Using Spark Streaming

Starting with CDH 5.5, dynamic allocation is enabled by default, which means that executors are removed when idle. However, dynamic allocation is not effective in Spark Streaming. In Spark Streaming, data comes in every batch, and executors run whenever data is available. If the executor idle timeout is less than the batch duration, executors are constantly being added and removed. However, if the executor idle timeout is greater than the batch duration, executors are never removed. Therefore, Cloudera recommends that you disable dynamic allocation by setting spark.dynamicAllocation.enabled to false when running streaming applications.

This example uses Kafka to deliver a stream of words to a Python word count program.

1. Install Kafka and create a Kafka service.
2. Create a Kafka topic wordcounttopic and pass in your ZooKeeper server:

   $ kafka-topics --create --zookeeper zookeeper_server:2181 --topic wordcounttopic \
   --partitions 1 --replication-factor 1

3. Create a Kafka word count Python program adapted from the Spark Streaming example kafka_wordcount.py. This version divides the input stream into batches of 10 seconds and counts the words in each batch:

   from __future__ import print_function
   
   import sys
   
   from pyspark import SparkContext
   from pyspark.streaming import StreamingContext
   from pyspark.streaming.kafka import KafkaUtils
   
   if __name__ == "__main__":
       if len(sys.argv) != 3:
           print("Usage: kafka_wordcount.py <zk> <topic>", file=sys.stderr)
           exit(-1)
   
       sc = SparkContext(appName="PythonStreamingKafkaWordCount")
       ssc = StreamingContext(sc, 10)
   
       zkQuorum, topic = sys.argv[1:]
       kvs = KafkaUtils.createStream(ssc, zkQuorum, "spark-streaming-consumer", {topic: 1})
       lines = kvs.map(lambda x: x[1])
       counts = lines.flatMap(lambda line: line.split(" ")).map(lambda word: (word, 1)).reduceByKey(lambda a, b: a+b)
       counts.pprint()
   
       ssc.start()
       ssc.awaitTermination()

4. Submit the application using spark-submit with dynamic allocation disabled and pass in your ZooKeeper server and topic wordcounttopic. To run locally, you must specify at least two worker threads: one to receive and one to process data.

   $ spark-submit --master local[2] --conf "spark.dynamicAllocation.enabled=false" \
   --jars SPARK_HOME/lib/spark-examples.jar kafka_wordcount.py \
   zookeeper_server:2181 wordcounttopic

   In a CDH deployment, SPARK_HOME defaults to /usr/lib/spark in package installations and /opt/cloudera/parcels/CDH/lib/spark in parcel installations. In a Cloudera Manager deployment, the shells are also available from /usr/bin.

   Alternatively, you can run on YARN as follows:

   $ spark-submit --master yarn --deploy-mode client --conf "spark.dynamicAllocation.enabled=false" \
   --jars SPARK_HOME/lib/spark-examples.jar kafka_wordcount.py \
   zookeeper_server:2181 wordcounttopic

5. In another window, start a Kafka producer that publishes to wordcounttopic:

   $ kafka-console-producer --broker-list kafka_broker:9092 --topic wordcounttopic

6. In the producer window, type the following:

   hello
   hello
   hello
   hello
   hello
   hello
   gb
   gb
   gb
   gb
   gb
   gb

   Depending on how fast you type, in the Spark Streaming application window you will see output like:

   -------------------------------------------
   Time: 2016-01-06 14:18:00
   -------------------------------------------
   (u'hello', 6)
   (u'gb', 2)
   
   -------------------------------------------
   Time: 2016-01-06 14:18:10
   -------------------------------------------
   (u'gb', 4)

If the driver host for a Spark Streaming application fails, it can lose data that has been received but not yet processed. To ensure that no data is lost, you can use Spark Streaming recovery. Spark writes incoming data to HDFS as it is received and uses this data to recover state if a failure occurs.

To enable Spark Streaming recovery:

1. Set the spark.streaming.receiver.writeAheadLog.enable parameter to true in the SparkConf object.
2. Create a StreamingContext instance using this SparkConf, and specify a checkpoint directory.
3. Use the getOrCreate method in StreamingContext to either create a new context or recover from an old context from the checkpoint directory:

   from __future__ import print_function
   
   import sys
   
   from pyspark import SparkContext, SparkConf
   from pyspark.streaming import StreamingContext
   from pyspark.streaming.kafka import KafkaUtils
   
   checkpoint = "hdfs://ns1/user/systest/checkpoint"
   
   # Function to create and setup a new StreamingContext
   def functionToCreateContext():
   
     sparkConf = SparkConf()
     sparkConf.set("spark.streaming.receiver.writeAheadLog.enable", "true")
     sc = SparkContext(appName="PythonStreamingKafkaWordCount",conf=sparkConf)
     ssc = StreamingContext(sc, 10)
   
     zkQuorum, topic = sys.argv[1:]
     kvs = KafkaUtils.createStream(ssc, zkQuorum, "spark-streaming-consumer", {topic: 1})
     lines = kvs.map(lambda x: x[1])
     counts = lines.flatMap(lambda line: line.split(" ")).map(lambda word: (word, 1)).reduceByKey(lambda a, b: a+b)
     counts.pprint()
   
     ssc.checkpoint(checkpoint)   # set checkpoint directory
     return ssc
   
   if __name__ == "__main__":
     if len(sys.argv) != 3:
       print("Usage: kafka_wordcount.py <zk> <topic>", file=sys.stderr)
       exit(-1)
   
     ssc = StreamingContext.getOrCreate(checkpoint, lambda: functionToCreateContext())
     ssc.start()
     ssc.awaitTermination()
   
   

For more information, see Checkpointing.

To prevent data loss if a receiver fails, receivers must be able to replay data from the original data sources if required.

• The Kafka receiver automatically replays if the spark.streaming.receiver.writeAheadLog.enable parameter is set to true.
• The receiverless Direct Kafka DStream does not require the spark.streaming.receiver.writeAheadLog.enable parameter and can function without data loss, even without Streaming recovery.
• Both Flume receivers packaged with Spark replay the data automatically on receiver failure.

For more information, see Spark Streaming + Kafka Integration Guide and Spark Streaming + Flume Integration Guide.

If you are using authenticated Spark communication, you must perform additional configuration steps for long-running Spark Streaming jobs. See Configuring Spark on YARN for Long-Running Applications.

When using Spark Streaming with a cloud service as the underlying storage layer, use ephemeral HDFS on the cluster to store the checkpoints, instead of the cloud store such as Amazon S3 or Microsoft ADLS.