Quorum：证人如何影响数据库可用性

美国宪法表示，大多数参议院构成了做生意的法定人数。那是51名参议员。在英国，议会40名成员（650分）需要出现在公约之家，以验证投票。在SQL Server数据库镜像中，仲裁为2，为3.对于具有自动故障转移的高安全模式，通常称为“高可用性”模式，Quorum确保一次只有一个合作伙伴拥有。在此模式中，有三个单独的SQL Server实例播放了主体，镜像和证人的角色。主体拥有数据库可更新的“Live”副本，镜像持有同步副本，并且证人对两者都密切关注，以确定需要故障转移。因此，“全权仲裁”意味着可以使用所有三个角色，所有这一切都是游泳（英国人） - 或桃子（美国）。主体数据库可供客户端使用，数据库受保护，生命是好的。所有三个角色都通过TCP通信的“端点”连接，以提供每个服务器的最新生命体征。校长上的更新在高安全模式下与镜像同步配置，以便在故障转移期间没有数据丢失。 So what happens when one of the roles fails and goes off-line? If just the Principal goes offline, we have what is called a “Witness to Partner” quorum, the Witness detects the failure and automatic failover occurs. The Mirror becomes the Principal and the clients redirect themselves to the new server using the Failover Partner field in their connection string. This all should take a matter of seconds. When the failed server comes back online, the Witness lets it know it is now the Mirror and it synchronizes with the new Principal. We then have a full quorum again. Failover back again or “failback” is not automatic but can be done manually. (Failback within 30 days will guarantee that Microsoft does not come looking for a SQL Server license for the Mirror if that’s its only role.) So what happens if the Mirror fails? Well, in this case we have again a “Witness to Partner” quorum but the Principal is fine so the Witness recognizes that no failover is necessary. The clients continue blissfully unaware of the issue. However, this state is referred to as “running exposed” since another quick failure will bring down the availability of the database. SSMS shows the Principal database with “DISCONNECTED” to indicate that the Mirror is offline. Hopefully, the Mirror comes back online soon and will automatically re-synchronize with the Principal and we have a full quorum again. And the Witness? If this fails, we have a “Partner to Partner” quorum. The Principal and Mirror databases continue as normal in a synchronized state and the clients are fine. However, the ability to perform an automatic failover is lost while the Witness is down. Manual failover can be performed but obviously we want to get the Witness up and running as soon as possible so we have a full quorum again. OK, on to Murphy’s Law: what about a double or triple-failure? Well, that means we have lost quorum. If the Principal and the Mirror are lost then obviously the database becomes unavailable and the clients will complain. If the Mirror comes up before the Principal, interestingly, the Witness will not invoke a failover since offline updates may have occurred and it will wait for the Principal to come back up to re-synchronize and the full quorum is back. It’s kind of what’s happening in the U.K. at the moment except it’s called a coalition. But that would be stretching the political analogy too far. Cheers Brian Detailed scenarios: http://technet.microsoft.com/en-us/library/ms189902.aspx