When you or Pega Cloud deploys a new Pega application, users working in the system may initially experience poor application performance. When the application starts, the core Pega Platform application orchestrates rules and the application code to retrieve and cache rules. This action is known as rule assembly, a system process for generating and compiling the Java code that corresponds to the application rules. Pega Platform performs rule assembly when the system starts new computing resources, deploys new code, or runs updated rules for the first time in the application. Pega refers to this action as First Use Assembly (FUA). By running the vast majority of rules using FUA, you optimize system performance because the process moves the code into a cache, which allows the system to process a rule in your application much faster.

A rule cache is an in-memory collection of recently used rules. Pega Platform maintains rule caches to improve application performance by avoiding unnecessary rule resolution and database interactions. When an application requests a rule, Pega Platform runs the rule if it is in the cache. If the rule is not in the cache, Pega Platform processes the rule and then adds the rule to the cache.

Pega recommends that application administrators run the static assembler after you import an application that contains a large number of new rules, after a software update, or after any significant change occurs during the application development cycle. Re-assembly of rules is optional.