Raft一致性框架_Copycat基础学习(一)

Copycat is a fault-tolerant state machine replication framework. Built
on the Raft consensus algorithm, it handles replication and
persistence and enforces strict ordering of inputs and outputs,
allowing developers to focus on single-threaded application logic. Its
event-driven model allows for efficient client communication with
replicated state machines, from simple key-value stores to wait-free
locks and leader elections. You supply the state machine and Copycat
takes care of the rest, making it easy to build robust, safe
distributed systems.

上面一段摘录于Copycat官网的介绍(http://atomix.io/copycat/),那么Copycat 是一个基于Raft一致性算法的编程框架，它能够为分布式应用中的状态提供一致性。本文主要基于Copycat官网给的示例进行学习.

1.首先在IDE里面创建一个maven工程，并在pom文件中加入依赖:

<dependency>
    <groupId>io.atomix.copycatgroupId>
    <artifactId>copycat-serverartifactId>
    <version>1.1.4version>
dependency>
<dependency>
    <groupId>io.atomix.copycatgroupId>
    <artifactId>copycat-clientartifactId>
    <version>1.1.4version>
dependency>
<dependency>
    <groupId>io.atomix.catalystgroupId>
    <artifactId>catalyst-nettyartifactId>
    <version>1.1.1version>
dependency>

2.自定义StateMachine以及Command

//自定了MapstateMachine，它继承框架提供的StateMachine类，MapstateMachine主要处理来自客户端的操作，如示例建的这个类，用于处理两个操作,put和get.put用于向map中写入键值，get用于获取值
public class MapstateMachine extends StateMachine implements Snapshottable {
    //此为copycat-server需要维护的一致性数据结构,本例使用的是MAP
    private Map map = new HashMap<>();

    //定义对map的put操作
    public Object put(Commit commit) {
        try {
            map.put(commit.operation().key(), commit.operation().value());
        } finally {
            commit.close();
        }
        return null;
    }
    //定义对map的get操作
    public Object get(Commit commit) {
        try {
            return map.get(commit.operation().key());
        } finally {
            commit.close();
        }
    }

    //以下两个方法来自于实现Snapshottable的接口，实现这个接口是用于copycat-server能够对本地状态日志进行压缩，并形成snapshot(快照),当copycat-server重启后，可以从快照恢复状态，如果有其它的server加入进来，可以将快照复制到其它server上.
    @Override
    public void snapshot(SnapshotWriter writer) {
        writer.writeObject(map);
    }

    @Override
    public void install(SnapshotReader reader) {
        map = reader.readObject();
    }
}

GetQuery类

package com.xkx.common;

import io.atomix.copycat.Query;

//定义对MapstateMachine查询的命令
public class GetQuery implements Query<Object> {

    private final Object key;

    public GetQuery(Object key){
        this.key = key;
    }

    public Object key(){
        return key;
    }
}

PutCommand类

package com.xkx.common;


import io.atomix.copycat.Command;

public class PutCommand implements Command