如何高效地将数据从flink管道写入redis

Question

我正在 Apache flink 中构建管道 sql api。管道进行简单的投影查询。但是，我需要在查询之前和查询之后再写一次元组（恰好是每个元组中的一些元素）。事实证明，我用来写入 redis 的代码严重降低了性能。即 flink 以非常小的数据速率产生背压。我的代码有什么问题，我该如何改进。请有任何建议。

当我停止写入redis前后，性能都非常出色。这是我的管道代码：

public class QueryExample {
    public static Long throughputCounterAfter=new Long("0");
    public static void main(String[] args) {
        int k_partitions = 10;
        reamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
        env.setStreamTimeCharacteristic(TimeCharacteristic.EventTime);
        env.setParallelism(5 * 32);
        Properties props = new Properties();
        props.setProperty("zookeeper.connect", "zookeeper-node-01:2181");
        props.setProperty("bootstrap.servers", "kafka-node-01:9092,kafka-node-02:9092,kafka-node-03:9092");
        // not to be shared with another job consuming the same topic
        props.setProperty("group.id", "flink-group");
        props.setProperty("enable.auto.commit","false");
        FlinkKafkaConsumer011<String> purchasesConsumer=new FlinkKafkaConsumer011<String>("purchases",
                new SimpleStringSchema(),
                props);

        DataStream<String> purchasesStream = env
                .addSource(purchasesConsumer)
                .setParallelism(Math.min(5 * 32, k_partitions));
        DataStream<Tuple4<Integer, Integer, Integer, Long>> purchaseWithTimestampsAndWatermarks =
                purchasesStream
                        .flatMap(new PurchasesParser())
                        .assignTimestampsAndWatermarks(new BoundedOutOfOrdernessTimestampExtractor<Tuple4<Integer, Integer, Integer, Long>>(Time.seconds(10)) {

                            @Override
                            public long extractTimestamp(Tuple4<Integer, Integer, Integer, Long> element) {
                                return element.getField(3);
                            }
                        });

        Table purchasesTable = tEnv.fromDataStream(purchaseWithTimestampsAndWatermarks, "userID, gemPackID,price, rowtime.rowtime");
        tEnv.registerTable("purchasesTable", purchasesTable);

        purchaseWithTimestampsAndWatermarks.flatMap(new WriteToRedis());
        Table result = tEnv.sqlQuery("SELECT  userID, gemPackID, rowtime from purchasesTable");
        DataStream<Tuple2<Boolean, Row>> queryResultAsDataStream = tEnv.toRetractStream(result, Row.class);
        queryResultAsDataStream.flatMap(new WriteToRedis());
        try {
            env.execute("flink SQL");

        } catch (Exception e) {
            e.printStackTrace();
        }
    }




/**
 * write to redis
 */
public static class WriteToRedis extends RichFlatMapFunction<Tuple4<Integer, Integer, Integer, Long>, String> {
    RedisReadAndWrite redisReadAndWrite;

    @Override
    public void open(Configuration parameters) {
        LOG.info("Opening connection with Jedis to {}", "redis");
        this.redisReadAndWrite = new RedisReadAndWrite("redis",6379);

    }

    @Override
    public void flatMap(Tuple4<Integer, Integer, Integer, Long> input, Collector<String> out) throws Exception {
        this.redisReadAndWrite.write(input.f0+":"+input.f3+"","time_seen", TimeUnit.NANOSECONDS.toMillis(System.nanoTime())+"");
    }
}
}


public class RedisReadAndWrite {
    private Jedis flush_jedis;

    public RedisReadAndWrite(String redisServerName , int port) {
        flush_jedis=new Jedis(redisServerName,port);
    }


    public void write(String key,String field, String value) {
        flush_jedis.hset(key,field,value);

    }
}

附加部分：我尝试了第二个实现过程函数，该函数使用 Jedis 批量写入 toredis。但是我收到以下错误。 org.apache.flink.runtime.client.JobExecutionException: redis.clients.jedis.exceptions.JedisConnectionException: java.net.SocketException: 套接字未连接。我试图将批处理消息的数量减少，但一段时间后仍然出现错误。

这里是进程函数的实现：

/** * 使用进程函数写入redis */

public static class WriteToRedisAfterQueryProcessFn extends ProcessFunction<Tuple2<Boolean, Row>, String> {
    Long timetoFlush;
    @Override
    public void open(Configuration parameters) {
        flush_jedis=new Jedis("redis",6379,1800);
        p = flush_jedis.pipelined();
        this.timetoFlush=System.currentTimeMillis()-initialTime;
    }

    @Override
    public void processElement(Tuple2<Boolean, Row> input, Context context, Collector<String> collector) throws Exception {
        p.hset(input.f1.getField(0)+":"+new Instant(input.f1.getField(2)).getMillis()+"","time_updated",TimeUnit.NANOSECONDS.toMillis(System.nanoTime())+"");
        throughputAccomulationcount++;
        System.out.println(throughputAccomulationcount);
        if(throughputAccomulationcount==50000){
            throughputAccomulationcount=0L;
            p.sync();
        }
    }
}

Answer 1

通常在写入外部服务时，这会成为 Flink 工作流的瓶颈。提高性能的最简单方法是通过 AsyncFunction. See this documentation 对工作流的该部分进行多线程处理以获取更多详细信息。

--肯

Answer 2

您遇到的性能不佳无疑是因为您为每次写入向 redis 发出同步请求。 @kkrugler 已经提到了 async i/o，这是针对这种情况的常见补救措施。那将需要切换到支持异步操作的 redis 客户端之一。

使用外部服务时常用的另一种解决方案是将写入组批处理在一起。对于绝地武士，您可以使用 pipelining。例如，您可以将 WriteToRedis RichFlatMapFunction 替换为 ProcessFunction，该 ProcessFunction 以一定大小的批次对 Redis 进行流水线写入，并根据需要依赖超时来刷新其缓冲区。你可以使用 Flink 的 ListState 作为缓冲区。

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