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Replication is a service that transfers data, objects, or events among appservers. Data replication service (DRS) is the internal WAS component that replicates data.

Use data replication to make data for...

...available across many appservers in a cluster.

Session manager Use memory-to-memory replication to maintain data about sessions across multiple appservers, preventing the loss of session data if a single application server fails.
Dynamic cache Improves performance by copying cache information across appservers in the cluster.
Stateful session beans Applications using stateful session beans are not limited by unexpected server failures.

You can define the number of replicas that DRS creates on remote appservers. A replica is a copy of the data that copies from one application server to another. The number of replicas that you configure affects the performance of your configuration. Smaller numbers of replicas result in better performance because the data does not have to copy many times. However, if you create more replicas, you have more redundancy in your system. By configuring more replicas, your system becomes more tolerant to possible failures of appservers in the system because the data is backed up in several locations.

By having a single replica configuration defined, one can avoid a single point of failure in the system. However, if your system must be tolerant to more failure, introduce extra redundancy in the system. Increase the number of replicas that you create for any HTTP session that is replicated with DRS. Any replication domain that is used by dynamic cache must use a full group replica.

Session manager, dynamic cache, and stateful session beans are the three consumers of replication. A consumer is a component that uses the replication service. When you configure replication, the same types of consumers belong to the same replication domain. For example, if you are configuring both session manager and dynamic cache to use DRS to replicate objects, create separate replication domains for each consumer. Create one replication domain for all the session managers on all the appservers and one replication domain for the dynamic cache on all the appservers. The only exception to this rule is to create one replication domain if you are configuring replication for HTTP sessions and stateful session beans. Configuring one replication domain in this case ensures that the backup state information is located on the same backup appservers.




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