Memory-to-memory
WebSphere V5 provides a third mechanism for session management: memory-to-memory replication using WebSphere Data Replication Service (DRS). There are three topological models for memory-to-memory replication:
- Buddy system with single replica
- Dedicated replication server
- Peer-to-peer replication (default)
If the original server fails, the HTTP WLM plug-in will route the user's request to another server in the server cluster (adoptive server). The session manager in the adoptive server will look whether there is a copy of the user's session locally. If not, it will go to another store to retrieve this user's session.
Considerations for configuring the replication topology are:
1. At least two replication stores are needed for high availability in case a single replicated copy is also unavailable 2. The message complexity of session replication should be minimized While the buddy system and the dedicated replication server may have fewer replication messages, they are also prone to failures if many servers fail at the same time. The peer-to-peer or client/server models provide more resistance to multiple server failures. Channel partitioning provided with WebSphere V5 can be used in an environment with a large number of servers to reduce the replication message complexity.
The HTTP WLM plug-in doesn't know which servers have the replicated session locally. It simply routes the user's request to an adoptive server in the cluster, according to server weights, without any session store consideration. With the peer-to-peer model, the session information is distributed across all appservers, that is, each application server retrieves sessions from other application servers, and each application server provides sessions to other application servers. Therefore, no matter which server the HTTP plug-in routes the user's request to, it can always get the session locally.
In order to make dynamic Web contents highly available, you also need to make LDAP, database, and file system highly available. This is discussed in detail in Chapter 12, WebSphere data management high availability and Chapter 13, High availability of LDAP, NFS, and firewall.
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