Quorum in database mirroring

Before a server can assume the role of primary server, it must have a quorum, which means that at least one other server must agree that a server can own the database. If the mirror server becomes unavailable while the primary server and arbiter are connected, the primary server continues to provide access to the database. If the primary server loses quorum, it can no longer permit access to the database. At that point, the primary server stops the mirrored database, attempts to restart it, and then waits to regain quorum before making the database available.

What happens when a database mirroring system starts

When you start a database mirroring system, the database servers go through a startup process to reach quorum and accept client connections. The following steps describe a typical sequence of events for this process. Assume that Server 1 and Server 2 are partners that were both stopped (Server 2 was the mirror and stopped first, and then Server 1 was stopped), and that the arbiter server is running.

The arbiter server waits for connections from Server 1 and Server 2.
Server 1 starts and tries to connect to Server 2 and the arbiter server.
Server 1 connects to the arbiter server.
Server 1 fails to connect to Server 2 because it is not running.
Server 1 negotiates with the arbiter to determine if Server 1 should be the primary or mirror server.
Quorum is reached when the arbiter and Server 1 agree that Server 1 should assume the role of the primary server. The arbiter and the state file for the Server 1 indicate that Server 1 was the most recent primary server before the partner servers stopped.
Server 1 starts accepting connections.
Server 2 starts and tries to connect to Server 1 and the arbiter.
Server 2 connects to the arbiter and to server 1.
Server 2 negotiates with Server 1 and the arbiter to determine if Server 2 should be the primary or mirror server.
All three servers agree Server 2 should be the mirror.
Server 2 starts accepting read-only connections.
Server 2 starts applying log file pages received from Server 1.

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