MSSQL

• Database Mirroring Terms and Definitions

Database mirroring is a solution for increasing the availability of a SQL Server database. Mirroring is implemented on a per-database basis and works only with databases that use the full recovery model.


automatic failover

    The process by which, when the principal server becomes unavailable, the mirror server to take over the role of principal server and brings its copy of the database online as the principal database.
 
    High-performance mode

    The database mirroring session operates asynchronously and uses only the principal server and mirror server. The only form of role switching is forced service (with possible data loss).

High-safety mode

    The database mirroring session operates synchronously and, optionally, uses a witness, as well as the principal server and mirror server.
 
    mirror database

    The copy of the database that is typically fully synchronized with the principal database.
 
    principal database

    In database mirroring, a read-write database whose transaction log records are applied to a read-only copy of the database (a mirror database).
 
    Witness

    For use only with high-safety mode, an optional instance of SQL Server that enables the mirror server to recognize when to initiate an automatic failover. Unlike the two failover partners, the witness does not serve the database. Supporting automatic failover is the only role of the witness.
 
    http://msdn.microsoft.com/en-us/library/ms189852.aspx

• Replication Agents Overview

SQL Server Agent
SQL Server Agent hosts and schedules the agents used in replication and provides an easy way to run replication agents.

Snapshot Agent
The Snapshot Agent is typically used with all types of replication

Log Reader Agent
The Log Reader Agent is used with transactional replication

Distribution Agent
The Distribution Agent is used with snapshot replication and transactional replication

Merge Agent
The Merge Agent is used with merge replication.

Queue Reader Agent
The Queue Reader Agent is used with transactional replication with the queued updating option

http://msdn.microsoft.com/en-us/library/ms152501.aspx