Structured Streaming + Kafka Integration Guide (Kafka broker version 0.10.0 or higher)

Structured Streaming integration for Kafka 0.10 to read data from and write data to Kafka.

Linking

For Scala/Java applications using SBT/Maven project definitions, link your application with the following artifact:

groupId = org.apache.spark
artifactId = spark-sql-kafka-0-10_2.12
version = 3.0.2

Please note that to use the headers functionality, your Kafka client version should be version 0.11.0.0 or up.

For Python applications, you need to add this above library and its dependencies when deploying your application. See the Deploying subsection below.

For experimenting on spark-shell, you need to add this above library and its dependencies too when invoking spark-shell. Also, see the Deploying subsection below.

Reading Data from Kafka

Creating a Kafka Source for Streaming Queries

// Subscribe to 1 topic
val df = spark
  .readStream
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .as[(String, String)]

// Subscribe to 1 topic, with headers
val df = spark
  .readStream
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1")
  .option("includeHeaders", "true")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)", "headers")
  .as[(String, String, Array[(String, Array[Byte])])]

// Subscribe to multiple topics
val df = spark
  .readStream
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1,topic2")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .as[(String, String)]

// Subscribe to a pattern
val df = spark
  .readStream
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribePattern", "topic.*")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .as[(String, String)]

// Subscribe to 1 topic
Dataset<Row> df = spark
  .readStream()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1")
  .load();
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");

// Subscribe to 1 topic, with headers
Dataset<Row> df = spark
  .readStream()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1")
  .option("includeHeaders", "true")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)", "headers");

// Subscribe to multiple topics
Dataset<Row> df = spark
  .readStream()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1,topic2")
  .load();
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");

// Subscribe to a pattern
Dataset<Row> df = spark
  .readStream()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribePattern", "topic.*")
  .load();
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");

# Subscribe to 1 topic
df = spark \
  .readStream \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("subscribe", "topic1") \
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")

# Subscribe to 1 topic, with headers
val df = spark \
  .readStream \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("subscribe", "topic1") \
  .option("includeHeaders", "true") \
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)", "headers")

# Subscribe to multiple topics
df = spark \
  .readStream \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("subscribe", "topic1,topic2") \
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")

# Subscribe to a pattern
df = spark \
  .readStream \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("subscribePattern", "topic.*") \
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")

Creating a Kafka Source for Batch Queries

If you have a use case that is better suited to batch processing, you can create a Dataset/DataFrame for a defined range of offsets.

// Subscribe to 1 topic defaults to the earliest and latest offsets
val df = spark
  .read
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .as[(String, String)]

// Subscribe to multiple topics, specifying explicit Kafka offsets
val df = spark
  .read
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1,topic2")
  .option("startingOffsets", """{"topic1":{"0":23,"1":-2},"topic2":{"0":-2}}""")
  .option("endingOffsets", """{"topic1":{"0":50,"1":-1},"topic2":{"0":-1}}""")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .as[(String, String)]

// Subscribe to a pattern, at the earliest and latest offsets
val df = spark
  .read
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribePattern", "topic.*")
  .option("startingOffsets", "earliest")
  .option("endingOffsets", "latest")
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .as[(String, String)]

// Subscribe to 1 topic defaults to the earliest and latest offsets
Dataset<Row> df = spark
  .read()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1")
  .load();
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");

// Subscribe to multiple topics, specifying explicit Kafka offsets
Dataset<Row> df = spark
  .read()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribe", "topic1,topic2")
  .option("startingOffsets", "{\"topic1\":{\"0\":23,\"1\":-2},\"topic2\":{\"0\":-2}}")
  .option("endingOffsets", "{\"topic1\":{\"0\":50,\"1\":-1},\"topic2\":{\"0\":-1}}")
  .load();
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");

// Subscribe to a pattern, at the earliest and latest offsets
Dataset<Row> df = spark
  .read()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("subscribePattern", "topic.*")
  .option("startingOffsets", "earliest")
  .option("endingOffsets", "latest")
  .load();
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");

# Subscribe to 1 topic defaults to the earliest and latest offsets
df = spark \
  .read \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("subscribe", "topic1") \
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")

# Subscribe to multiple topics, specifying explicit Kafka offsets
df = spark \
  .read \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("subscribe", "topic1,topic2") \
  .option("startingOffsets", """{"topic1":{"0":23,"1":-2},"topic2":{"0":-2}}""") \
  .option("endingOffsets", """{"topic1":{"0":50,"1":-1},"topic2":{"0":-1}}""") \
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")

# Subscribe to a pattern, at the earliest and latest offsets
df = spark \
  .read \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("subscribePattern", "topic.*") \
  .option("startingOffsets", "earliest") \
  .option("endingOffsets", "latest") \
  .load()
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")

Each row in the source has the following schema:

The following options must be set for the Kafka source for both batch and streaming queries.

The following configurations are optional:

Consumer Caching

It’s time-consuming to initialize Kafka consumers, especially in streaming scenarios where processing time is a key factor. Because of this, Spark pools Kafka consumers on executors, by leveraging Apache Commons Pool.

The caching key is built up from the following information:

• Topic name
• Topic partition
• Group ID

The following properties are available to configure the consumer pool:

The size of the pool is limited by spark.kafka.consumer.cache.capacity, but it works as “soft-limit” to not block Spark tasks.

Idle eviction thread periodically removes consumers which are not used longer than given timeout. If this threshold is reached when borrowing, it tries to remove the least-used entry that is currently not in use.

If it cannot be removed, then the pool will keep growing. In the worst case, the pool will grow to the max number of concurrent tasks that can run in the executor (that is, number of task slots).

If a task fails for any reason, the new task is executed with a newly created Kafka consumer for safety reasons. At the same time, we invalidate all consumers in pool which have same caching key, to remove consumer which was used in failed execution. Consumers which any other tasks are using will not be closed, but will be invalidated as well when they are returned into pool.

Along with consumers, Spark pools the records fetched from Kafka separately, to let Kafka consumers stateless in point of Spark’s view, and maximize the efficiency of pooling. It leverages same cache key with Kafka consumers pool. Note that it doesn’t leverage Apache Commons Pool due to the difference of characteristics.

The following properties are available to configure the fetched data pool:

Writing Data to Kafka

Here, we describe the support for writing Streaming Queries and Batch Queries to Apache Kafka. Take note that Apache Kafka only supports at least once write semantics. Consequently, when writing—either Streaming Queries or Batch Queries—to Kafka, some records may be duplicated; this can happen, for example, if Kafka needs to retry a message that was not acknowledged by a Broker, even though that Broker received and wrote the message record. Structured Streaming cannot prevent such duplicates from occurring due to these Kafka write semantics. However, if writing the query is successful, then you can assume that the query output was written at least once. A possible solution to remove duplicates when reading the written data could be to introduce a primary (unique) key that can be used to perform de-duplication when reading.

The Dataframe being written to Kafka should have the following columns in schema:

* The topic column is required if the “topic” configuration option is not specified.

The value column is the only required option. If a key column is not specified then a null valued key column will be automatically added (see Kafka semantics on how null valued key values are handled). If a topic column exists then its value is used as the topic when writing the given row to Kafka, unless the “topic” configuration option is set i.e., the “topic” configuration option overrides the topic column. If a “partition” column is not specified (or its value is null) then the partition is calculated by the Kafka producer. A Kafka partitioner can be specified in Spark by setting the kafka.partitioner.class option. If not present, Kafka default partitioner will be used.

The following options must be set for the Kafka sink for both batch and streaming queries.

The following configurations are optional:

Creating a Kafka Sink for Streaming Queries

// Write key-value data from a DataFrame to a specific Kafka topic specified in an option
val ds = df
  .selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .writeStream
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("topic", "topic1")
  .start()

// Write key-value data from a DataFrame to Kafka using a topic specified in the data
val ds = df
  .selectExpr("topic", "CAST(key AS STRING)", "CAST(value AS STRING)")
  .writeStream
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .start()

// Write key-value data from a DataFrame to a specific Kafka topic specified in an option
StreamingQuery ds = df
  .selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .writeStream()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("topic", "topic1")
  .start();

// Write key-value data from a DataFrame to Kafka using a topic specified in the data
StreamingQuery ds = df
  .selectExpr("topic", "CAST(key AS STRING)", "CAST(value AS STRING)")
  .writeStream()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .start();

# Write key-value data from a DataFrame to a specific Kafka topic specified in an option
ds = df \
  .selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)") \
  .writeStream \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("topic", "topic1") \
  .start()

# Write key-value data from a DataFrame to Kafka using a topic specified in the data
ds = df \
  .selectExpr("topic", "CAST(key AS STRING)", "CAST(value AS STRING)") \
  .writeStream \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .start()

Writing the output of Batch Queries to Kafka

// Write key-value data from a DataFrame to a specific Kafka topic specified in an option
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .write
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("topic", "topic1")
  .save()

// Write key-value data from a DataFrame to Kafka using a topic specified in the data
df.selectExpr("topic", "CAST(key AS STRING)", "CAST(value AS STRING)")
  .write
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .save()

// Write key-value data from a DataFrame to a specific Kafka topic specified in an option
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .write()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .option("topic", "topic1")
  .save();

// Write key-value data from a DataFrame to Kafka using a topic specified in the data
df.selectExpr("topic", "CAST(key AS STRING)", "CAST(value AS STRING)")
  .write()
  .format("kafka")
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
  .save();

# Write key-value data from a DataFrame to a specific Kafka topic specified in an option
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)") \
  .write \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .option("topic", "topic1") \
  .save()

# Write key-value data from a DataFrame to Kafka using a topic specified in the data
df.selectExpr("topic", "CAST(key AS STRING)", "CAST(value AS STRING)") \
  .write \
  .format("kafka") \
  .option("kafka.bootstrap.servers", "host1:port1,host2:port2") \
  .save()

Producer Caching

Given Kafka producer instance is designed to be thread-safe, Spark initializes a Kafka producer instance and co-use across tasks for same caching key.

The caching key is built up from the following information:

• Kafka producer configuration

This includes configuration for authorization, which Spark will automatically include when delegation token is being used. Even we take authorization into account, you can expect same Kafka producer instance will be used among same Kafka producer configuration. It will use different Kafka producer when delegation token is renewed; Kafka producer instance for old delegation token will be evicted according to the cache policy.

The following properties are available to configure the producer pool:

Idle eviction thread periodically removes producers which are not used longer than given timeout. Note that the producer is shared and used concurrently, so the last used timestamp is determined by the moment the producer instance is returned and reference count is 0.

Kafka Specific Configurations

Kafka’s own configurations can be set via DataStreamReader.option with kafka. prefix, e.g, stream.option("kafka.bootstrap.servers", "host:port"). For possible kafka parameters, see Kafka consumer config docs for parameters related to reading data, and Kafka producer config docs for parameters related to writing data.

Note that the following Kafka params cannot be set and the Kafka source or sink will throw an exception:

• group.id: Kafka source will create a unique group id for each query automatically. The user can set the prefix of the automatically generated group.id’s via the optional source option groupIdPrefix, default value is “spark-kafka-source”. You can also set “kafka.group.id” to force Spark to use a special group id, however, please read warnings for this option and use it with caution.
• auto.offset.reset: Set the source option startingOffsets to specify where to start instead. Structured Streaming manages which offsets are consumed internally, rather than rely on the kafka Consumer to do it. This will ensure that no data is missed when new topics/partitions are dynamically subscribed. Note that startingOffsets only applies when a new streaming query is started, and that resuming will always pick up from where the query left off.
• key.deserializer: Keys are always deserialized as byte arrays with ByteArrayDeserializer. Use DataFrame operations to explicitly deserialize the keys.
• value.deserializer: Values are always deserialized as byte arrays with ByteArrayDeserializer. Use DataFrame operations to explicitly deserialize the values.
• key.serializer: Keys are always serialized with ByteArraySerializer or StringSerializer. Use DataFrame operations to explicitly serialize the keys into either strings or byte arrays.
• value.serializer: values are always serialized with ByteArraySerializer or StringSerializer. Use DataFrame operations to explicitly serialize the values into either strings or byte arrays.
• enable.auto.commit: Kafka source doesn’t commit any offset.
• interceptor.classes: Kafka source always read keys and values as byte arrays. It’s not safe to use ConsumerInterceptor as it may break the query.

Deploying

As with any Spark applications, spark-submit is used to launch your application. spark-sql-kafka-0-10_2.12 and its dependencies can be directly added to spark-submit using --packages, such as,

./bin/spark-submit --packages org.apache.spark:spark-sql-kafka-0-10_2.12:3.0.2 ...

For experimenting on spark-shell, you can also use --packages to add spark-sql-kafka-0-10_2.12 and its dependencies directly,

./bin/spark-shell --packages org.apache.spark:spark-sql-kafka-0-10_2.12:3.0.2 ...

See Application Submission Guide for more details about submitting applications with external dependencies.

Security

Kafka 0.9.0.0 introduced several features that increases security in a cluster. For detailed description about these possibilities, see Kafka security docs.

It’s worth noting that security is optional and turned off by default.

Spark supports the following ways to authenticate against Kafka cluster:

• Delegation token (introduced in Kafka broker 1.1.0)
• JAAS login configuration

Delegation token

This way the application can be configured via Spark parameters and may not need JAAS login configuration (Spark can use Kafka’s dynamic JAAS configuration feature). For further information about delegation tokens, see Kafka delegation token docs.

The process is initiated by Spark’s Kafka delegation token provider. When spark.kafka.clusters.${cluster}.auth.bootstrap.servers is set, Spark considers the following log in options, in order of preference:

• JAAS login configuration, please see example below.

• Keytab file, such as,

  ./bin/spark-submit \
      --keytab <KEYTAB_FILE> \
      --principal <PRINCIPAL> \
      --conf spark.kafka.clusters.${cluster}.auth.bootstrap.servers=<KAFKA_SERVERS> \
      ...
  

• Kerberos credential cache, such as,

  ./bin/spark-submit \
      --conf spark.kafka.clusters.${cluster}.auth.bootstrap.servers=<KAFKA_SERVERS> \
      ...
  

The Kafka delegation token provider can be turned off by setting spark.security.credentials.kafka.enabled to false (default: true).

Spark can be configured to use the following authentication protocols to obtain token (it must match with Kafka broker configuration):

• SASL SSL (default)
• SSL
• SASL PLAINTEXT (for testing)

After obtaining delegation token successfully, Spark distributes it across nodes and renews it accordingly. Delegation token uses SCRAM login module for authentication and because of that the appropriate spark.kafka.clusters.${cluster}.sasl.token.mechanism (default: SCRAM-SHA-512) has to be configured. Also, this parameter must match with Kafka broker configuration.

When delegation token is available on an executor Spark considers the following log in options, in order of preference:

• JAAS login configuration, please see example below.
• Delegation token, please see spark.kafka.clusters.${cluster}.target.bootstrap.servers.regex parameter for further details.

When none of the above applies then unsecure connection assumed.

Configuration

Delegation tokens can be obtained from multiple clusters and ${cluster} is an arbitrary unique identifier which helps to group different configurations.

Kafka Specific Configurations

Kafka’s own configurations can be set with kafka. prefix, e.g, --conf spark.kafka.clusters.${cluster}.kafka.retries=1. For possible Kafka parameters, see Kafka adminclient config docs.

Caveats

• Obtaining delegation token for proxy user is not yet supported (KAFKA-6945).

JAAS login configuration

JAAS login configuration must placed on all nodes where Spark tries to access Kafka cluster. This provides the possibility to apply any custom authentication logic with a higher cost to maintain. This can be done several ways. One possibility is to provide additional JVM parameters, such as,

./bin/spark-submit \
    --driver-java-options "-Djava.security.auth.login.config=/path/to/custom_jaas.conf" \
    --conf spark.executor.extraJavaOptions=-Djava.security.auth.login.config=/path/to/custom_jaas.conf \
    ...