Mapper is a facility embedded in the Application Server version of JSPservlet. The diagram above shows the problem that it addresses.

Requestor 1 and 2 call a Presentation entry page. They know only its well-known URL, therefore they are routed to the same PageBox A, which is not the best one for Requestor 1 and 2. The difference can be significant. Let say PageBox A is at 300ms of Requestor 1 and at 200ms of Requestor 2, whereas PageBox B is at 25ms of Requestor 1 and PageBox C is at 30ms of Requestor 2.

Mapper has recorded the location of all PageBoxes that have subscribed and therefore host the presentation.
When PageBox A receives a request, Mapper gets its IP address and analyzes its location. Then it looks in subscribers list for the closest PageBoxes and selects one of them.
Next JSPservlet updates the URL in the entry page with the selected PageBox URL.

So when requestor 1 makes a subsequent request, it is routed to PageBox B. When requestor 2 makes a subsequent request, it is routed to PageBox C.

Note that Mapper includes a load balancing facility. If requestor 3 - a neighbor of 1 makes a request, it can be routed to PageBox D, in the same area as PageBox B. With that facility, PageBox not only enhances the performance, it also provides a scalability only limited by backends systems that PageBoxes invoke.

You can browse Mapper documentation for more information.

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