Apache provides an application programming interface (API) to modules in order to insulate them from the mechanics of the HTTP protocol and from each other. In this chapter, we explore the main concepts of the API and provide a detailed listing of the functions available to the module author.
The most important thing to understand about the Apache API is the idea of a pool. This is a grouped collection of resources (i.e., file handles, memory, child programs, sockets, pipes, and so on) that are released when the pool is destroyed. Almost all resources used within Apache reside in pools, and their use should only be avoided with careful thought.
An interesting feature of pool resources is that many of them can be released only by destroying the pool. Pools may contain subpools, and subpools may contain subsubpools, and so on. When a pool is destroyed, all its subpools are destroyed with it.
Naturally enough, Apache creates a pool at startup, from which all other pools are derived. Configuration information is held in this pool (so it is destroyed and created anew when the server is restarted with a kill). The next level of pool is created for each connection Apache receives and is destroyed at the end of the connection. Since a connection can span several requests, a new pool is created (and destroyed) for each request. In the process of handling a request, various modules create their own pools, and some also create subrequests, which are pushed through the API machinery as if they were real requests. Each of these pools can be accessed through the corresponding structures (i.e., the connect structure, the request structure, and so on).
With this in mind, we can more clearly state when you should not use a pool: when the lifetime of the resource in question does not match the lifetime of a pool. If you need temporary storage (or files, or whatever), you can create a subpool of a convenient pool (the request pool is the most likely candidate) and destroy it when you are done, so having a lifetime that is shorter than the pool's is not normally a good enough excuse. The only example we can think of where there is no appropriate pool is the code for handling listeners (copy_listeners() and close_unused_listeners() in http_main.c), which have a lifetime longer than the topmost pool!
There are a number of advantages to this approach, the most obvious being that modules can use resources without having to worry about when and how to release them. This is particularly useful when Apache handles an error condition. It simply bails out, destroying the pool associated with the erroneous request, confident that everything will be neatly cleaned up. Since each instance of Apache may handle many requests, this functionality is vital to the reliability of the server. Unsurprisingly, pools come into almost every aspect of Apache's API, as we shall see in this chapter. They are defined in alloc.h :
typedef struct pool pool;
The actual definition of struct pool can be found in alloc.c, but no module should ever need to use it. All modules ever see of a pool is a pointer to it, which they then hand on to the pool APIs.
Like many other aspects of Apache, pools are configurable, in the sense that you can add your own resource management to a pool, mainly by registering cleanup functions (see the pool API later in this chapter).
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