Request Ready Checklist


The cluster node maintains a ready checklist which maps a key and
a callback operate. The secret is chosen relying on the precise
standards to invoke the callback.
For instance if it must be invoked at any time when
a message from different cluster node is obtained, it may be the
Correlation Identifer of the message.
Within the case of Replicated Log it’s the
Excessive-Water Mark. The callback handles
the response and decides if the shopper request might be fulfilled.

Take into account the instance of a key-value retailer the place, knowledge is replicated
on a number of servers. Right here, Quorum can be utilized
to determine when a replication might be thought of profitable
to provoke a response to the shopper.
The cluster node then tracks the requests despatched to different cluster nodes,
and a callback is registered with every request.
Every request is marked with a Correlation Identifer,
which is used to map response to the request.
The ready checklist is then notified to invoke the callback
when the response from different cluster nodes are obtained.

For the sake of this instance, let’s name our three cluster nodes
athens, byzantium and cyrene.
The shopper connects with athens to retailer “title” as “Microservices”.
Athens replicates it on byzantium and cyrene; so it sends
a request to itself to retailer the key-value and sends
requests to each byzantium and cyrene concurrently.
To trace responses, athens creates a WriteQuorumResponseCallback
and provides it to the ready checklist for every of the requests despatched.

For each response obtained, the WriteQuorumResponseCallback is
invoked to deal with the response. It checks whether or not the required quantity
of responses have been obtained.
As soon as the response is obtained from byzantium, the quorum is reached
and the pending shopper request is accomplished.
Cyrene can reply later, however the response might be despatched to the shopper
with out ready for it.

The code appears to be like just like the pattern under:
Notice that each cluster node maintains its personal occasion of a ready checklist.
The ready checklist tracks the important thing and related callback and
shops the timestamp at which the callback was registered.
The timestamp is used to verify whether or not the callbacks must be expired
if responses have not been obtained inside the anticipated time.

public class RequestWaitingList<Key, Response> {
    non-public Map<Key, CallbackDetails> pendingRequests = new ConcurrentHashMap<>();
    public void add(Key key, RequestCallback<Response> callback) {
        pendingRequests.put(key, new CallbackDetails(callback, clock.nanoTime()));
class CallbackDetails {
    RequestCallback requestCallback;
    lengthy createTime;

    public CallbackDetails(RequestCallback requestCallback, lengthy createTime) {
        this.requestCallback = requestCallback;
        this.createTime = createTime;

    public RequestCallback getRequestCallback() {
        return requestCallback;

    public lengthy elapsedTime(lengthy now) {
        return now - createTime;
public interface RequestCallback<T> {
    void onResponse(T r);
    void onError(Throwable e);

It’s requested to deal with the response or error
as soon as the response has been obtained from the opposite cluster node.

class RequestWaitingList…

  public void handleResponse(Key key, Response response) {
      if (!pendingRequests.containsKey(key)) {
      CallbackDetails callbackDetails = pendingRequests.take away(key);


class RequestWaitingList…

  public void handleError(int requestId, Throwable e) {
      CallbackDetails callbackDetails = pendingRequests.take away(requestId);

The ready checklist can then be used to deal with quorum responses
with the implementation wanting one thing like this:

static class WriteQuorumCallback implements RequestCallback<RequestOrResponse> {
    non-public ultimate int quorum;
    non-public risky int expectedNumberOfResponses;
    non-public risky int receivedResponses;
    non-public risky int receivedErrors;
    non-public risky boolean completed;

    non-public ultimate RequestOrResponse request;
    non-public ultimate ClientConnection clientConnection;

    public WriteQuorumCallback(int totalExpectedResponses, RequestOrResponse clientRequest, ClientConnection clientConnection) {
        this.expectedNumberOfResponses = totalExpectedResponses;
        this.quorum = expectedNumberOfResponses / 2 + 1;
        this.request = clientRequest;
        this.clientConnection = clientConnection;

    public void onResponse(RequestOrResponse response) {
        if (receivedResponses == quorum && !completed) {
            completed = true;

    public void onError(Throwable t) {
        if (receivedErrors == quorum && !completed) {
            completed = true;

    non-public void respondToClient(String response) {
        clientConnection.write(new RequestOrResponse(RequestId.SetValueResponse.getId(), response.getBytes(), request.getCorrelationId()));

Each time a cluster node sends requests to different nodes,
it provides a callback to the ready checklist mapping with the Correlation Identifer
of the request despatched.

class ClusterNode…

  non-public void handleSetValueClientRequestRequiringQuorum(Checklist<InetAddressAndPort> replicas, RequestOrResponse request, ClientConnection clientConnection) {
      int totalExpectedResponses = replicas.measurement();
      RequestCallback requestCallback = new WriteQuorumCallback(totalExpectedResponses, request, clientConnection);
      for (InetAddressAndPort duplicate : replicas) {
          int correlationId = nextRequestId();
          requestWaitingList.add(correlationId, requestCallback);
          attempt {
              SocketClient shopper = new SocketClient(duplicate);
              shopper.sendOneway(new RequestOrResponse(RequestId.SetValueRequest.getId(), request.getMessageBodyJson(), correlationId, listenAddress));
          } catch (IOException e) {
              requestWaitingList.handleError(correlationId, e);

As soon as the response is obtained, the ready checklist is requested to deal with it:

class ClusterNode…

  non-public void handleSetValueResponse(RequestOrResponse response) {
      requestWaitingList.handleResponse(response.getCorrelationId(), response);

The ready checklist will then invoke the related WriteQuorumCallback.
The WriteQuorumCallback occasion verifies if
the quorum responses have been obtained and invokes the callback
to answer the shopper.

Expiring Lengthy Pending Requests

Typically, responses from the opposite cluster nodes are
delayed. In these cases the ready checklist typically has
a mechanism to run out requests after a timeout:

class RequestWaitingList…

  non-public SystemClock clock;
  non-public ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
  non-public lengthy expirationIntervalMillis = 2000;
  public RequestWaitingList(SystemClock clock) {
      this.clock = clock;
      executor.scheduleWithFixedDelay(this::expire, expirationIntervalMillis, expirationIntervalMillis, MILLISECONDS);

  non-public void expire() {
      lengthy now = clock.nanoTime();
      Checklist<Key> expiredRequestKeys = getExpiredRequestKeys(now); -> {
          CallbackDetails request = pendingRequests.take away(expiredRequestKey);
          request.requestCallback.onError(new TimeoutException("Request expired"));

  non-public Checklist<Key> getExpiredRequestKeys(lengthy now) {
      return pendingRequests.entrySet().stream().filter(entry -> entry.getValue().elapsedTime(now) > expirationIntervalMillis).map(e -> e.getKey()).gather(Collectors.toList());

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