User's Guide Part III. Using SQL Anywhere Chapter 27. Introduction to SQL Remote ReplicationData replication is the sharing of data among physically distinct databases. Changes made to shared data at any one database are replicated to the other databases in the replication setup. The SQL Remote data replication system enables replication of data among SQL Anywhere databases.
One of the key benefits of a data replication system is that data is made available locally, rather than through potentially expensive, less reliable, and slow connections to a single central database. Data is accessible locally even in the absence of any connection to a central server, so that you are not cut off from data in the event of a failure of a long-distance network connection.
Replication improves response times for data requests for two reasons. Requests are processed on a local server without accessing some wide area network, so that transfer rates are faster. Also, local processing offloads work from a central database server so that competition for processor time is decreased.
One of the challenges of any replication system is to ensure that each database retains data integrity at all times. Today's replication systems, such as Sybase Replication Server and SQL Remote, replicate portions of the transaction log in such a way that transactions are replicated atomically: either a whole transaction is replicated, or none of it is replicated. This ensures data integrity at each database in the setup.
Another challenge to replication systems is to maintain data consistency throughout the setup. SQL Anywhere and other replication systems maintain a loose consistency in the setup as a whole: that is, all changes are replicated to each site over time in a consistent manner, but because of the time lag different sites may have different copies of data at any instant.
SQL Anywhere supports two forms of replication. SQL Anywhere databases can take part in Sybase Replication Server setups. Replication Server is a powerful, high-performance, technology for maintaining replicated data at multiple sites on a network. Replication Server permits replication among heterogeneous databases and database management systems. Replication Server can replicate data to SQL Anywhere databases using the Open Server Gateway that is part of this package. To replicate data from SQL Anywhere databases to other databases using Replication Server, you need a SQL Anywhere Replication Agent, which is available as a separate product.
SQL Remote is a system for data replication between SQL Anywhere databases only. SQL Remote is less flexible and configurable than Replication Server; for example, it has strict requirements for object names and attributes to be identical at different databases in the replication setup. Rather, SQL Remote is designed to be easy to administer, suitable for widespread deployment, capable of supporting replication to many replicate sites from one database, and is designed to replicate data even between databases not directly connected to each other. The following sections describe SQL Remote in more detail.