Abstract

Squid is a WWW Cache application developed by the National Laboratory for Applied Network Research and members of the Web Caching community. Squid is implemented as a single, non-blocking process based around a BSD select() loop. This document describes the operation of the Squid source code and is intended to be used by others who wish to customize or improve it.

Introduction

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 while we do make some effort to document code as we go some parts of the documentation may be left out. If you find any inconsistencies, please feel free to notify https://www.squid-cache.org/Support/contact.dyn the Squid Developers.