基于Redis的CAS集群

单点登录（SSO）是复杂应用系统的基本需求，Yale CAS是目前常用的开源解决方案。CAS认证中心，基于其特殊作用，自然会成为整个应用系统的核心，所有应用系统的认证工作，都将请求到CAS来完成。因此CAS服务器是整个应用的关键节点，CAS发生故障，所有系统都将陷入瘫痪。同时，CAS的负载能力要足够强，能够承担所有的认证请求响应。利用负载均衡和集群技术，不仅能克服CAS单点故障，同时将认证请求分布到多台CAS服务器上，有效减轻单台CAS服务器的请求压力。下面将基于CAS 3.4.5来讨论下CAS集群。

CAS的工作原理，主要是基于票据(Ticket)来实现的（参见 CAS基本原理）。CAS票据，存储在TicketRegistry中，因此要想实现CAS Cluster, 必须要多台CAS之间共享所有的Ticket，采用统一的TicketRegistry，可以达到此目的。  缺省的CAS实现中，TicketRegistry在内存中实现，不同的CAS服务器有自己单独的TicketRegistry，因此是不支持分布式集群的。但CAS提供了支持TicketRegistry分布式的接口 org.jasig.cas.ticket.registry.AbstractDistributedTicketRegistry，我们可以实现这个接口实现多台CAS服务器TicketRegistry共享，从而实现CAS集群。

同时，较新版本CAS使用SpringWebFlow作为认证流程，而webflow需要使用session存储流程相关信息，因此实现CAS集群，我们还得需要让不同服务器的session进行共享。

我们采用内存数据库Redis来实现TicketRegistry,让多个CAS服务器共用同一个TicketRegistry。同样方法，我们让session也存储在Redis中，达到共享session的目的。下面就说说如何用 Redis来实现TicketRegistry，我们使用Java调用接口Jedis来操作Redis，代码如下：

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import java.io.ByteArrayInputStream;  import java.io.ByteArrayOutputStream;  import java.io.ObjectInputStream;  import java.io.ObjectOutputStream;  import java.util.Collection;      import org.jasig.cas.ticket.Ticket;  import org.jasig.cas.ticket.TicketGrantingTicket;  import org.jasig.cas.ticket.registry.AbstractDistributedTicketRegistry;          import redis.clients.jedis.Jedis;  import redis.clients.jedis.JedisPool;  import redis.clients.jedis.JedisPoolConfig;          /*   *  TicketRegistry using Redis, to solve CAS Cluster.   *     *  @author ZL   *    */      public class RedisTicketRegistry extends AbstractDistributedTicketRegistry {                  private static int redisDatabaseNum;      private static String hosts;      private static int port;           private static int st_time;  //ST最大空闲时间            private static int tgt_time; //TGT最大空闲时间              private static JedisPool cachePool;              static {                  redisDatabaseNum = PropertiesConfigUtil.getPropertyInt("redis_database_num");          hosts = PropertiesConfigUtil.getProperty("hosts");          port = PropertiesConfigUtil.getPropertyInt("port");          st_time = PropertiesConfigUtil.getPropertyInt("st_time");          tgt_time = PropertiesConfigUtil.getPropertyInt("tgt_time");          cachePool = new JedisPool(new JedisPoolConfig(), hosts, port);                  }              public void addTicket(Ticket ticket) {                          Jedis jedis = cachePool.getResource();          jedis.select(redisDatabaseNum);                                int seconds = 0;                        String key = ticket.getId() ;                      if(ticket instanceof TicketGrantingTicket){              //key = ((TicketGrantingTicket)ticket).getAuthentication().getPrincipal().getId();              seconds = tgt_time/1000;          }else{              seconds = st_time/1000;          }                            ByteArrayOutputStream bos = new ByteArrayOutputStream();          ObjectOutputStream oos = null;          try{              oos = new ObjectOutputStream(bos);              oos.writeObject(ticket);                         }catch(Exception e){              log.error("adding ticket to redis error.");          }finally{              try{                   if(null!=oos) oos.close();              }catch(Exception e){                  log.error("oos closing error when adding ticket to redis.");              }          }          jedis.set(key.getBytes(), bos.toByteArray());          jedis.expire(key.getBytes(), seconds);                      cachePool.returnResource(jedis);                  }              public Ticket getTicket(final String ticketId) {          return getProxiedTicketInstance(getRawTicket(ticketId));      }                      private Ticket getRawTicket(final String ticketId) {                      if(null == ticketId) return null;                      Jedis jedis = cachePool.getResource();          jedis.select(redisDatabaseNum);                      Ticket ticket = null;                      ByteArrayInputStream bais = new ByteArrayInputStream(jedis.get(ticketId.getBytes()));          ObjectInputStream ois = null;                      try{              ois = new ObjectInputStream(bais);              ticket = (Ticket)ois.readObject();           }catch(Exception e){              log.error("getting ticket to redis error.");          }finally{              try{                  if(null!=ois)  ois.close();              }catch(Exception e){                  log.error("ois closing error when getting ticket to redis.");              }          }                      cachePool.returnResource(jedis);                      return ticket;      }                         public boolean deleteTicket(final String ticketId) {                      if (ticketId == null) {              return false;          }                                  Jedis jedis = cachePool.getResource();          jedis.select(redisDatabaseNum);                          jedis.del(ticketId.getBytes());                      cachePool.returnResource(jedis);                      return true;              }           public Collection<Ticket> getTickets() {                      throw new UnsupportedOperationException("GetTickets not supported.");          }          protected boolean needsCallback() {          return false;      }              protected void updateTicket(final Ticket ticket) {          addTicket(ticket);      }       }

同时，我们在ticketRegistry.xml配置文件中，将TicketRegistry实现类指定为上述实现。即修改下面的class值

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<!-- Ticket Registry -->     <bean id="ticketRegistry" class="org.jasig.cas.util.RedisTicketRegistry" />       <!--     <bean id="ticketRegistry" class="org.jasig.cas.ticket.registry.DefaultTicketRegistry" />  -->

因为使用了Redis的expire功能，注释掉如下代码：

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<!-- TICKET REGISTRY CLEANER -->  lt;!--  <bean id="ticketRegistryCleaner" class="org.jasig.cas.ticket.registry.support.DefaultTicketRegistryCleaner"      p:ticketRegistry-ref="ticketRegistry" />      <bean id="jobDetailTicketRegistryCleaner" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean"      p:targetObject-ref="ticketRegistryCleaner"      p:targetMethod="clean" />      <bean id="triggerJobDetailTicketRegistryCleaner" class="org.springframework.scheduling.quartz.SimpleTriggerBean"      p:jobDetail-ref="jobDetailTicketRegistryCleaner"      p:startDelay="20000"      p:repeatInterval="5000000" /> -->

通过上述实现TicketRegistry,多台CAS服务器就可以共用同一个 TicketRegistry。对于如何共享session，我们可以采用现成的第三方工具tomcat-redis-session-manager直接集成即可。对于前端web服务器(如nginx)，做好负载均衡配置，将认证请求分布转发给后面多台CAS，实现负载均衡和容错目的。