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This chapter is from the book

SQL Server Replication Enhancements

SQL Server Replication enables synchronization of data in distributed and mobile deployments, supporting application in all lines of business, from e-commerce to customer relationship management. Microsoft has shipped replication with previous versions of SQL Server.

SQL Server 2005 Replication Services includes the following new features and improvements:

  • Programmability. A set of CLR classes is provided for configuring, managing, scripting, and monitoring replication. You can use these object models to programmatically control replication objects such as publication and subscription.
  • Manageability. Many enhancements to the setup and management of replication topologies are introduced. These include multiple subscription creation with the Create Subscription Wizard, a lightweight merge Subscriber option that stores much less metadata, and an enhanced ability to do a transactional subscribe from a backup. Also, the Replication Monitor is enhanced to help you oversee replication operations across your entire enterprise from one console. The Replication Monitor has also been redesigned so that users can better understand the state of replication actions.
  • Availability. Improvements to snapshot generation, schema replication, and metadata maintenance are introduced. Snapshots interrupted during delivery can automatically resume without resending files that already completely transferred. Also, a much broader range of Data Definition Language (DDL) changes can now be replicated without using special stored procedures. Finally, Merge Replication now features automatic retention-based metadata cleanup, providing easier maintenance of merge topologies.
  • Business intelligence/data warehousing. Support for Oracle as a first-class transaction replication publisher is introduced. Oracle data can be published directly to SQL Server, where you can take advantage of the business intelligence and data warehousing tools and technology provided by SQL Server and the .NET platform.
  • Mobility. Message-based replication is introduced, which is the ability to replicate data over the HTTP or SOAP protocol. Message-based replication is well suited to replication over the Internet and for topologies that include mobile subscribers such as Microsoft Windows CE devices.
  • Scalability and performance. New performance optimizations are introduced, including partition groups for filtered merge replication publications and new article types for customers who want little or no change tracking on certain portions of their publication. Also, transaction replication performance is improved by using multiple connections to retrieve and apply commands.

The SQL Server 2005 Replication Monitor is enhanced to help you oversee replication operations across your entire enterprise from one console. Replication Monitor helps you discover performance problems before they become critical. When problems occur, the new Replication Monitor provides more detailed statistics and history to help you troubleshoot and solve problems faster.

Replication Monitor allows you to do the following:

  • View a summary of replication activity across an enterprise in one display and drill down to more detailed information as needed.
  • Set warning levels so that Replication Monitor can alert you when the delivery of data to subscriptions takes too long.
  • Leave Replication Monitor running in a small background window that pops up notifications when synchronization slows down or an error occurs.
  • Control synchronization schedules, properties, and notifications from a central location.
  • Receive notification through a replication alert when an event occurs on a replication agent.
  • Validate subscriptions to ensure that data values are the same at the publisher and subscribers.
  • Reinitialize one or all subscriptions to a publication as needed.

Peer-to-Peer Replication

Peer-to-peer transaction replication is designed for applications that might read or modify the data at any of the databases participating in replication. For example, an online shopping application is well suited for peer-to-peer replication. You can improve application performance by spreading out queries that read data across multiple databases. Additionally, if any of the servers hosting the databases are unavailable, the application can be programmed to route traffic to the remaining servers, which contain identical copies of the data. Read performance is improved because activity can be spread across all nodes. Aggregate update, insert, and delete performance for the topology is similar to a single node, because ultimately all changes are propagated to all nodes.

All nodes in a peer-to-peer topology are peers: each node publishes and subscribes to the same schema and data. Changes (inserts, updates, and deletes) can be made at all nodes. Replication recognizes when a change has been applied to a given node, preventing changes from cycling through the nodes more than once.

Two basic topologies are recommended for use with peer-to-peer replication:

  • The first topology is typical of an e-commerce or web-based application platform. In this case, each peer node server has a database that participates in the transaction replication modality. Microsoft recommends dividing the workload among the peers so that one is the read server and the other is the write server. The writes are passed to the read server. Although both servers can function as a read database, transaction replication requires that only one database receive writes.
  • The second topology is one in which the peers span a significant time zone difference, and thus no overlaps occur in write activity. In this case, all the peers receive the transaction updates across the topology and, due to the difference in time zone and business hours, no contention occurs. Each database is independent from the other. Because the databases’ peak load hours are isolated, transaction contention is minimized.

Oracle Publication

With Microsoft SQL Server 2005, you can include Oracle Publishers in your replication topology, starting with Oracle version 8.0.5. Publishing servers can be deployed on any Oracle-supported hardware and operating system. The feature is built on the well-established foundation of SQL Server snapshot replication and transaction replication, providing similar performance and usability. The main purpose of Oracle as a publisher is to get data out of Oracle and into SQL Server Analysis Services. In many customer scenarios, legacy data used for reporting resides in Oracle. The Oracle publishing functionality is focused on a few scenarios and is not an attempt to duplicate Oracle functionality. If you’re considering using the Oracle Publisher feature, it’s highly recommended that you read the Books Online topics.

Web-Based Replication Update

One of the features that many replication customers asked for was the ability to perform merge replication via Hypertext Transfer Protocol/Secure (HTTP/S). The ability to have a subscriber request a merge through port 80 allows for customer scenarios wherein a VPN connection is not possible. The Internet Server Application Programming Interface (ISAPI) service on the server acts as a storage location for articles that are exchanged in merge replication. Simply put, replication provides the data to a virtual server in Internet Information Services (IIS), which the client application (typically a Windows Pocket PC or laptop computer) then connects to. The connection is accomplished over the same protocols as any other HTTP command. The client application use scenario is one in which the user of the database is not connected to the main data store. The user typically modifies the data on his or her local source and updates the main database upon connection/ synchronization. For example, a salesperson may keep her list of customers on a local machine and add new customers and orders while she is in the field. At the end of a period, she synchs the data by connecting her laptop to the corporate back-end data.

Performance and Scalability

Both merge and transaction replication provide new features for increased performance. Replication has three main areas of focus to improve performance and scalability. Both merge and transaction replication have increased support for more concurrent subscribers (greater concurrency is one area of improvement). With more subscribers, the ability to scale through higher levels of throughput is expected through new features—the second area of improvement. The third area of improvement is better control and design through the ability to define package sets in a more granular fashion. Table 3-2 describes the new performance and scalability features.

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