Three low-cost approaches to high-availability databases

Service provider takeaway: Customers looking for a low-cost approach to ensuring high-availability databases can turn to value-added resellers for help choosing the technology that best fits their shop.

One of the most difficult tasks faced by a company's database staff is choosing the right approach to ensure high-availability databases, since there are many factors that play into the decision. For large mission-critical databases, a solution involving hardware database mirroring, clustering and geoclustering (an approach in which servers are hosted in different geographic locations) is required. While such solutions are very expensive, the cost of downtime justifies the investment. But if your customers don't have the budget for such sophisticated technologies, there are cheaper ways to keep databases running. Here we examine the pros and cons of these low-cost options to help you simplify the choices for your customers.

The three low-cost methods to reduce downtime on a relational database management system (RDBMS) are log shipping, replication and database mirroring. When working with customers, it's important to stress that:

Now that we've established those basic gotchas, here are the advantages and disadvantages of each approach.

Log shipping

Log shipping is a technology in which the backup is copied from the production server to a standby server, restored there, and then transaction logs backed up on the production server are copied or shipped to the

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standby server and applied there. This process keeps the two databases synchronized.
The main advantages of log shipping are:

The disadvantages of log shipping are:

Replication

Replication is a technology that captures transactions occurring on the source database (called a publisher) and applies them on the destination database (subscriber).

The advantages of replication are:

The disadvantages of replication are:

Database mirroring

Database mirroring is a feature included in SQL Server 2005, and companies such as EMC and Double-Take Software have database mirroring tools. Conceptually, database mirroring can be considered continuous log shipping.

The advantages of database mirroring are:

The disadvantages are:

With this breakdown of the low-cost ways to ensure high-availability databases, you'll have an easier time helping your customers make the right choice.

About the author
Hilary Cotter has been involved in IT for more than 20 years as a Web and database consultant. Microsoft first awarded Cotter the Microsoft SQL Server MVP award in 2001. Cotter received his bachelor of applied science degree in mechanical engineering from the University of Toronto and subsequently studied economics at the University of Calgary and computer science at UC Berkeley. He is the author of a book on SQL Server transactional replication and is currently working on books on merge replication and Microsoft search technologies.