7.10 The epoxy layer

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Epoxy is a special interprocess communication layer designed to offset the inevitable degradation in performance that you can expect when work is divided across multiple processes. Interprocess communication normally results in many context switches, which are expensive under Windows. Epoxy uses shared memory to manage the data that must pass between the Store and IIS, and the Exchange development team claims that this technique reduces the performance loss to less than 1 percent of normal interprocess communication. The development team believes that any performance loss is bad, but the slight degradation is offset by improvements made elsewhere within the Store. Epoxy cannot operate across remote links, implying that the IIS must be local or installed on each server that hosts Store databases.

On a strategic level, creating a division between protocol access and storage helps toward the long-term goal of scaling Exchange to be able to handle the user and message loads generated by typical ISP environments. Based on today's technology, even the largest clustered server is probably not going to be able to handle hundreds of thousands of mailboxes and the attendant load generated by tens of thousands of concurrent user connections. Building a virtual server composed of front-end protocol servers and back-end storage servers is a good solution, and you cannot do that if storage and protocol access is totally integrated.

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