如何使用Apache Flink读取Cassandra？

Question

我的flink程序应该为每个输入记录做一个Cassandra查找，并根据结果做一些进一步的处理。

ClusterBuilder secureCassandraSinkClusterBuilder = new ClusterBuilder() {
        @Override
        protected Cluster buildCluster(Cluster.Builder builder) {
            return builder.addContactPoints(props.getCassandraClusterUrlAll().split(","))
                    .withPort(props.getCassandraPort())
                    .withAuthProvider(new DseGSSAPIAuthProvider("HTTP"))
                    .withQueryOptions(new QueryOptions().setConsistencyLevel(ConsistencyLevel.LOCAL_QUORUM))
                    .build();
        }
    };

    for (int i=1; i<5; i++) {
        CassandraInputFormat<Tuple2<String, String>> cassandraInputFormat =
                new CassandraInputFormat<>("select * from test where id=hello" + i, secureCassandraSinkClusterBuilder);
        cassandraInputFormat.configure(null);
        cassandraInputFormat.open(null);
        Tuple2<String, String> out = new Tuple8<>();
        cassandraInputFormat.nextRecord(out);
        System.out.println(out);
    }

但问题是，每次查找需要将近10秒，换句话说，这个for循环需要50秒来执行。

我如何加快这个操作？或者，在Flink中有没有其他的方法可以查到Cassandra呢？

Answer 1

我想出了一个在使用流数据查询Cassandra时相当快的解决方案。会对有同样问题的人有用。

首先,可以用很少的代码查询Cassandra,

Session session = secureCassandraSinkClusterBuilder.getCluster().connect();
ResultSet resultSet = session.execute("SELECT * FROM TABLE");

但这样做的问题是，创建session是一个非常耗时的操作，并且每个键空间应该执行一次。您只创建一次session并将其用于所有读取查询。

public class CassandraSessionSingleton {
    private static CassandraSessionSingleton cassandraSessionSingleton = null;

    public Session session;

    private CassandraSessionSingleton(ClusterBuilder clusterBuilder) {
        Cluster cluster = clusterBuilder.getCluster();
        session = cluster.connect();
    }

    public static CassandraSessionSingleton getInstance(ClusterBuilder clusterBuilder) {
        if (cassandraSessionSingleton == null)
            cassandraSessionSingleton = new CassandraSessionSingleton(clusterBuilder);
        return cassandraSessionSingleton;
    }

}

public class SomeProcessFunction implements ProcessFunction <Object, ResultSet> {
    ClusterBuilder secureCassandraSinkClusterBuilder;

    // Constructor
    public SomeProcessFunction (ClusterBuilder secureCassandraSinkClusterBuilder) {
        this.secureCassandraSinkClusterBuilder = secureCassandraSinkClusterBuilder;
    }

    @Override
    public void  ProcessElement (Object obj) throws Exception {
        ResultSet resultSet = CassandraLookUp.cassandraLookUp("SELECT * FROM TEST", secureCassandraSinkClusterBuilder);
        return resultSet;
    }
}

注意，您可以将clusterbuilder传递给processfunction，因为它是可序列化的。现在来看cassandralookup方法，我们在该方法中执行查询。

public class CassandraLookUp {
    public static ResultSet cassandraLookUp(String query, ClusterBuilder clusterBuilder) {
        CassandraSessionSingleton cassandraSessionSingleton = CassandraSessionSingleton.getInstance(clusterBuilder);
        Session session = cassandraSessionSingleton.session;
        ResultSet resultSet = session.execute(query);
        return resultSet;
    }
}

singleton对象只在第一次运行查询时创建，之后重用相同的对象，因此查找没有延迟。