MCITP SQL Server 2005 Database Developer All-in-One Exam Guide in .NET

Create QR Code in .NET MCITP SQL Server 2005 Database Developer All-in-One Exam Guide

MCITP SQL Server 2005 Database Developer All-in-One Exam Guide
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Any user databases can be configured with database mirroring The master, msdb, tempdb, and model databases cannot be configured as database mirrors
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One of the great benefits of database mirroring is that it s a simple solution to implement It doesn t require any special hardware (such as a clustering solution would require), yet can provide high availability If configured with a witness server, database mirroring can provide high-availability benefits for a database While database mirrors can t be queried directly, when they are used with database snapshots users can query the mirror In other words, if a database mirror is created without a database snapshot, it can t be queried If you want the ability to query the database mirror, a database snapshot, can be created on the mirror and users will then be able to query the mirror via the snapshot Without the snapshot you provide a layer of redundancy to your solution With the snapshot you add high availability
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A third server can also be configured with a database mirror, as shown in Figure 10-35 This third server is known as the witness server
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Figure 10-35 A database mirror with a witness server
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Mirror SQL server
Witness SQL server
The only purpose of the witness server is to enable automatic failover, or to enable highavailability mode If this is not your goal, it s recommended to leave the witness set to Off For the database mirroring session to support automatic failover with a witness server, the session must be in synchronous-operating mode The two choices are synchronous and asynchronous They are also referred to as transaction safety on (synchronous) and transaction safety off (asynchronous) Synchronous mode provides high availability, while asynchronous mode provides high performance These modes are outlined in Table 10-9
Mode Transaction Safety Result
Synchronous Asynchronous
On Off
High availability High performance
Table 10-9 Database Mirror Operating Modes
10: Disaster Recovery
TIP If you are implementing automatic failover on a database mirror, you must have a witness server configured This is really the only reason to configure a witness server High-availability mode (automatic failover) also requires the session to be set to synchronous mode The witness server monitors the state between the principal and mirror servers, and can initiate a change in roles if the principal server goes down All three servers exchange PING messages with each other to verify connectivity and operation If a witness server is not implemented or the mode is not configured in synchronous mode, then automatic failover is not possible However, the mirror still functions as a warm spare and can be manually switched over to the principal role
Database Mirroring Endpoints
SQL Server 2005 uses endpoints to manage connections An endpoint is a SQL Server object, and it is used by SQL Server to communicate over the network with other SQL Server instances A database mirroring endpoint is used specifically to facilitate communication within database mirroring connections Database mirroring endpoints identify a unique listener port A server instance can have only one database mirroring endpoint, but multiple database mirrors can use the same endpoint In other words, if we create a second database mirror on an instance, we would not re-create a new endpoint, but instead use the first one Each database-mirroring server instance requires a unique listener port for mirroring connections To identify the port for a server instance to use, you need a unique database mirroring endpoint for that instance A server instance can have only one endpoint for mirroring, and all of the mirroring sessions to be supported by that server instance must use the same endpoint When creating the endpoint, a system administrator specifies the authentication and encryption methods of the server instance
Endpoints and Ports If you re a system administrator, this language about ports may sound familiar For example, you may know that port 80 is the well-known port for HTTP, 25 for SMTP, 20 and 21 for FTP, and so on IANAorg has identified the protocols used with the first 1024 ports as well-known ports However, the possible ports actually go up to 65,536 An IP address combined with a port number is sometimes referred to as a packet The IP address tells the packet how to get to the specific computer However, once it gets to the computer, the port number tells the computer what to do with the data For example, port 80 tells the computer to pass this onto the HTTP process to respond to the packet When we create a database mirroring endpoint, we re telling the computer that whenever a packet is received that is addressed to this port, pass it to the process managing the database mirroring session
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