AlwaysOn Availability Groups (SQL Server)

参考了这两篇文章：

http://technet.microsoft.com/en-us/library/ff877884.aspx

http://technet.microsoft.com/en-us/library/aa427606-8422-4656-b205-c9e665ddc8c1

什么是AlwaysOn Availability Groups, 下面这句话说的很明了：

The AlwaysOn Availability Groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. Introduced in SQL Server 2012.

Availability Groups的结构

首先Deploying AlwaysOn Availability Groups requires a Windows Server Failover Clustering (WSFC) cluster. Each availability replica of a given availability group must reside on a different node of the same WSFC cluster.

A WSFC resource group is created for every availability group that you create. The WSFC cluster monitors this resource group to evaluate the health of the primary replica. The quorum for AlwaysOn Availability Groups is based on all nodes in the WSFC cluster regardless of whether a given cluster node hosts any availability replicas.

( 所以SQL Server Instance必须在不同的cluster node上，只有这样才能实现failover。同时理论上一个node上可以装不同属于不同availability的replica)

下面是上图中的名词解释：

availability group

A container for a set of databases, availability databases, that fail over together.

availability replica

An instantiation of an availability group that is hosted by a specific instance of SQL Server and maintains a local copy of each availability database that belongs to the availability group. Two types of availability replicas exist: a single primary replica and one to four secondary replicas.

primary replica

The availability replica that makes the primary databases available for read-write connections from clients and, also, sends transaction log records for each primary database to every secondary replica.

虽然叫副本，但其实就是我们希望保护的那个最重要的数据库实例。

secondary replica

An availability replica that maintains a secondary copy of each availability database, and serves as a potential failover targets for the availability group. Optionally, a secondary replica can support read-only access to secondary databases can support creating backups on secondary databases.

因此相当于一个主SQL Server instance，拥有4个从SQL Server  instances。当primary replica失败时，可以failover到任何一个从replica上。failover 在replica之间发生。

primary replica和secondary replica之间的同步方式有两种：同步和异步。且同步是数据库级别的，数据库之间不会相互影响。

availability group listener

A server name to which clients can connect in order to access a database in a primary or secondary replica of an AlwaysOn availability group. Availability group listeners direct incoming connections to the primary replica or to a read-only secondary replica.

availability database

A database that belongs to an availability group. For each availability database, the availability group maintains a single read-write copy (the primary database) and one to four read-only copies (secondary databases).

primary database

The read-write copy of an availability database. 就是上图中primary replica中的一个个数据库。

secondary database

A read-only copy of an availability database. 就是上图中secondary replica中的一个个数据库。

提供三种不同的Failover--automatic, manual, and forced (with possible data loss).具体参考原文吧。