Kafkaconsumer is not safe for multi-threading access

Question

I am using below code to read from Kafka topic , and process the data.

JavaDStream<Row> transformedMessages = messages.flatMap(record -> processData(record))
                .transform(new Function<JavaRDD<Row>, JavaRDD<Row>>() {
                    //JavaRDD<Row> records = ss.emptyDataFrame().toJavaRDD();
                    StructType schema = DataTypes.createStructType(fields);

                    public JavaRDD<Row> call(JavaRDD<Row> rdd) throws Exception {
                        records = rdd.union(records);
                        return rdd;
                    }
        });

       transformedMessages.foreachRDD(record -> {
            //System.out.println("Aman" +record.count());
            StructType schema = DataTypes.createStructType(fields);

            Dataset ds = ss.createDataFrame(records, schema);
            ds.createOrReplaceTempView("trades");
            System.out.println(ds.count());
            ds.show();

        });

While running the code, i am getting below exception :

Caused by: java.util.ConcurrentModificationException: KafkaConsumer is not safe for multi-threaded access
    at org.apache.kafka.clients.consumer.KafkaConsumer.acquire(KafkaConsumer.java:1624)
    at org.apache.kafka.clients.consumer.KafkaConsumer.seek(KafkaConsumer.java:1197)
    at org.apache.spark.streaming.kafka010.CachedKafkaConsumer.seek(CachedKafkaConsumer.scala:95)
    at org.apache.spark.streaming.kafka010.CachedKafkaConsumer.get(CachedKafkaConsumer.scala:69)
    at org.apache.spark.streaming.kafka010.KafkaRDD$KafkaRDDIterator.next(KafkaRDD.scala:228)
    at org.apache.spark.streaming.kafka010.KafkaRDD$KafkaRDDIterator.next(KafkaRDD.scala:194)
    at scala.collection.Iterator$$anon$12.nextCur(Iterator.scala:434)
    at scala.collection.Iterator$$anon$12.hasNext(Iterator.scala:440)
    at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:408)
    at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:408)
    at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIterator.agg_doAggregateWithoutKey$(Unknown Source)
    at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIterator.processNext(Unknown Source)
    at org.apache.spark.sql.execution.BufferedRowIterator.hasNext(BufferedRowIterator.java:43)
    at org.apache.spark.sql.execution.WholeStageCodegenExec$$anonfun$8$$anon$1.hasNext(WholeStageCodegenExec.scala:377)
    at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:408)
    at org.apache.spark.shuffle.sort.BypassMergeSortShuffleWriter.write(BypassMergeSortShuffleWriter.java:126)
    at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:96)
    at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:53)
    at org.apache.spark.scheduler.Task.run(Task.scala:99)
    at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:322)

The fact that i only have one DStream, i am not sure why i am getting this exception. I am reading from 3 partitions in a Kafka topic. I assume that the "createDirectStream" will be creating 3 consumers to read the data.

Below is the code for for KafkaConsumer, acquire method:

 private void acquire() {
        this.ensureNotClosed();
        long threadId = Thread.currentThread().getId();
        if(threadId != this.currentThread.get() && !this.currentThread.compareAndSet(-1L, threadId)) {
            throw new ConcurrentModificationException("KafkaConsumer is not safe for multi-threaded access");
        } else {
            this.refcount.incrementAndGet();
        }
    }

Answer 1

Spark 2.2.0 has a workaround using no cache. Just use spark.streaming.kafka.consumer.cache.enabled to false. Take a look on this pull request

Answer 2

This is a similar problem of java.util.ConcurrentModificationException: KafkaConsumer is not safe for multi-threaded access , you have more than one thread running with the same consumer and Kafka does not support multithreading. Also make sure you are not using spark.speculation=true as it will cause the error mentioned above.