Kafka Input Operator

Introduction

This is an input operator that consumes data from Kafka messaging system for further processing in Apex. Kafka Input Operator is a fault-tolerant and scalable Malhar Operator.

Why is it needed ?

Kafka is a pull-based and distributed publish subscribe messaging system, topics are partitioned and replicated across nodes. Kafka input operator is needed when you want to read data from multiple partitions of a Kafka topic in parallel in an Apex application.

AbstractKafkaInputOperator

This is the abstract implementation that serves as base class for consuming messages from Kafka messaging system. This class doesn’t have any ports.

Configuration Parameters

Parameter	Description
maxTuplesPerWindow	Controls the maximum number of messages emitted in each streaming window from this operator. Minimum value is 1. Default value = MAX_VALUE
idempotentStorageManager	This is an instance of IdempotentStorageManager. Idempotency ensures that the operator will process the same set of messages in a window before and after a failure. For example, let's say the operator completed window 10 and failed somewhere between window 11. If the operator gets restored at window 10 then it will process the same messages again in window 10 which it did in the previous run before the failure. Idempotency is important but comes with higher cost because at the end of each window the operator needs to persist some state with respect to that window. Default Value = com.datatorrent.lib.io.IdempotentStorageManager. NoopIdempotentStorageManager
strategy	Operator supports two types of partitioning strategies, ONE_TO_ONE and ONE_TO_MANY. ONE_TO_ONE: If this is enabled, the AppMaster creates one input operator instance per Kafka topic partition. So the number of Kafka topic partitions equals the number of operator instances. ONE_TO_MANY: The AppMaster creates K = min(initialPartitionCount, N) Kafka input operator instances where N is the number of Kafka topic partitions. If K is less than N, the remaining topic partitions are assigned to the K operator instances in round-robin fashion. If K is less than initialPartitionCount, the AppMaster creates one input operator instance per Kafka topic partition. For example, if initialPartitionCount = 5 and number of Kafka partitions(N) = 2 then AppMaster creates 2 Kafka input operator instances. Default Value = ONE_TO_ONE
msgRateUpperBound	Maximum messages upper bound. Operator repartitions when the msgProcessedPS exceeds this bound. msgProcessedPS is the average number of messages processed per second by this operator.
byteRateUpperBound	Maximum bytes upper bound. Operator repartitions when the bytesPS exceeds this bound. bytesPS is the average number of bytes processed per second by this operator.
offsetManager	This is an optional parameter that is useful when the application restarts or start at specific offsets (offsets are explained below)
repartitionInterval	Interval specified in milliseconds. This value specifies the minimum time required between two repartition actions. Default Value = 30 Seconds
repartitionCheckInterval	Interval specified in milliseconds. This value specifies the minimum interval between two offset updates. Default Value = 5 Seconds
initialPartitionCount	When the ONE_TO_MANY partition strategy is enabled, this value indicates the number of Kafka input operator instances. Default Value = 1
consumer	This is an instance of com.datatorrent.contrib.kafka.KafkaConsumer. Default Value = Instance of SimpleKafkaConsumer.

Abstract Methods

void emitTuple(Message message): Abstract method that emits tuples extracted from Kafka message.

KafkaConsumer

This is an abstract implementation of Kafka consumer. It sends the fetch requests to the leading brokers of Kafka partitions. For each request, it receives the set of messages and stores them into the buffer which is ArrayBlockingQueue. SimpleKafkaConsumer which extends KafkaConsumer and serves the functionality of Simple Consumer API and HighLevelKafkaConsumer which extends KafkaConsumer and  serves the functionality of High Level Consumer API.

Pre-requisites

This operator referred the Kafka Consumer API of version 0.8.1.1. So, this operator will work with any 0.8.x and 0.7.x version of Apache Kafka.

Configuration Parameters

Parameter	Type	Default	Description
zookeeper	String		Specifies the zookeeper quorum of Kafka clusters that you want to consume messages from. zookeeper  is a string in the form of hostname1:port1,hostname2:port2,hostname3:port3  where hostname1,hostname2,hostname3 are hosts and port1,port2,port3 are ports of zookeeper server.  If the topic name is the same across the Kafka clusters and want to consume data from these clusters, then configure the zookeeper as follows: c1::hs1:p1,hs2:p2,hs3:p3;c2::hs4:p4,hs5:p5,c3::hs6:p6 where c1,c2,c3 indicates the cluster names, hs1,hs2,hs3,hs4,hs5,hs6 are zookeeper hosts and p1,p2,p3,p4,p5,p6 are corresponding ports. Here, cluster name is optional in case of single cluster
cacheSize	int	1024	Maximum of buffered messages hold in memory.
topic	String	default_topic	Indicates the name of the topic.
initialOffset	String	latest	Indicates the type of offset i.e, “earliest or latest”. If initialOffset is “latest”, then the operator consumes messages from latest point of Kafka queue. If initialOffset is “earliest”, then the operator consumes messages starting from message queue. This can be overridden by OffsetManager.

Abstract Methods

1. void commitOffset(): Commit the offsets at checkpoint.
2. Map <KafkaPartition, Long> getCurrentOffsets(): Return the current offset status.
3. resetPartitionsAndOffset(Set <KafkaPartition> partitionIds, Map <KafkaPartition, Long> startOffset): Reset the partitions with partitionIds and offsets with startOffset.

Configuration Parameters for SimpleKafkaConsumer

Parameter	Type	Default	Description
bufferSize	int	1 MB	Specifies the maximum total size of messages for each fetch request.
metadataRefreshInterval	int	30 Seconds	Interval in between refresh the metadata change(broker change) in milliseconds. Enabling metadata refresh guarantees an automatic reconnect when a new broker is elected as the host. A value of -1 disables this feature.
metadataRefreshRetryLimit	int	-1	Specifies the maximum brokers' metadata refresh retry limit. -1 means unlimited retry.

OffsetManager

This is an interface for offset management and is useful when consuming data from specified offsets. Updates the offsets for all the Kafka partitions periodically. Below is the code snippet:        

public interface OffsetManager
{
  public Map<KafkaPartition, Long> loadInitialOffsets();
  public void updateOffsets(Map<KafkaPartition, Long> offsetsOfPartitions);
}

Abstract Methods

Map <KafkaPartition, Long> loadInitialOffsets(): Specifies the initial offset for consuming messages; called at the activation stage.

updateOffsets(Map <KafkaPartition, Long> offsetsOfPartitions):  This method is called at every repartitionCheckInterval to update offsets.

Partitioning

The logical instance of the KafkaInputOperator acts as the Partitioner as well as a StatsListener. This is because the AbstractKafkaInputOperator implements both the com.datatorrent.api.Partitioner and com.datatorrent.api.StatsListener interfaces and provides an implementation of definePartitions(...) and processStats(...) which makes it auto-scalable.

Response processStats(BatchedOperatorStats stats)

The application master invokes this method on the logical instance with the stats (tuplesProcessedPS, bytesPS, etc.) of each partition. Re-partitioning happens based on whether any new Kafka partitions added for the topic or bytesPS and msgPS cross their respective upper bounds.

DefinePartitions

Based on the repartitionRequired field of the Response object which is returned by processStats(...) method, the application master invokes definePartitions(...) on the logical instance which is also the partitioner instance. Dynamic partition can be disabled by setting the parameter repartitionInterval value to a negative value.

AbstractSinglePortKafkaInputOperator

This class extends AbstractKafkaInputOperator and having single output port, will emit the messages through this port.

Ports

outputPort <T>: Tuples extracted from Kafka messages are emitted through this port.

Abstract Methods

T getTuple(Message msg) : Converts the Kafka message to tuple.

Concrete Classes

1. KafkaSinglePortStringInputOperator : This class extends AbstractSinglePortKafkaInputOperator and getTuple() method extracts string from Kafka message.

2. KafkaSinglePortByteArrayInputOperator: This class extends AbstractSinglePortKafkaInputOperator and getTuple() method extracts byte array from Kafka message.

Application Example

This section builds an Apex application using Kafka input operator. Below is the code snippet:

@ApplicationAnnotation(name = "KafkaApp")
public class ExampleKafkaApplication implements StreamingApplication
{
@Override
public void populateDAG(DAG dag, Configuration entries)
{
  KafkaSinglePortByteArrayInputOperator input =  dag.addOperator("MessageReader", new KafkaSinglePortByteArrayInputOperator());

  ConsoleOutputOperator output = dag.addOperator("Output", new ConsoleOutputOperator());

  dag.addStream("MessageData", input.outputPort, output.input);
}
}

Below is the configuration for using the earliest offset, “test” as the topic name and “localhost:2181” as the zookeeper forum:

<property>
  <name>dt.operator.MessageReader.prop.initialOffset</name>
  <value>earliest</value>
</property>

<property>
<name>dt.operator.MessageReader.prop.topic</name>
<value>test</value>
</property>

<property>
<name>dt.operator.MessageReader.prop.zookeeper</nam>
<value>localhost:2181</value>
</property>

Please note that MessageReader is the string passed as the first argument to the addOperator() call. The above stanza sets these parameters for this operator regardless of the application it resides in; if you want to set them on a per-application basis, you can use this instead (where KafkaApp is the name of the application):

<property>
  <name>dt.application.KafkaApp.operator.MessageReader.prop.initialOffset</name>
  <value>earliest</value>
</property>

<property>
  <name>dt.application.KafkaApp.operator.MessageReader.prop.topic</name>
  <value>test-topic</value>
</property>

<property>
  <name>dt.application.KafkaApp.operator.MessageReader.prop.zookeeper</name>
  <value>node21:2181</value>
</property>