5.4.  Create and configure Data Replication Service (DRS) replication domains.


Data replication

Replication is a service that transfers data, objects, or events among application servers. Data replication service (DRS) is the internal WebSphere Application Server component that replicates data.

Use data replication to make data for session manager, dynamic cache, and stateful session beans available across many application servers in a cluster. The benefits of using replication vary depending on the component that you configure to use replication:

Important: When you use the replication services, ensure that the "Propagate security attributes" option is enabled. Security attribute propagation is enabled, by default.

You can define the number of replicas that DRS creates on remote application servers. 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 application servers in the system because the data is backed up in several locations.

Defining a single replica configuration helps you to 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. The "Number of replicas" property for any replication domain that is used by the dynamic cache service must be set to "Entire domain".

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 application servers and one replication domain for the dynamic cache on all the application servers. 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 application servers.

Configuring cache replication

Use this task to improve performance by configuring the data replication service (DRS) to replicate data from the dynamic cache service across the consumers in a replication domain.

You should have a replication domain created for the dynamic cache service. Configure a different replication domain for each type of consumer of the replication domain. For example, configure two different replication domains for dynamic cache and session manager. There are two ways to configure replication domains:

  1. In the administrative console, click "Servers > Application servers > server_name > Container services > Dynamic cache service".

    Figure 5.10. Dynamic cache service replication

    Dynamic cache service replication

  2. To enable replication, select "Enable cache replication".

  3. Full group replication domain: Choose a replication domain. Use different replication domains for each type of consumer. For example, dynamic cache should use a different replication domain than session manager. The only replication domains that you can select in this panel include replication domains that are configured to use full-group replication. In a full-group configuration, every cache entry is replicated to every other cache that is configured in the servers that are in the replication domain. If none of the replication domains in your configuration meet these requirements, the list is empty. In this case, create a replication domain or alter an existing replication domain so that you have a replication domain that can perform full-group replication.

  4. Define the dynamic cache replication settings:

    Replication type: Select appropriate replication type where:

    • Not Shared: Cache entries for this object are not shared among different application servers. These entries can contain non-serializable data. For example, a cached servlet can place non-serializable objects into the request attributes, if the class type supports it.

    • PUSH: Cache entries for this object are automatically distributed to the dynamic caches in other application servers or cooperating Java Virtual Machines (JVMs). Each cache has a copy of the entry at the time it is created. These entries cannot store non-serializable data.

    • PULL: Cache entries for this object are shared between application servers on demand. If an application server gets a cache miss for this object, it queries the cooperating application servers to see if they have the object. If no application server has a cached copy of the object, the original application server runs the request and generates the object. These entries cannot store non-serializable data. This mode of sharing is NOT recommended.

    • PUSH_PULL: Cache entries for this object are shared between application servers on demand. When an application server generates a cache entry, it broadcasts the cache ID of the created entry to all cooperating application servers. Each server then knows whether an entry exists for any given cache ID. On a given request for that entry, the application server knows whether to generate the entry or pull it from somewhere else. These entries cannot store non-serializable data.

    Push Frequency: You can define when and how often data is replicated across the dynamic cache replication domain

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