flink并行度与kafa分区(partition)设置

flink Connector-kafka

//TODO文章 connector系统文章，等待编写

原理

采用取模运算；平衡 kafka partition与并行度关系。

取模运算原理见取模运算理解

计算公式

kafkaPartition mod 并行度总数 = 分配到并行度中的partition

例子：partition 个数为 6；并行度为 3

图示如下：

partition取模.png

如上分析，如果并行度 大于 partition总数，那么多余的并行度分配不到 partition，该并行度也就不会有数据
如下图：3个kafka partition，flink设置4个并行度为例，编号为3的并行度将获取不到数据

partition取模1.png

源码分析

由于源码比较多，为了代码便于阅读，只抽取关键的代码

FlinkKafkaConsumerBase

public abstract class FlinkKafkaConsumerBase<T> extends RichParallelSourceFunction<T> implements
        CheckpointListener,
        ResultTypeQueryable<T>,
        CheckpointedFunction {
            
    /** 
      * The partition discoverer, used to find new partitions.
      * 分区 discover
      */
    private transient volatile AbstractPartitionDiscoverer partitionDiscoverer;
            
    /** 
      * Describes whether we are discovering partitions for fixed topics or a topic pattern.
      * topic 描述
      */
    private final KafkaTopicsDescriptor topicsDescriptor;
    
    //构造器
    public FlinkKafkaConsumerBase(
            List<String> topics,
            Pattern topicPattern,
            KafkaDeserializationSchema<T> deserializer,
            long discoveryIntervalMillis,
            boolean useMetrics) {
         // topicsDescriptor 创建
        this.topicsDescriptor = new KafkaTopicsDescriptor(topics, topicPattern);
        //...
    }
    
    @Override
    public void open(Configuration configuration) throws Exception {
        // create the partition discoverer
        this.partitionDiscoverer = createPartitionDiscoverer(
                topicsDescriptor,
                getRuntimeContext().getIndexOfThisSubtask(),//当前并行度 id
                getRuntimeContext().getNumberOfParallelSubtasks());//所有并行度总数
        this.partitionDiscoverer.open();
         //获取当前并行度 分配的 kafka partitions
         final List<KafkaTopicPartition> allPartitions = partitionDiscoverer.discoverPartitions();
         //...
    }
            
    /**
     * Creates the partition discoverer that is used to find new partitions for this subtask.
     *
     * @param topicsDescriptor Descriptor that describes whether we are discovering partitions for fixed topics or a topic pattern.
     * @param indexOfThisSubtask The index of this consumer subtask.
     * @param numParallelSubtasks The total number of parallel consumer subtasks.
     *
     * @return The instantiated partition discoverer
     */
    protected abstract AbstractPartitionDiscoverer createPartitionDiscoverer(
            KafkaTopicsDescriptor topicsDescriptor,
            int indexOfThisSubtask,
            int numParallelSubtasks);
        
}

AbstractPartitionDiscoverer : 该类为抽象类，有些方法实现在各个版本的kafka实现类中

public abstract class AbstractPartitionDiscoverer {
    
    /**
      * 当前并行度 id
      * Index of the consumer subtask that this partition discoverer belongs to.
      */
    private final int indexOfThisSubtask;

    /** 
      * 所有并行度总数
      * The total number of consumer subtasks.
      */
    private final int numParallelSubtasks;
    
    public List<KafkaTopicPartition> discoverPartitions() throws WakeupException, ClosedException {
    
        //获取 所有 partitions
        List<KafkaTopicPartition> newDiscoveredPartitions = 
            //各版本的kafka实现类中
            getAllPartitionsForTopics(topicsDescriptor.getFixedTopics());
        //移除掉不属于该并行度 中的 partition
        Iterator<KafkaTopicPartition> iter = newDiscoveredPartitions.iterator();
        KafkaTopicPartition nextPartition;
        while (iter.hasNext()) {
            nextPartition = iter.next();
            if (!setAndCheckDiscoveredPartition(nextPartition)) {
                iter.remove();
            }
        }
        
    }
    
    //判断是否是当前并行度的 任务
    public boolean setAndCheckDiscoveredPartition(KafkaTopicPartition partition) {
        if (isUndiscoveredPartition(partition)) {
            discoveredPartitions.add(partition);

            return KafkaTopicPartitionAssigner.assign(partition, numParallelSubtasks) == indexOfThisSubtask;
        }

        return false;
    }
}

KafkaTopicPartitionAssigner

public class KafkaTopicPartitionAssigner {
    //取模算法
    public static int assign(KafkaTopicPartition partition, int numParallelSubtasks) {
        int startIndex = ((partition.getTopic().hashCode() * 31) & 0x7FFFFFFF) % numParallelSubtasks;
        return (startIndex + partition.getPartition()) % numParallelSubtasks;
    }
}