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Uses of Replication
Data replication is the capability of a system to automatically make copies of its data and application objects in remote locations. Any changes to the original or to the copies are propagated to all other copies. Data replication allows users to make changes to data offline at remote locations. Changes to either the original or the remote data are synchronized with other instances of the database.The original database is referred to as the design master . Changes to definitions of tables or other application objects can be made only to the design master. The design master is used to make special copies called replicas . Although there is only one design master, replicas can be made from other replicas. The process of the design master and replicas sharing changes is referred to as synchronization . The design master and replicas that participate in the synchronization process are collectively referred to as a replica set .To see an example of data replication at work, suppose that you have a team of salespeople who are out on the road all day. At the end of the day, each salesperson logs on to one of the company's Windows NT servers through Dial-Up Networking (DUN) or Remote Access Services (RAS). Each salesperson's transactions are sent to the server. If necessary, any changes to the server data are also sent to the salesperson. In addition to data being replicated, if the developers in the organization are busily adding forms, reports, and modules to the database's master copy, any changes to the application components are also updated in the remote copies as users log on to the system.This example illustrates just one of the several valuable uses of replication. In a nutshell, data replication is used to improve the availability and integrity of data throughout an organization or enterprise. The practical uses of data replication are many; they can be categorized into five general areas, explained in the following sections.
Sharing Data Among Offices
In today's global economy, it's the norm for companies to have many offices distributed throughout the country, or even the world. Before Access 95, it was difficult to implement an Access application that would support sharing data among several offices. However, with replication, each office can have a replica of the database. Periodically throughout the day, each office can synchronize its changes into data at corporate headquarters. How often the synchronization happens depends on the frequency required for data at each location to be current at any given moment.
Sharing Data Among Dispersed Users
Sharing data among dispersed users is illustrated by the salesperson example used earlier. This implementation of replication generally involves mobile users who connect to the network after modifying data out on the road. Because only incremental changes are transferred from the design master (the original) to the replicas (the copies), and from the replicas to the design master, this form of replication makes the mobile computing scenario economically feasible.
Reducing Network Load
Replication can be very effective in reducing network traffic loads. The design master can be replicated onto one or more additional servers. Distributed users can then make changes to one of the additional servers, which significantly improves performance by distributing the processing load throughout the network. Changes made to the data on the additional servers can be synchronized with the main server periodically during the day.
Distributing Application Updates
Replication is an excellent vehicle for distributing application updates. Design changes can be made only to the design master; therefore, as users throughout the organization log on to synchronize with the design master, any structural changes to the application are sent to the user. This is much more efficient and effective than giving every user an entirely new copy of the application database each time a minor change is made to the application's schema.
Backing Up the Data in Your Application
Many people don't think of replication as a means of backing up application data, but replication is extremely well suited for this task. Ordinarily, to back up an Access database, everyone must log off the system, but that's not necessary with replication. The synchronization process can occur periodically during the day while users are still logged on to the system, and all changes are replicated. Not only is this more efficient than backing up the entire database, it also ensures that you can quickly be up and running on a backup server if there's a problem on a main server.
