private KvReply perform(KvOperation operation) {
final int MAX_ITERATIONS = 10000;
KvReply reply = new KvReply();
for (int i = 0; i < MAX_ITERATIONS; i++) {
// So it have been processed yet, do not advance to make
// at-most-once semantic
String last = lastOperationUUID.get(operation.clientId);
if (last != null && last.compareTo(operation.uuid) == 0) {
reply.status = KvReply.Status.Success;
reply.payload = lastOperation.get(operation.clientId);
return reply;
}
int instanceId = completedOperationId + 1;
Paxos.Status status = paxos.getStatus(instanceId);
KvOperation result = null;
if (status.accepted) {
result = (KvOperation) status.value;
} else {
paxos.start(instanceId, operation);
result = waitAgreement(instanceId);
// If empty, please skip the apply operation
if (result == null) continue;
}
// Apply (result, instanceId);
apply(instanceId, result);
if (operation.uuid.compareTo(result.uuid) == 0) {
reply.status = KvReply.Status.Success;
reply.payload = lastOperation.get(operation.clientId);
return reply;
}
}
reply.status = KvReply.Status.InternalError;
return reply;
}
private void apply(int instanceId, KvOperation operation) {
if (engineInstance.containsKey(operation.key)) {
lastOperation.put(operation.clientId, engineInstance.get(operation.key));
} else {
lastOperation.put(operation.clientId, null);
}
lastOperationUUID.put(operation.clientId, operation.uuid);
if (operation.command == KvOperation.Command.PutOperation) {
engineInstance.put(operation.key, operation.value);
}
completedOperationId++;
paxos.done(instanceId);
}