mirror of
https://github.com/matrix-construct/construct
synced 2024-11-16 06:51:08 +01:00
65 lines
3.3 KiB
Markdown
65 lines
3.3 KiB
Markdown
# Architectural Philosophy
|
|
|
|
### libircd
|
|
|
|
##### Single-threaded✝
|
|
|
|
The design of `libircd` is fully-asynchronous, oriented around a single-thread
|
|
event-loop. 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 run by the
|
|
executable at its discretion; typically the embedder's call to `ios.run()` is
|
|
the only place the process will _block_.
|
|
|
|
The single-threaded approach ensures there is an _uninterrupted_, _uncontended_,
|
|
_predictable_ execution which is easy for developers to reason about intuitively
|
|
with sequential-consistency. This is ideal for the I/O-bound application being
|
|
facilitated. 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 through passing the application's own
|
|
messages.
|
|
|
|
✝ 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 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 (this is a hybrid system).
|
|
|
|
Consider coroutines like "macro-ops" and asynchronous callbacks like
|
|
"micro-ops." The pattern tends to use a coroutine to perform a large and
|
|
complex operation which may involve many micro-ops behind the scenes. This
|
|
approach relegates the asynchronous callback pattern to simple tasks contained
|
|
within specific units which require scale, encapsulating the complexity away
|
|
from the rest of the project.
|
|
|
|
##### 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.
|
|
|
|
##### Formal grammars, RTTI, exceptions
|
|
|
|
We utilize the `boost::spirit` system of parsing and printing through
|
|
compile-time 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.
|