construct/ircd

IRCd Library Definitions

This directory contains definitions and linkage for libircd

The purpose of libircd is to facilitate the execution of a server which handles requests from end-users. The library hosts a set of pluggable modules which may introduce the actual application features (or the "business logic") of the server.

The executable linking and invoking libircd may be referred to as the "embedding" or just the "executable" interchangably in this documentation.

libircd is single-threaded✝

The design of libircd is fully-asynchronous, single-thread-oriented. No code in the library blocks the process. All operations are conducted on top of a single boost::asio::io_service which must be supplied by the executable linking to libircd. That io_service must be orchestrated by the executable at its discretion; typically the embedder's call to ios.run() is the only place the process will block.

Applications are limited by one or more of the following bounds:

• Computing: program is limited by the efficiency of the CPU over time.
• Space: program is limited by the space available for its dataset.
• I/O: program is limited by external events, disks, and networks.

libircd is dominated by the I/O bound. Its design is heavily optimized for this assumption with its single-thread orientation. This methodology ensures there is an uninterrupted, uncontended, predictable execution which is easy for developers to reason about intuitively with sequential-consistency in a cooperative coroutine model.

If there are periods of execution which are computationally intense like parsing, hashing, cryptography, etc: this is absorbed in lieu of thread synchronization and bus contention. This system achieves scale through running multiple independent instances which synchronize at the application-logic level.

✝ However, don't start assuming a truly threadless execution for the entire address space. If there is ever a long-running background computation or a call to a 3rd party library which will do IO and block the event loop, we may use an additional std::thread to "offload" such an operation. Thus we do have a threading model, but it is heterogeneous.

libircd introduces userspace threading✝

IRCd presents an interface introducing stackful coroutines, a.k.a. userspace context switching, a.k.a. green threads, a.k.a. fibers. The library avoids callbacks as the way to break up execution when waiting for events. Instead, we harken back to the simple old ways of synchronous programming where control flow and data are easy to follow.

✝ If there are certain cases where we don't want a stack to linger which may jeopardize the c10k'ness of the daemon the asynchronous pattern is still used.

libircd can be embedded in your application with very minimal overhead.

Linking to libircd from your executable allows you to customize and extend the functionality of the server and have control over its execution, or, simply use library routines provided by the library without any daemonization.

libircd runs only one server at a time.

Keeping with the spirit of simplicity of the original architecture, libircd continues to be a "singleton" object which uses globals and keeps actual server state in the library itself. In other words, only one IRC daemon can exist within a process's address space at a time. Whether or not this was a pitfall of the original design, it has emerged over the decades as a very profitable decision for making IRCd an accessible open source internet project.

libircd leverages formal grammars

We utilize the boost::spirit system of parsing and printing through formal grammars, rather than writing our own parsers manually. In addition, we build several tools on top of such formal devices like a type-safe format string library acting as a drop-in for ::sprintf(), but accepting objects like std::string without .c_str() and prevention of outputting unprintable/unwanted characters that may have been injected into the system somewhere prior.

Overview

libircd is designed specifically as a shared object library. The purpose of its shared'ness is to facilitate IRCd's modular design: IRCd ships with many other shared objects which introduce the "business logic" and features of the daemon. If libircd was not a shared object, every single module would have to include large amounts of duplicate code drawn from the static library. This would be a huge drag on both compilation and the runtime performance.

                           (module)   (module)
                               |         |
                               |         |
                               V         V
                             |-------------|
----------------------       |             | < ---- (module)
|                    |       |             |
|  User's executable | <---- |   libircd   |
|                    |       |             |
----------------------       |             | < ---- (module)
                             |-------------|
                               ^         ^
                               |         |
                               |         |
                           (module)   (module)

The user (which we may also refer to as the "embedder" elsewhere in documentation) only deals directly with libircd and not the modules. libircd is generally loaded with its symbols bound globally in the executable and on most platforms cannot be unloaded (or even loaded) manually and has not been tested to do so. As an aside, we do not summarily dismiss the idea of reload capability and would like to see it made possible.