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From hanm <...@git.apache.org>
Subject [GitHub] zookeeper pull request #628: ZOOKEEPER-3140: Allow Followers to host Observe...
Date Tue, 06 Nov 2018 00:37:02 GMT
Github user hanm commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/628#discussion_r230965386
  
    --- Diff: zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/ObserverMaster.java
---
    @@ -0,0 +1,514 @@
    +/**
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you under the Apache License, Version 2.0 (the
    + * "License"); you may not use this file except in compliance
    + * with the License.  You may obtain a copy of the License at
    + *
    + *     http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.zookeeper.server.quorum;
    +
    +import org.apache.zookeeper.jmx.MBeanRegistry;
    +import org.apache.zookeeper.server.Request;
    +import org.apache.zookeeper.server.ZKDatabase;
    +
    +import java.io.BufferedInputStream;
    +import java.io.ByteArrayInputStream;
    +import java.io.DataInputStream;
    +import java.io.IOException;
    +import java.net.ServerSocket;
    +import java.net.Socket;
    +import java.net.SocketAddress;
    +import java.util.ArrayList;
    +import java.util.Arrays;
    +import java.util.Collections;
    +import java.util.Iterator;
    +import java.util.Set;
    +import java.util.concurrent.ConcurrentHashMap;
    +import java.util.concurrent.ConcurrentLinkedQueue;
    +import java.util.concurrent.Executors;
    +import java.util.concurrent.ScheduledExecutorService;
    +import java.util.concurrent.TimeUnit;
    +import java.util.concurrent.atomic.AtomicLong;
    +
    +import org.apache.zookeeper.server.quorum.auth.QuorumAuthServer;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +/**
    + * Used by Followers to host Observers. This reduces the network load on the Leader process
by pushing
    + * the responsibility for keeping Observers in sync off the leading peer.
    + *
    + * It is expected that Observers will continue to perform the initial vetting of clients
and requests.
    + * Observers send the request to the follower where it is received by an ObserverMaster.
    + *
    + * The ObserverMaster forwards a copy of the request to the ensemble Leader and inserts
it into its own
    + * request processor pipeline where it can be matched with the response comes back. All
commits received
    + * from the Leader will be forwarded along to every Learner connected to the ObserverMaster.
    + *
    + * New Learners connecting to a Follower will receive a LearnerHandler object and be
party to its syncing logic
    + * to be brought up to date.
    + *
    + * The logic is quite a bit simpler than the corresponding logic in Leader because it
only hosts observers.
    + */
    +public class ObserverMaster implements LearnerMaster, Runnable {
    +    private static final Logger LOG = LoggerFactory.getLogger(ObserverMaster.class);
    +
    +    //Follower counter
    +    private final AtomicLong followerCounter = new AtomicLong(-1);
    --- End diff --
    
    I have a question about what variable this for. I see this value will never get incremented,
as the only function references it is `getAndDecrementFollowerCounter`.  Might worth to put
more comments here.


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