Oracle7 Server Distributed Systems Volume II: Replicated Data

Distributed vs. Replicated Data

If you determine that you prefer to use synchronous data propagation, you must next decide if you need to distribute or replicate your data. This choice will be primarily determined by whether you need all data at all sites or if data can be clearly divided between sites.

Distributed Data

A distributed system consists of multiple servers, each responsible for their own data. This data can be accessed using a network, and appears to the user to be a single server. Although the data can be accessed from multiple locations, there is only one physical copy of each piece of data.

Distributing your data can improve your performance by providing faster updates of the local subset of your data. It can also improve your availability. If one site in a distributed system becomes unavailable, you can continue to query and update the data at the remaining sites.

If you frequently perform queries that access multiple remote sites, you may experience some performance degradation because all of the data is at a single location. Additionally, if one of these sites becomes unavailable, you will not be able to complete this transaction until the site becomes available again.

For these reasons, a distributed model is most appropriate when you frequently query and update a distinct subset of your data from a single location, and seldom query or update the remaining portions. Oracle7 Server Distributed Systems, Volume I provides more detail on distributing your data.

Replicated Data

In a replicated environment, each site contains a copy of all necessary data for that site, with multiple sites potentially having multiple updatable copies of the same data. Maintaining multiple copies of your data at multiple sites will require greater system resources, and is therefore most appropriate in situations requiring frequent local access to data from multiple locations.

Replicating your data can improve your performance by providing faster queries of all of your data. It can also provide improved availability to all of your data for queries. Even if your local site goes down, you can still access a complete copy of your data at another replicated location.

Synchronously replicating your data can decrease the availability of your data for updates, however. If one site becomes unavailable, you cannot update any other replicas until the downed site either becomes available or is dropped from the replicated environment. For this reason, you may prefer to use "near real-time" asynchronous replication, if you determine that you need to replicate, rather than distribute your data.