Lesson 1: Understanding Replication Types in .NET

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Lesson 1: Understanding Replication Types
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and consistency of the data. A publication simplifies management by providing a single point for subscribing to a group of articles, making the replication easier to configure. At the same time, a publication preserves the integrity of the information SQL Server replicates by grouping data that must be kept together. For example, the OrderDetail and OrderHeader tables are probably better published as a single unit to assure the consistency of the related information.
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Server Roles
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The publishing metaphor extends to the roles a server can play in the replication topology. You can configure a server as Publisher, Distributor, and/or Subscriber. The relationship between these three servers is shown in Figure 19-1. The Publisher is the original owner of the information that is published. In some scenarios, a Publisher is the only place where data can be modified. However, some replication types allow the propagation of changes from other servers.
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Figure 19-1 Server roles in replication
You also need to configure a server in the role of Distributor. The Distributor is responsible for managing the distribution database, which stores replication status data, metadata, and, in some replication scenarios, the actual data that SQL Server replicates. A single database server instance can act as both the Publisher and the Distributor, in which case the Distributor is called a local Distributor. When you configure the Publisher and the Distributor on separate database server instances, the Distributor is called a remote Distributor. The server that then receives copies of the publication and provides the data to end users and applications is called the Subscriber. In some configurations, Subscribers
19
Managing Replication
have a read-only copy of the database; in other configurations, you can update information in the Subscribers and replicate those changes back to the Publisher.
Push and Pull Subscriptions
In replication terminology, you find two types of subscriptions, or ways for Subscribers to get the publication: push subscriptions and pull subscriptions (shown in Figures 19-2 and 19-3). With a push subscription, the Distributor copies the data to the Subscriber database. With a pull subscription, the Subscriber retrieves the data from the Distributor.
Agent
Data
Distributor
Subscriber
Figure 19-2 Push subscription
Agent
Data Distributor Subscriber
Figure 19-3 Pull subscription
When your communication infrastructure has stable and permanent connections among replication servers, a push subscription offers the advantage of providing a central management point to all replication agents that manage the replication process. This results in less administrative overhead and easier troubleshooting procedures. However, when the replication configuration includes many Subscribers, the distribution process for push subscriptions can tax the hardware resources of a Distributor. Furthermore, servers or clients that connect on demand are better configured as pull subscriptions.
Lesson 1: Understanding Replication Types
Replication Types
With this terminology foundation laid, let s look at each of the three types of replication that SQL Server 2005 provides. These types are illustrated in Figure 19-4. Snapshot replication is the easiest replication type to understand because it is conceptually similar to a full backup and restore. With snapshot replication, the server copies an entire set of data to the Subscribers at scheduled times, rewriting the data at the Subscribers with each copy operation. However, snapshot replication does not operate on the complete database as backup and restore does; snapshot replication copies only the specified articles from the Publisher to the Subscribers. Keep in mind that because snapshot replication copies the entire data set every time it runs, you should use this replication type only when the amount of data is small and rather static.
Publisher Distributor
Data
Data
Subscriber
Subscriber
Subscriber
Figure 19-4 Snapshot replication
For more volatile scenarios, transactional replication (illustrated in Figure 19-5) provides a better solution because it makes an initial complete copy of the data, and then all subsequent copies transfer modified data only. Transactional replication uses the transaction log to apply to the destination data the same transactions performed on the source data. Because the same modifications are applied at both ends, the information is identical at the Publisher and the Subscriber. This replication type is frequently used for transactional tables, such as an Order Details table in a retail database.
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