Cell affinity function

Using the cell affinity function, you can configure unbridged, on demand router (ODR) topologies to preserve sessions even in the event of on demand router (ODR) outages. With this function, you can configure your topology in such a way that, when an ODR receives misrouted in-session traffic, the ODR reroutes the traffic back to a working ODR in the original cell. Thus, you can configure an IBM® HTTP Server to route to ODRs in multiple cells and still preserve session affinity.

The cell affinity function prevents the loss of sessions when there are multiple ODRs within multiple, unbridged cells, and the IBM HTTP Server is configured to forward session traffic, either through load-balancing or failover, to more than one ODR. For example, in a network configuration where ODRs are situated between the IBM HTTP Server and the back-end application servers, the IBM HTTP Server is not able to recognize the servers identified on the JSESSIONID cookies contained within in-session traffic because it is configured to recognize and route to the ODRs. Thus, the IBM HTTP Server generally selects different ODRs and sprays the session requests. If the IBM HTTP Server selects a router within the same cell as the hosting application server, or if all application servers share session data through a common database, the risk of losing sessions is not a concern. However, without cell affinity, if the IBM HTTP Server selects an ODR within another cell, the ODR does not recognize the server ID, does not route the request, and consequently the session is lost. The cell affinity function helps you avoid session loss.

Aspects of the cell affinity function

There are two aspects of the cell affinity function. The first aspect enables the IBM HTTP Server to affine or always to route to a specific ODR after a session is established. Configuring the IBM HTTP Server to preserve session affinity through a particular ODR is accomplished by enabling cell affinity, producing a plugin-cfg.xml, moving the plugin-cfg.xml to the IBM HTTP Server, and restarting that server. The produced plugin-cfg.xml instructs the IBM HTTP Server plug-in to use the ODRSESSIONID cookie for its session ID, enabling session affinity to the ODRs.

The second aspect of cell affinity is the ability to route session traffic across cell boundaries in order to direct misrouted traffic into the correct cell. To enable this function, in addition to enabling cell affinity, generic server clusters (GSCs) must be configured for each cell for which an ODR can receive traffic. The GSC members must be the ODRs in the remote cells. When an ODR receives misrouted session traffic and cell affinity is enabled, it checks the GSC lists to find the ODR associated with the ODR session ID. If a match is identified, the traffic is rerouted to the appropriate GSC. If the reroute is successful, the final ODR adopts the session and routes the traffic to the appropriate back-end server for the session.