper 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 #

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.
  1. 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.

Upgrading to the Latest Connector Version #

The generic upgrade steps are outlined in upgrading jobs and Flink versions guide. For Kafka, you additionally need to follow these steps:

  • Do not upgrade Flink and the Kafka Connector version at the same time.
  • Make sure you have a group.id configured for your Consumer.
  • Set setCommitOffsetsOnCheckpoints(true) on the consumer so that read offsets are committed to Kafka. It’s important to do this before stopping and taking the savepoint. You might have to do a stop/restart cycle on the old connector version to enable this setting.
  • Set setStartFromGroupOffsets(true) on the consumer so that we get read offsets from Kafka. This will only take effect when there is no read offset in Flink state, which is why the next step is very important.
  • Change the assigned uid of your source/sink. This makes sure the new source/sink doesn’t read state from the old source/sink operators.
  • Start the new job with --allow-non-restored-state because we still have the state of the previous connector version in the savepoint.

Troubleshooting #

If you have a problem with Kafka when using Flink, keep in mind that Flink only wraps KafkaConsumer or KafkaProducer and your problem might be independent of Flink and sometimes can be solved by upgrading Kafka brokers, reconfiguring Kafka brokers or reconfiguring KafkaConsumer or KafkaProducer in Flink. Some examples of common problems are listed below.

Data loss #

Depending on your Kafka configuration, even after Kafka acknowledges writes you can still experience data loss. In particular keep in mind about the following properties in Kafka config:

  • acks
  • log.flush.interval.messages
  • log.flush.interval.ms
  • log.flush.*

Default values for the above options can easily lead to data loss. Please refer to the Kafka documentation for more explanation.

UnknownTopicOrPartitionException #

One possible cause of this error is when a new leader election is taking place, for example after or during restarting a Kafka broker. This is a retriable exception, so Flink job should be able to restart and resume normal operation. It also can be circumvented by changing retries property in the producer settings. However this might cause reordering of messages, which in turn if undesired can be circumvented by setting max.in.flight.requests.per.connection to 1.

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