Sql-server – What happens when the mirroring server goes offline (for a short while)

The article linked uses the dreaded WMI middle-man for delivering the notifications. That is so error prone that is not worth talking about. Cut out the WMI middle man (that also looses events...) and go straight to the source: create an event notification for the Database Mirroring State Change Event. Deliver the notifications locally to your [msdb] database. Then you can react in your activation procedure directly to the state change notification. Yo do not need any TCP or other communication, each mirroring partner can do this monitoring independently and each partner reacts to the local state change.

For a C# application to react to the notification simply have it monitor a queue (WAITFOR(RECEIVE...)) where, again, you have the system deliver event notifications for mirroring state change event. You can even use the external activation app.

The Pro SQL Server 2008 Mirroring book has an example showing exactly this technique.

Just don't use WMI... and absolutely don't connect to client back from server via TCP.

SQL Server's native database mirroring includes something really slick: automatic page repair. If the principle detects a corrupt page on disk, it fetches a clean copy of the page from the mirror automatically without taking the database down. You don't get that level of database repair with any third party product. That database awareness is one of the reasons I prefer the native tools - well, that and I just don't trust third party filter drivers on database servers. I've seen too many blue screens of death due to third party filter driver bugs.

Bad news: database mirroring is deprecated in SQL 2012. This means it's still around for this version, but will be removed in the future.

Fantastic news: SQL Server 2012 introduced AlwaysOn Availability Groups to replace mirroring, and they're awesome. You can actually query or back up from the mirror, and you get up to four mirrors. Some can be synch and some can be asynch. This gives you all kinds of flexibility over database mirroring, and it blows away the third party tools.