The Squid source code has evolved more from empirical observation and tinkering, rather than a solid design process. It carries a legacy of being ``touched'' by numerous individuals, each with somewhat different techniques and terminology.
Squid is a single-process proxy server. Every request is handled by the main process, with the exception of FTP. However, Squid does not use a ``threads package'' such has Pthreads. While this might be easier to code, it suffers from portability and performance problems. Instead Squid maintains data structures and state information for each active request.
The code is often difficult to follow because there are no explicit state variables for the active requests. Instead, thread execution progresses as a sequence of ``callback functions'' which get executed when I/O is ready to occur, or some other event has happened. As a callback function completes, it is responsible for registering the next callback function for subsequent I/O.
Note there is only a pseudo-consistent naming scheme. In
most cases functions are named like moduleFooBar().
However, there are also some functions named like
module_foo_bar().
Note that the Squid source changes rapidly, and some parts of this document may become out-of-date. If you find any inconsistencies, please feel free to notify the Squid Developers.
Function names and file names will be written in a courier
font, such as store.c and storeRegister(). Data
structures and their members will be written in an italicized
font, such as StoreEntry.