spark-streaming：如何将流数据输出到cassandra

Question

I am reading kafka streaming messages using spark-streaming. Now I want to set Cassandra as my output. I have created a table in cassandra "test_table" with columns "key:text primary key" and "value:text" I have mapped the data successfully into JavaDStream<Tuple2<String,String>> data like this:

JavaSparkContext sc = new JavaSparkContext("local[4]", "SparkStream",conf);
JavaStreamingContext jssc = new JavaStreamingContext(sc, new Duration(3000));

JavaPairReceiverInputDStream<String, String> messages = KafkaUtils.createStream(jssc, args[0], args[1], topicMap );
JavaDStream<Tuple2<String,String>> data = messages.map(new Function< Tuple2<String,String>, Tuple2<String,String> >() 
{
    public Tuple2<String,String> call(Tuple2<String, String> message)
    {
        return new Tuple2<String,String>( message._1(), message._2() );
    }
}
);  

Then I have created a List:

List<TestTable> list = new ArrayList<TestTable>();

where TestTable is my custom class having the same structure as my Cassandra table, with members "key" and "value":

class TestTable
{
    String key;
    String val;

    public TestTable() {}

    public TestTable(String k, String v)
    {
        key=k;
        val=v;
    }

    public String getKey(){
        return key;
    }

    public void setKey(String k){
        key=k;
    }

    public String getVal(){
        return val;
    }

    public void setVal(String v){
        val=v;
    }

    public String toString(){
        return "Key:"+key+",Val:"+val;
    }
}

Please suggest a way how to I add the data from JavaDStream<Tuple2<String,String>> data into the List<TestTable> list . I am doing this so that I can subsequently use

JavaRDD<TestTable> rdd = sc.parallelize(list); 
javaFunctions(rdd, TestTable.class).saveToCassandra("testkeyspace", "test_table"); 

to save the RDD data into Cassandra.

I had tried coding this way:

messages.foreachRDD(new Function<Tuple2<String,String>, String>()
                        {
                            public List<TestTable> call(Tuple2<String,String> message)
                            {
                                String k = message._1();
                                String v = message._2();
                                TestTable tbl = new TestTable(k,v);
                                list.put(tbl);
                            }
                        }
                    );

but seems some type mis-match happenning. Please help.

Answer 1

Assuming that the intention of this program is to save the streaming data from kafka into Cassandra, it's not necessary to dump the JavaDStream<Tuple2<String,String>> data into a List<TestTable> list.

The Spark-Cassandra connector by DataStax supports this functionality directly through the Spark Streaming extensions .

It should be sufficient to use such extensions on the JavaDStream :

javaFunctions(data).writerBuilder("testkeyspace", "test_table", mapToRow(TestTable.class)).saveToCassandra();

instead of draining data on an intermediary list.