kafka中的回调函数

kafka客户端中使用了很多的回调方式处理请求。基本思路是将回调函数暂存到ClientRequest中，而ClientRequest会暂存到inFlightRequests中，当返回response的时候，从inFlightRequests中读取对应的ClientRequest，并调用request中的回调函数完成处理。

inFlightRequests是请求和响应处理的桥梁.

1. 接口和抽象类

无论是producer还是consumer，回调函数类都是实现了RequestCompletionHandler接口。

public interface RequestCompletionHandler {
    public void onComplete(ClientResponse response);
}

consumer的回调函数类不但实现了RequestCompletionHandler，还继承了RequestFuture。RequestFuture是一个有状态的类，在调用中会设置响应的状态，可以持有RequestFuture的引用，用来判断请求的状态。

public class RequestFuture<T> {

    private boolean isDone = false;
    private T value;
    private RuntimeException exception;
    private List<RequestFutureListener<T>> listeners = new ArrayList<>();
    // 省略其他方法
}

2. producer

producer是在sender线程中创建的ClientRequest，如下：

private List<ClientRequest> createProduceRequests(Map<Integer, List<RecordBatch>> collated, long now) {
        List<ClientRequest> requests = new ArrayList<ClientRequest>(collated.size());
        for (Map.Entry<Integer, List<RecordBatch>> entry : collated.entrySet())
            requests.add(produceRequest(now, entry.getKey(), acks, requestTimeout, entry.getValue()));
        return requests;
}

// 创建request
 private ClientRequest produceRequest(long now, int destination, short acks, int timeout, List<RecordBatch> batches) {
        Map<TopicPartition, ByteBuffer> produceRecordsByPartition = new HashMap<TopicPartition, ByteBuffer>(batches.size());
        final Map<TopicPartition, RecordBatch> recordsByPartition = new HashMap<TopicPartition, RecordBatch>(batches.size());
        for (RecordBatch batch : batches) {
            TopicPartition tp = batch.topicPartition;
            produceRecordsByPartition.put(tp, batch.records.buffer());
            recordsByPartition.put(tp, batch);
        }
        ProduceRequest request = new ProduceRequest(acks, timeout, produceRecordsByPartition);
        RequestSend send = new RequestSend(Integer.toString(destination),
                                           this.client.nextRequestHeader(ApiKeys.PRODUCE),
                                           request.toStruct());

        // 回调函数
        RequestCompletionHandler callback = new RequestCompletionHandler() {
            public void onComplete(ClientResponse response) {
                handleProduceResponse(response, recordsByPartition, time.milliseconds());
            }
        };

        // 回调函数保存到request中, 然后request被保存到了inFlightRequests
        return new ClientRequest(now, acks != 0, send, callback);
}

在NetworkClient#poll(..)最后会处理会调用对应的回调函数

public List<ClientResponse> poll(long timeout, long now) {
        long metadataTimeout = metadataUpdater.maybeUpdate(now);
        try {
            this.selector.poll(Utils.min(timeout, metadataTimeout, requestTimeoutMs));
        } catch (IOException e) {
            log.error("Unexpected error during I/O", e);
        }

        // process completed actions
        long updatedNow = this.time.milliseconds();
        List<ClientResponse> responses = new ArrayList<>();
        handleCompletedSends(responses, updatedNow);
        handleCompletedReceives(responses, updatedNow);
        handleDisconnections(responses, updatedNow);
        handleConnections();
        handleTimedOutRequests(responses, updatedNow);

        // invoke callbacks
        for (ClientResponse response : responses) { // response中封装了request中的回调函数
            if (response.request().hasCallback()) {
                try {
                    response.request().callback().onComplete(response); //调用回调函数
                } catch (Exception e) {
                    log.error("Uncaught error in request completion:", e);
                }
            }
        }

        return responses;
    }

3. Consumer

consumer使用回调函数和producer使用方式类似，但是比producer复杂一些。前面说了Consumer的回调函数不但实现了RequestCompletionHandler，还继承了RequestFuture。

public static class RequestFutureCompletionHandler
            extends RequestFuture<ClientResponse>
            implements RequestCompletionHandler {

        @Override
        public void onComplete(ClientResponse response) {
            if (response.wasDisconnected()) {
                ClientRequest request = response.request();
                RequestSend send = request.request();
                ApiKeys api = ApiKeys.forId(send.header().apiKey());
                int correlation = send.header().correlationId();
                log.debug("Cancelled {} request {} with correlation id {} due to node {} being disconnected",
                        api, request, correlation, send.destination());
                raise(DisconnectException.INSTANCE);
            } else {
                complete(response); // 关键， complete方法会设置RequestFuture的状态
            }
        }
    }
}

public void complete(T value) { // 设置RequestFuture状态
        if (isDone)
            throw new IllegalStateException("Invalid attempt to complete a request future which is already complete");
        this.value = value;
        this.isDone = true;
        fireSuccess(); // 循环调用RequestFuture中的listeners
    }

private void fireSuccess() {
        for (RequestFutureListener<T> listener : listeners)
            listener.onSuccess(value);
    }

    private void fireFailure() {
        for (RequestFutureListener<T> listener : listeners)
            listener.onFailure(exception);
    }

与producer类似，请求被放到一个map中，不过名字是unsent。如下ConsumerNetworkClient#send(..):

public RequestFuture<ClientResponse> send(Node node,
                                              ApiKeys api,
                                              AbstractRequest request) {
        long now = time.milliseconds();
        RequestFutureCompletionHandler future = new RequestFutureCompletionHandler(); // 回调函数
        RequestHeader header = client.nextRequestHeader(api);
        RequestSend send = new RequestSend(node.idString(), header, request.toStruct());
        put(node, new ClientRequest(now, true, send, future)); // request方法哦unsent中
        return future; // 并返回回调函数类的引用
    }

在调用ConsumerNetworkClient#send(..)后又紧接着调用了Future#compose(..)。如下：

private RequestFuture<Void> sendGroupCoordinatorRequest() {
        Node node = this.client.leastLoadedNode();
        if (node == null) {
            return RequestFuture.noBrokersAvailable();
        } else {
            log.debug("Sending coordinator request for group {} to broker {}", groupId, node);
            GroupCoordinatorRequest metadataRequest = new GroupCoordinatorRequest(this.groupId);
            return client.send(node, ApiKeys.GROUP_COORDINATOR, metadataRequest) // send后返回FutureRequest，然后又调用compose方法
                    .compose(new RequestFutureAdapter<ClientResponse, Void>() {
                        @Override
                        public void onSuccess(ClientResponse response, RequestFuture<Void> future) {
                            handleGroupMetadataResponse(response, future);
                        }
                    });
        }
    }

Future#compose(..)方法又两个作用

1. 添加FutureRequest的listeners
2. 返回一个新的FutureRequest，用新FutureRequest来判断状态

public <S> RequestFuture<S> compose(final RequestFutureAdapter<T, S> adapter) {
        final RequestFuture<S> adapted = new RequestFuture<S>(); // 返回新的RequestFuture
        addListener(new RequestFutureListener<T>() { // 添加到原先FutureRequest中的listeners中
            @Override
            public void onSuccess(T value) {
                adapter.onSuccess(value, adapted); // 返回response后会调用listeners，从而会设置新的RequestFuture状态，我们就可以根据这个新的RequestFuture来判断response处理状态。
            }

            @Override
            public void onFailure(RuntimeException e) {
                adapter.onFailure(e, adapted);
            }
        });
        return adapted;
    }

所以将ClientRequest放到map中后，最终我们持有的是compose中新建的FutureRequest，如AbstractCoordinator#ensureCoordinatorReady(..):

public void ensureCoordinatorReady() {
        while (coordinatorUnknown()) {
            RequestFuture<Void> future = sendGroupCoordinatorRequest();// 最终返回compose返回的future。
            client.poll(future); // 在poll中不停的轮训future的状态

            if (future.failed()) {
                if (future.isRetriable())
                    client.awaitMetadataUpdate();
                else
                    throw future.exception();
            } else if (coordinator != null && client.connectionFailed(coordinator)) {
                coordinatorDead();
                time.sleep(retryBackoffMs);
            }

        }
    }

public void poll(RequestFuture<?> future) {
        while (!future.isDone()) // 轮训future状态，当response做相应处理会调用回调函数，从而设置future相应状态。
            poll(Long.MAX_VALUE);
    }

总结

kafka客户端中使用了大量的回调函数做请求的处理，理解回调函数很重要，附回调函数链接：

http://www.cnblogs.com/set-cookie/p/8996951.html