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Michael Hotek Since I formally entered the database industry more than a decade ago, my primary focus has been on building systems to predictably achieve high levels of availability. In the 1980s and 1990s, this was a rather difficult task. It usually involved complex architectures, complicated components, and a large dose of custom coding. The tools and technologies were immature at best and nonexistent at worst. You could divide the availability systems that we designed back then into two basic categories. Client-oriented systems would receive a transaction and then write it to multiple destinations. And we generally built server-oriented systems around code to transfer backups between one or more systems. As technology matured, we gained additional tools that allowed basic data duplication across multiple environments in a timely manner. However, this advancement also introduced latency between the primary and secondary databases, which could lead to data loss. We learned to deal with the potential problem because we simply could not eliminate it.
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Before You Begin
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What we needed was an integrated database technology, transparent to applications, that would maintain a duplicate copy of the data without incurring latency. If the technology could also provide mechanisms to automatically fail over to the secondary database upon failure of the primary database, it would be a significant evolutionary advancement in availability technologies. With the addition of database mirroring in SQL Server 2005, we finally have a technology that fills a significant gap in availability solutions and does not require custom coding. I can now make a single database within an instance highly available, with automatic failover, no latency between the primary and secondary databases, and transparency to the application. It isn t a perfect solution yet, but database mirroring is well on the way to fulfilling an availability requirement that I ve had for well over ten years.
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17
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Implementing Database Mirroring
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Lesson 1: Understanding Database Mirroring Roles
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All new technologies add new terminology to our vocabulary, and database mirroring is no different. In this lesson, you learn many of the key terms for database mirroring, including principal database, mirror database, and witness server. And you see how these different database mirroring roles interact with each other in a database mirroring session.
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After this lesson, you will be able to:
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Define what the principal database does. Define what the mirror database does. Define what the witness server does. Understand how the database mirroring roles work together.
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Estimated lesson time: 10 minutes
Database Mirroring Roles
Database mirroring comprises two mandatory roles and a third optional role. You must define a database in a principal role and another database in a mirror role. You can also optionally define a SQL Server instance in the role of witness server to govern automatic failover from the primary to the mirror database. Figure 17-1 shows a reference diagram for a database mirroring configuration.
Witness
Application
SQL Server
SQL Server
Principal
Mirror
Figure 17-1 Database mirroring components
Lesson 1: Understanding Database Mirroring Roles
Principal Role
The database you configure in the principal role becomes the source of all transactions in a data mirroring session. The primary database is recovered, it allows connections, and applications can read data to and write data from it. Note that you must specify the Full recovery model for the database to participate in a database mirroring session, a requirement that Lesson 2 covers in more detail.
NOTE
Serving the database
When an instance has a database that allows transactions to be processed against it, it is said to be serving the database.
Mirror Role
The database you define in the mirror role is the partner of the primary database and continuously receives transactions from the principal database. The database mirroring process is constantly replaying transactions from the primary database into the transaction log and flushing the transaction log to the data files on the mirror database so that the mirror database includes the same data as the primary database. The mirror database is in a recovering state, so it does not allow connections of any kind, and transactions cannot be written directly to it. However, you can perform a database snapshot against a mirror database to give users read-only access to the database s data at a specific point in time. (See 16, Managing Database Snapshots, for information about database snapshots.)
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