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Mounted Partitions

Partitions have to be mapped to cluster nodes.
The mapping additionally must be saved and made accessible to the shoppers.
It is common to make use of a devoted Constant Core; this
handles each. The devoted Constant Core acts as a coordinator which
retains monitor of all nodes within the cluster and maps partitions to nodes.
It additionally shops the mapping in a fault tolerant means through the use of a
Replicated Log. The grasp cluster in YugabyteDB
or controller implementation in Kafka are each
good examples of this.

Peer-to-peer programs like Akka or Hazelcast
additionally want a selected cluster node to behave as an coordinator.
They use Emergent Chief because the coordinator.

Methods like [kubernetes] use a generic
Constant Core like [etcd].
They should elect one of many cluster nodes to play the position of
coordinator as mentioned right here.

Monitoring Cluster Membership

Every cluster node will register itself with the consistent-core.
It additionally periodically sends a HeartBeat to permit
the Constant Core detect node failures.

class KVStore…

  public void begin() {
      community.sendAndReceive(coordLeader, new RegisterClusterNodeRequest(generateMessageId(), listenAddress));
          community.ship(coordLeader, new HeartbeatMessage(generateMessageId(), listenAddress));
      }, 200, 200, TimeUnit.MILLISECONDS);


The coordinator handles the registration after which shops member info.

class ClusterCoordinator…

  ReplicatedLog replicatedLog;
  Membership membership = new Membership();
  TimeoutBasedFailureDetector failureDetector = new TimeoutBasedFailureDetector(Period.ofMillis(TIMEOUT_MILLIS));

  non-public void handleRegisterClusterNodeRequest(Message message) {
      logger.information("Registering node " + message.from);
      CompletableFuture completableFuture = registerClusterNode(message.from);
      completableFuture.whenComplete((response, error) -> {
          logger.information("Sending register response to node " + message.from);
          community.ship(message.from, new RegisterClusterNodeResponse(message.messageId, listenAddress));

  public CompletableFuture registerClusterNode(InetAddressAndPort deal with) {
      return replicatedLog.suggest(new RegisterClusterNodeCommand(deal with));

When a registration is dedicated within the Replicated Log,
the membership will likely be up to date.

class ClusterCoordinator…

  non-public void applyRegisterClusterNodeEntry(RegisterClusterNodeCommand command) {

class ClusterCoordinator…

  non-public void updateMembership(InetAddressAndPort deal with) {
      membership = membership.addNewMember(deal with);
      failureDetector.heartBeatReceived(deal with);

The coordinator maintains an inventory of all nodes which are a part of the cluster:

class Membership…

  public class Membership {
      Checklist<Member> liveMembers = new ArrayList<>();
      Checklist<Member> failedMembers = new ArrayList<>();
      public boolean isFailed(InetAddressAndPort deal with) {
          return failedMembers.stream().anyMatch(m -> m.deal with.equals(deal with));

class Member…

  public class Member implements Comparable<Member> {
      InetAddressAndPort deal with;
      MemberStatus standing;

The coordinator will detect cluster node failures utilizing a
mechanism just like
If a cluster node stops sending the heartbeat, the node
will likely be marked as failed.

class ClusterCoordinator…

  public void onBecomingLeader() {
      scheduledTask = executor.scheduleWithFixedDelay(this::checkMembership,

  non-public void checkMembership() {
      Checklist<Member> failedMembers = getFailedMembers();
      if (!failedMembers.isEmpty()) {
          replicatedLog.suggest(new MemberFailedCommand(failedMembers));

  non-public Checklist<Member> getFailedMembers() {
      Checklist<Member> liveMembers = membership.getLiveMembers();
      return liveMembers.stream()
              .filter(m -> failureDetector.isMonitoring(m.getAddress()) && !failureDetector.isAlive(m.getAddress()))

An instance situation

Take into account that there are three knowledge servers athens, byzantium and cyrene.
Contemplating there are 9 partitions, the circulate seems to be like following.

The consumer can then use the partition desk to map a given key
to a selected cluster node.

Now a brand new cluster node – ‘ephesus’ – is added to the cluster.
The admin triggers a reassignment and the coordinator
checks which nodes are underloaded by checking the partition desk.
It figures out that ephesus is the node which is underloaded,
and decides to allocate partition 7 to it, transferring it from athens.
The coordinator shops the migrations after which sends the
request to athens to maneuver partition 7 to ephesus.
As soon as the migration is full, athens lets the coordinator know.
The coordinator then updates the partition desk.

Assigning Partitions To Cluster Nodes

The coordinator assigns partitions to cluster nodes that are recognized at
that cut-off date. If it is triggered each time a brand new cluster node is added,
it would map partitions too early till the cluster reaches a steady state.
That is why the coordinator ought to be configured to attend till
the cluster reaches a minimal dimension.

The primary time the partition project is completed, it may possibly merely
be carried out in a spherical robin style.

class ClusterCoordinator…

  CompletableFuture assignPartitionsToClusterNodes() {
      if (!minimumClusterSizeReached()) {
          return CompletableFuture.failedFuture(new NotEnoughClusterNodesException(MINIMUM_CLUSTER_SIZE));
      return initializePartitionAssignment();

  non-public boolean minimumClusterSizeReached() {
      return membership.getLiveMembers().dimension() >= MINIMUM_CLUSTER_SIZE;
  non-public CompletableFuture initializePartitionAssignment() {
      partitionAssignmentStatus = PartitionAssignmentStatus.IN_PROGRESS;
      PartitionTable partitionTable = arrangePartitions();
      return replicatedLog.suggest(new PartitiontableCommand(partitionTable));

  public PartitionTable arrangePartitions() {
      PartitionTable partitionTable = new PartitionTable();
      Checklist<Member> liveMembers = membership.getLiveMembers();
      for (int partitionId = 1; partitionId <= noOfPartitions; partitionId++) {
          int index = partitionId % liveMembers.dimension();
          Member member = liveMembers.get(index);
          partitionTable.addPartition(partitionId, new PartitionInfo(partitionId, member.getAddress(), PartitionStatus.ASSIGNED));
      return partitionTable;

The replication log makes the partition desk persistent.

class ClusterCoordinator…

  PartitionTable partitionTable;
  PartitionAssignmentStatus partitionAssignmentStatus = PartitionAssignmentStatus.UNASSIGNED;

  non-public void applyPartitionTableCommand(PartitiontableCommand command) {
      this.partitionTable = command.partitionTable;
      partitionAssignmentStatus = PartitionAssignmentStatus.ASSIGNED;
      if (isLeader()) {

As soon as the partition project is continued, the coordinator
sends messages to all cluster nodes to inform every node which partitions
it now owns.

class ClusterCoordinator…

  Checklist<Integer> pendingPartitionAssignments = new ArrayList<>();

  non-public void sendMessagesToMembers(PartitionTable partitionTable) {
      Map<Integer, PartitionInfo> partitionsTobeHosted = partitionTable.getPartitionsTobeHosted();
      partitionsTobeHosted.forEach((partitionId, partitionInfo) -> {
          HostPartitionMessage message = new HostPartitionMessage(requestNumber++, this.listenAddress, partitionId);
          logger.information("Sending host partition message to " + partitionInfo.hostedOn + " partitionId=" + partitionId);
          scheduler.execute(new RetryableTask(partitionInfo.hostedOn, community, this, partitionId, message));

The controller will hold making an attempt to achieve nodes repeatedly till
its message is profitable.

class RetryableTask…

  static class RetryableTask implements Runnable {
      Logger logger = LogManager.getLogger(RetryableTask.class);
      InetAddressAndPort deal with;
      Community community;
      ClusterCoordinator coordinator;
      Integer partitionId;
      int try;
      non-public Message message;

      public RetryableTask(InetAddressAndPort deal with, Community community, ClusterCoordinator coordinator, Integer partitionId, Message message) {
          this.deal with = deal with;
          this.community = community;
          this.coordinator = coordinator;
          this.partitionId = partitionId;
          this.message = message;

      public void run() {
          attempt {
              //cease making an attempt if the node is failed.
              if (coordinator.isSuspected(deal with)) {
              logger.information("Sending " + message + " to=" + deal with);
              community.ship(deal with, message);
          } catch (Exception e) {
              logger.error("Error making an attempt to ship ");

      non-public void scheduleWithBackOff() {
          scheduler.schedule(this, getBackOffDelay(try), TimeUnit.MILLISECONDS);

      non-public lengthy getBackOffDelay(int try) {
          lengthy baseDelay = (lengthy) Math.pow(2, try);
          lengthy jitter = randomJitter();
          return baseDelay + jitter;

      non-public lengthy randomJitter() {
          int i = new Random(1).nextInt();
          i = i < 0 ? i * -1 : i;
          lengthy jitter = i % 50;
          return jitter;


When cluster node receives the request to create the partition,
it creates one with the given partition id.
If we think about this occurring inside a easy key-value retailer,
its implementation will look one thing like this:

class KVStore…

  Map<Integer, Partition> allPartitions = new ConcurrentHashMap<>();
  non-public void handleHostPartitionMessage(Message message) {
      Integer partitionId = ((HostPartitionMessage) message).getPartitionId();
      logger.information("Including partition " + partitionId + " to " + listenAddress);
      community.ship(message.from, new HostPartitionAcks(message.messageId, this.listenAddress, partitionId));

  public void addPartitions(Integer partitionId) {
      allPartitions.put(partitionId, new Partition(partitionId));


class Partition…

  SortedMap<String, String> kv = new TreeMap<>();
  non-public Integer partitionId;

As soon as the coordinator receives the message that the partition
has been efficiently created,
it persists it within the replicated log and updates the partition standing to be on-line.

class ClusterCoordinator…

  non-public void handleHostPartitionAck(Message message) {
      int partitionId = ((HostPartitionAcks) message).getPartitionId();
      pendingPartitionAssignments.take away(Integer.valueOf(partitionId));
      logger.information("Obtained host partition ack from " + message.from + " partitionId=" + partitionId + " pending=" + pendingPartitionAssignments);
      CompletableFuture future = replicatedLog.suggest(new UpdatePartitionStatusCommand(partitionId, PartitionStatus.ONLINE));
      future.be a part of();

As soon as the Excessive-Water Mark is reached,
and the report is utilized, the partition’s standing will likely be up to date.

class ClusterCoordinator…

  non-public void updateParitionStatus(UpdatePartitionStatusCommand command) {
      logger.information("Altering standing for " + command.partitionId + " to " + command.standing);
      partitionTable.updateStatus(command.partitionId, command.standing);
Consumer Interface

If we once more take into account the instance of a easy key and worth retailer,
if a consumer must retailer or get a price for a selected key,
it may possibly accomplish that by following these steps:

  • The consumer applies the hash perform to the important thing and finds
    the related partition based mostly on the whole variety of partitions.
  • The consumer will get the partition desk from the coordinator
    and finds the cluster node that’s internet hosting the partition.
    The consumer additionally periodically refreshes the partition desk.

Purchasers fetching a partition desk from the coordinator can
rapidly result in bottlenecks,
particularly if all requests are being served by a
single coordinator chief. That’s the reason it’s common apply to
hold metadata accessible on all cluster nodes.
The coordinator can both push metadata to cluster nodes,
or cluster nodes can pull it from the coordinator.
Purchasers can then join with any cluster node to refresh
the metadata.

That is typically applied contained in the consumer library supplied by the important thing worth retailer,
or by consumer request dealing with (which occurs on the cluster nodes.)

class Consumer…

  public void put(String key, String worth) throws IOException {
      Integer partitionId = findPartition(key, noOfPartitions);
      InetAddressAndPort nodeAddress = getNodeAddressFor(partitionId);
      sendPutMessage(partitionId, nodeAddress, key, worth);

  non-public InetAddressAndPort getNodeAddressFor(Integer partitionId) {
      PartitionInfo partitionInfo = partitionTable.getPartition(partitionId);
      InetAddressAndPort nodeAddress = partitionInfo.getAddress();
      return nodeAddress;

  non-public void sendPutMessage(Integer partitionId, InetAddressAndPort deal with, String key, String worth) throws IOException {
      PartitionPutMessage partitionPutMessage = new PartitionPutMessage(partitionId, key, worth);
      SocketClient socketClient = new SocketClient(deal with);
      socketClient.blockingSend(new RequestOrResponse(RequestId.PartitionPutKV.getId(),
  public String get(String key) throws IOException {
      Integer partitionId = findPartition(key, noOfPartitions);
      InetAddressAndPort nodeAddress = getNodeAddressFor(partitionId);
      return sendGetMessage(partitionId, key, nodeAddress);

  non-public String sendGetMessage(Integer partitionId, String key, InetAddressAndPort deal with) throws IOException {
      PartitionGetMessage partitionGetMessage = new PartitionGetMessage(partitionId, key);
      SocketClient socketClient = new SocketClient(deal with);
      RequestOrResponse response = socketClient.blockingSend(new RequestOrResponse(RequestId.PartitionGetKV.getId(), JsonSerDes.serialize(partitionGetMessage)));
      PartitionGetResponseMessage partitionGetResponseMessage = JsonSerDes.deserialize(response.getMessageBodyJson(), PartitionGetResponseMessage.class);
      return partitionGetResponseMessage.getValue();
Shifting partitions to newly added members

When new nodes are added to a cluster, some partitions may be moved to
different nodes. This may be carried out mechanically as soon as a brand new cluster node is added.
However it may possibly contain plenty of knowledge being moved throughout the cluster node,
which is why an administrator will usually set off the repartitioning.
One easy technique to do that is to calculate the common variety of partitions
every node ought to host after which transfer the extra partitions
to the brand new node.
For instance, if the variety of partitions is 30 and there are three current nodes
within the cluster, every node ought to host 10 partitions.
If a brand new node is added, the common per node is about 7. The coordinator
will due to this fact attempt to transfer three partitions from every cluster node
to the brand new one.

class ClusterCoordinator…

  Checklist<Migration> pendingMigrations = new ArrayList<>();

  boolean reassignPartitions() {
      if (partitionAssignmentInProgress()) {
          logger.information("Partition project in progress");
          return false;
      Checklist<Migration> migrations = repartition(this.partitionTable);
      CompletableFuture proposalFuture = replicatedLog.suggest(new MigratePartitionsCommand(migrations));
      proposalFuture.be a part of();
      return true;
public Checklist<Migration> repartition(PartitionTable partitionTable) {
    int averagePartitionsPerNode = getAveragePartitionsPerNode();
    Checklist<Member> liveMembers = membership.getLiveMembers();
    var overloadedNodes = partitionTable.getOverloadedNodes(averagePartitionsPerNode, liveMembers);
    var underloadedNodes = partitionTable.getUnderloadedNodes(averagePartitionsPerNode, liveMembers);

    var migrations = tryMovingPartitionsToUnderLoadedMembers(averagePartitionsPerNode, overloadedNodes, underloadedNodes);
    return migrations;

non-public Checklist<Migration> tryMovingPartitionsToUnderLoadedMembers(int averagePartitionsPerNode,
                                                                Map<InetAddressAndPort, PartitionList> overloadedNodes,
                                                                Map<InetAddressAndPort, PartitionList> underloadedNodes) {
    Checklist<Migration> migrations = new ArrayList<>();
    for (InetAddressAndPort member : overloadedNodes.keySet()) {
        var partitions = overloadedNodes.get(member);
        var toMove = partitions.subList(averagePartitionsPerNode, partitions.getSize());
        overloadedNodes.put(member, partitions.subList(0, averagePartitionsPerNode));
        ArrayDeque<Integer> moveQ = new ArrayDeque<Integer>(toMove.partitionList());
        whereas (!moveQ.isEmpty() && nodeWithLeastPartitions(underloadedNodes, averagePartitionsPerNode).isPresent()) {
            assignToNodesWithLeastPartitions(migrations, member, moveQ, underloadedNodes, averagePartitionsPerNode);
        if (!moveQ.isEmpty()) {
    return migrations;

int getAveragePartitionsPerNode() {
    return noOfPartitions / membership.getLiveMembers().dimension();

The coordinator will persist the computed migrations within the replicated log
after which ship requests to maneuver partitions throughout the cluster nodes.

non-public void applyMigratePartitionCommand(MigratePartitionsCommand command) {
    logger.information("Dealing with partition migrations " + command.migrations);
    for (Migration migration : command.migrations) {
        RequestPartitionMigrationMessage message = new RequestPartitionMigrationMessage(requestNumber++, this.listenAddress, migration);
        if (isLeader()) {
            scheduler.execute(new RetryableTask(migration.fromMember, community, this, migration.getPartitionId(), message));

When a cluster node receives a request emigrate, it would mark
the partition as migrating.
This stops any additional modifications to the partition.
It can then ship your complete partition knowledge to the goal node.

class KVStore…

  non-public void handleRequestPartitionMigrationMessage(RequestPartitionMigrationMessage message) {
      Migration migration = message.getMigration();
      Integer partitionId = migration.getPartitionId();
      InetAddressAndPort toServer = migration.getToMember();
      if (!allPartitions.containsKey(partitionId)) {
          return;// The partition just isn't accessible with this node.
      Partition partition = allPartitions.get(partitionId);
      community.ship(toServer, new MovePartitionMessage(requestNumber++, this.listenAddress, toServer, partition));

The cluster node that receives the request will add
the brand new partition to itself and
return an acknowledgement.

class KVStore…

  non-public void handleMovePartition(Message message) {
      MovePartitionMessage movePartitionMessage = (MovePartitionMessage) message;
      Partition partition = movePartitionMessage.getPartition();
      allPartitions.put(partition.getId(), partition);
      community.ship(message.from, new PartitionMovementComplete(message.messageId, listenAddress,
              new Migration(movePartitionMessage.getMigrateFrom(), movePartitionMessage.getMigrateTo(),  partition.getId())));

The cluster node beforehand owned the partition will then
ship the migration full message
to the cluster coordinator.

class KVStore…

  non-public void handlePartitionMovementCompleteMessage(PartitionMovementComplete message) {
      allPartitions.take away(message.getMigration().getPartitionId());
      community.ship(coordLeader, new MigrationCompleteMessage(requestNumber++, listenAddress,

The cluster coordinator will then mark the migration as full.
The change will likely be saved within the replicated log.

class ClusterCoordinator…

  non-public void handleMigrationCompleteMessage(MigrationCompleteMessage message) {
      MigrationCompleteMessage migrationCompleteMessage = message;
      CompletableFuture suggest = replicatedLog.suggest(new MigrationCompletedCommand(message.getMigration()));
      suggest.be a part of();

class ClusterCoordinator…

  non-public void applyMigrationCompleted(MigrationCompletedCommand command) {
      pendingMigrations.take away(command.getMigration());
      logger.information("Accomplished migration " + command.getMigration());
      logger.information("pendingMigrations = " + pendingMigrations);

class PartitionTable…

  public void migrationCompleted(Migration migration) {
      this.addPartition(migration.partitionId, new PartitionInfo(migration.partitionId, migration.toMember, ClusterCoordinator.PartitionStatus.ONLINE));


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