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108 lines
5.2 KiB
Markdown
108 lines
5.2 KiB
Markdown
# Architectural Philosophy
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### libircd
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##### Single-threaded✝
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The design of `libircd` is fully-asynchronous, single-thread-oriented. No code
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in the library _blocks_ the process. All operations are conducted on top of
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a single `boost::asio::io_service` which must be supplied by the executable
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linking to `libircd`. That `io_service` must be orchestrated by the executable
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at its discretion; typically the embedder's call to `ios.run()` is the only
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place the process will _block_.
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> Generally, applications are limited by one or more of the following bounds:
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> Computing, Memory (Space), Memory (Peripheral I/O)
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>
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> `libircd` is dominated by the **I/O bound**.
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Our effort is rooted in the above assumption. The single-threaded approach
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ensures there is an _uninterrupted_, _uncontended_, _predictable_ execution
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which is easy for developers to reason about intuitively with
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sequential-consistency in a cooperative coroutine model. If there are periods
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of execution which are computationally intense like parsing, hashing,
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cryptography, etc: this is absorbed in lieu of thread synchronization and bus
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contention.
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This system achieves scale through running multiple independent instances which
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synchronize at the application-logic level through passing the application's own
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messages.
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✝ However, do not assume a truly threadless execution for the entire address
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space. If there is ever a long-running background computation or a call to a
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3rd party library which will block the event loop, we may use an additional
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`std::thread` to "offload" such an operation. Thus we do have a threading model,
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but it is heterogeneous.
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##### Introduces userspace threading
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IRCd presents an interface introducing stackful coroutines, a.k.a. userspace
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context switching, a.k.a. green threads, a.k.a. fibers. The library avoids callbacks
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as the way to break up execution when waiting for events. Instead, we harken back
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to the simple old ways of synchronous programming where control flow and data are
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easy to follow.
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If there are certain cases where we don't want a stack to linger which may
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jeopardize the c10k'ness of the daemon the asynchronous pattern is still used,
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thus this is a hybrid system.
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Consider coroutines like "macro-ops" and asynchronous callbacks like
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"micro-ops." The pattern tends to use a coroutine to perform a large and
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complex operation which may involve many micro-ops behind the scenes. This
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approach relegates the asynchronous callback pattern to simple tasks contained
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within specific units which require scale, encapsulating the complexity away
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from the rest of the project.
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##### Can be embedded in your application with very minimal overhead.
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Linking to libircd from your executable allows you to customize and extend the
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functionality of the server and have control over its execution, or, simply use
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library routines provided by the library without any daemonization. Users of
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the library should never pay for what they don't use. The library should also
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minimize conflicts with other libraries sharing the address space.
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##### Runs only one server at a time.
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Keeping with the spirit of simplicity of the original architecture, `libircd`
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continues to be a "singleton" object which uses globals and keeps actual server
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state in the library itself. In other words, **only one IRC daemon can exist
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within a process's address space at a time.** Whether or not this was a pitfall
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of the original design, it has emerged over the decades as a very profitable
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decision for making IRCd an accessible open source internet project.
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##### Leverages formal grammars
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We utilize the `boost::spirit` system of parsing and printing through formal grammars,
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rather than writing our own parsers manually. In addition, we build several tools
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on top of such formal devices like a type-safe format string library acting as a
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drop-in for `::sprintf()`, but accepting objects like `std::string` without `.c_str()`
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and prevention of outputting unprintable/unwanted characters that may have been
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injected into the system somewhere prior.
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##### Modular design
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`libircd` is designed specifically as a shared object library. The purpose of its
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shared'ness is to facilitate IRCd's modular design: IRCd ships with many other
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shared objects which introduce the "business logic" and features of the daemon. If
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`libircd` was not a shared object, every single module would have to include large
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amounts of duplicate code drawn from the static library. This would be a huge drag
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on both compilation and the runtime performance.
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```
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|-------------|
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---------------------- | | < ---- (module)
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| User's executable | <---- | libircd | < ---- (module)
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---------------------- | | < ---- (module)
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|-------------|
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```
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The user (which we may also refer to as the "embedder" elsewhere in
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documentation) only deals directly with `libircd` and not the modules.
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`libircd` is generally loaded with its symbols bound globally in the executable
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and on most platforms cannot be unloaded (or even loaded) manually and has not
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been tested to do so. As an aside, we do not summarily dismiss the idea of
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reload capability and would like to see it made possible.
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