Arbotext Publishing Engine Programmer's Guide (《Arbortext Publishing Engine 程序员指南》) > Arbortext Publishing Engine and its Components > Technical Overview of Arbortext Publishing Engine > Arbortext PE 子进程 Pools
  
Arbortext PE 子进程 Pools
The primary purpose of Arbortext Publishing Engine is the manipulation of documents using XML and related technologies. The Arbortext PE Request Manager acts as the facilitator handling requests and responses, but it has no knowledge of XML. XML processing is performed by Arbortext PE 子进程.
Each Arbortext PE 子进程 is a running instance of Arbortext’s XML processing engine, with the graphic user interface disabled and additional publishing and performance capabilities enabled. Groups of Arbortext PE 子进程 are organized into Arbortext PE 子进程 pools. When a dynamic Arbortext PE Request Manager component determines that it needs the services of an Arbortext PE 子进程, it invokes a method of the Arbortext Publishing Engine Request Context to obtain an Arbortext PE 子进程 from a particular pool, uses it to perform one or more operations, and then releases it so that the Arbortext PE Request Manager can use it to fulfill other requests.
An Arbortext PE 子进程 pool is a collection of identical Arbortext PE 子进程 that are associated by the following:
a set of request selectors that specify criteria for determining whether the Arbortext PE 子进程 in the pool should process a given transaction
a set of parameters that determine how many Arbortext PE 子进程 are in the pool, how often Arbortext PE 子进程 should be terminated and restarted, and other behaviors (explained in Arbortext PE 子进程 Pool Management).
Having multiple Arbortext PE 子进程 pools offer two major advantages:
Pools of Arbortext PE 子进程 can be initialized differently.
Arbortext PE 子进程 pools can be configured to process different types of requests.
When a dynamic component (most frequently, a Request Handler) determines that it needs an Arbortext PE 子进程, it calls an Arbortext Publishing Engine Request Context routine and passes a reference to the HTTP request being processed. The Arbortext Publishing Engine Request Context offers the request to each Arbortext PE 子进程 pool until one replies that it's configured to handle that type of request. The last Arbortext PE 子进程 pool queried is always the default pool, which can allocate an Arbortext PE 子进程 to fulfill any request not already handled.