Tomcat 8如何配置Session Replication实现Session共享

部署服务器集群离不开Session共享，实现Session共享方式有很多种，对于小型集群（5台服务器内）我们可以采用Session Replication来实现Session共享，这样实现起来容易，又可以
优点：
1，实现简单。
2，不需要使用共享数据库，节点少，降低了故障率。
缺点：
1，同步Session数据造成了网络带宽的开销。只要Session数据有变化，就需要将数据同步到所有其他机器上，机器越多，同步带来的网络带宽开销就越大。
2，每台Web服务器都要保存所有Session数据，如果整个集群的Session数据很多（很多人同时访问网站）的话，每台机器用于保存Session数据的内容占用会很严重。

这里我们使用tomcat8举例说明
测试环境使用两台linux服务器，分别部署tomcat。
服务器1 ip:172.16.0.102
服务器2 ip:172.16.0.104

1，修改tomcat的server.xml文件，增加Cluster配置，如果定义在Engine容器中，则表示对所有主机均启动用集群功能。如果定义在某Host容器中，则表示仅对此主机启用集群功能。此外，需要注意的是，Receiver中的address=”auto”一项的值最好改为当前部署tomcat主机所对应的网络接口的IP地址。配置如下：

<Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster" channelSendOptions="8">

    <Manager className="org.apache.catalina.ha.session.DeltaManager"
                   expireSessionsOnShutdown="false"
                   notifyListenersOnReplication="true"/>

            <Channel className="org.apache.catalina.tribes.group.GroupChannel">
                <Membership className="org.apache.catalina.tribes.membership.McastService"
                            address="228.0.0.4"
                            port="45564"
                            frequency="500"
                            dropTime="3000"/>
                <Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
                          address="172.16.0.102"
                          port="4000"
                          autoBind="100"
                          selectorTimeout="5000"
                          maxThreads="6"/>

                <Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter">
                  <Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/>
                Sender>
                <Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/>
                <Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatchInterceptor"/>
            Channel>

            <Valve className="org.apache.catalina.ha.tcp.ReplicationValve" filter=""/>
            <Valve className="org.apache.catalina.ha.session.JvmRouteBinderValve"/>

            <Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer"
                    tempDir="/tmp/war-temp/"
                    deployDir="/tmp/war-deploy/"
                    watchDir="/tmp/war-listen/"
                    watchEnabled="false"/>

            <ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/>
        Cluster>

按此配置好所有的tomcat，除了Receiver中的address，其它值不变。

2，更改web应用程序根目录下“WEB-INF”目录下的web.xml文件，必须添加这个元素

3，在服务器上开启Membership和Receiver使用的端口，使数据可以传输。

完成上述步骤后，挨个启动tomcat。查看tomcat日志，可以看到以下信息。
服务器1的日志

11-Oct-2017 10:24:39.368 INFO [main] org.apache.catalina.ha.tcp.SimpleTcpCluster.startInternal Cluster is about to start
11-Oct-2017 10:24:39.372 INFO [main] org.apache.catalina.tribes.transport.ReceiverBase.bind Receiver Server Socket bound to:/172.16.0.102:4000
11-Oct-2017 10:24:39.379 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.setupSocket Setting cluster mcast soTimeout to 500
11-Oct-2017 10:24:39.381 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Sleeping for 1000 milliseconds to establish cluster membership, start level:4
11-Oct-2017 10:24:40.382 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Done sleeping, membership established, start level:4
11-Oct-2017 10:24:40.383 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Sleeping for 1000 milliseconds to establish cluster membership, start level:8
11-Oct-2017 10:24:41.383 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Done sleeping, membership established, start level:8
11-Oct-2017 10:24:41.384 SEVERE [main] org.apache.catalina.ha.deploy.FarmWarDeployer.start FarmWarDeployer can only work as host cluster subelement!

服务器2的日志

11-Oct-2017 10:25:24.956 INFO [main] org.apache.catalina.ha.tcp.SimpleTcpCluster.startInternal Cluster is about to start
11-Oct-2017 10:25:24.960 INFO [main] org.apache.catalina.tribes.transport.ReceiverBase.bind Receiver Server Socket bound to:/172.16.0.104:4000
11-Oct-2017 10:25:24.967 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.setupSocket Setting cluster mcast soTimeout to 500
11-Oct-2017 10:25:24.970 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Sleeping for 1000 milliseconds to establish cluster membership, start level:4
11-Oct-2017 10:25:25.002 INFO [Membership-MemberAdded.] org.apache.catalina.ha.tcp.SimpleTcpCluster.memberAdded Replication member added:org.apache.catalina.tribes.membership.MemberImpl[tcp://{172, 16, 0, 102}:4000,{172, 16, 0, 102},4000, alive=45620, securePort=-1, UDP Port=-1, id={12 42 -64 76 118 -60 72 -75 -116 12 -52 74 71 -108 -100 23 }, payload={}, command={}, domain={}, ]
11-Oct-2017 10:25:25.970 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Done sleeping, membership established, start level:4
11-Oct-2017 10:25:25.971 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Sleeping for 1000 milliseconds to establish cluster membership, start level:8
11-Oct-2017 10:25:25.978 INFO [Tribes-Task-Receiver[Catalina-Channel]-1] org.apache.catalina.tribes.io.BufferPool.getBufferPool Created a buffer pool with max size:104857600 bytes of type: org.apache.catalina.tribes.io.BufferPool15Impl
11-Oct-2017 10:25:26.972 INFO [main] org.apache.catalina.tribes.membership.McastServiceImpl.waitForMembers Done sleeping, membership established, start level:8
11-Oct-2017 10:25:26.972 SEVERE [main] org.apache.catalina.ha.deploy.FarmWarDeployer.start FarmWarDeployer can only work as host cluster subelement!

可以看出两台服务器已经联系上了，再回过头来看第一台服务器的日志，

11-Oct-2017 10:25:25.007 INFO [Tribes-Task-Receiver[Catalina-Channel]-1] org.apache.catalina.tribes.io.BufferPool.getBufferPool Created a buffer pool with max size:104857600 bytes of type: org.apache.catalina.tribes.io.BufferPool15Impl
11-Oct-2017 10:25:25.974 INFO [Membership-MemberAdded.] org.apache.catalina.ha.tcp.SimpleTcpCluster.memberAdded Replication member added:org.apache.catalina.tribes.membership.MemberImpl[tcp://{172, 16, 0, 104}:4000,{172, 16, 0, 104},4000, alive=1006, securePort=-1, UDP Port=-1, id={5 52 67 -61 66 126 73 -8 -108 76 20 81 -92 58 27 108 }, payload={}, command={}, domain={}, ]

接下来我们来测试session复制是否成功。在jsp文件中加上

<%= session.getId() %>

这样可以打印出sessionId。
先通过浏览器访问172.16.0.102上的应用，https://172.16.0.102/ljzx/login，从页面上获取sessionId后，在url后加上;jsessionid=SESSION_ID访问172.16.0.104上的应用，https://ljzd.riskraiders.com/ljzx/login;jsessionid=12235D32C91EB8DDF585D0E6B03BB32C
此时如果页面上显示的sessionId和我们传入的sessionId一致，说明session复制已成功。

经过测试发现session复制未成功，查看日志发现以下错误信息

11-Oct-2017 10:47:54.029 SEVERE [http-nio-443-exec-1] org.apache.catalina.ha.session.DeltaManager.requestCompleted Unable to serialize delta request for sessionid [C27AFF6847CB9F76FF93090E55C6A355]
 java.io.NotSerializableException: com.srd.ljzd.entity.client.ClientCompany

这是因为保存到session中的对象没有实现Serializable接口，修改相关对象后重新部署测试后可以实现session复制，不过需要注意通过url后加jsessionid值的方式测试的时候，因为cookie中没有jsessionid的，所以第一次请求后服务器会生成一个新的jsessionid给客户端，刷新页面后显示出来的sessionid的值就变了。