mirror of
https://github.com/matrix-construct/construct
synced 2024-11-14 22:11:06 +01:00
106 lines
5.1 KiB
Markdown
106 lines
5.1 KiB
Markdown
# Architectural Philosophy
|
|
|
|
### libircd
|
|
|
|
##### 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 with message passing.
|
|
|
|
✝ However, do not assume 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.
|
|
|
|
##### 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.
|
|
|
|
##### 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.
|
|
|
|
##### 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.
|
|
|
|
##### 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.
|
|
|
|
##### Modular design
|
|
|
|
`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.
|