The HTTP syntax rules are simple to enforce when the parser is a state machine that works byte-by-byte. To write a faster parser, we have to use SIMD instructions. However, enforcing HTTP syntax rules becomes considerably more complex with SIMD instructions. That's why many parsers deliberately loosen HTTP syntax rules to employ them.

Kourier uses SIMD instructions extensively on its parser while maintaining strict adherence to HTTP syntax rules.

Kourier's parser is a performance powerhouse, capable of processing 12.1 million HTTP requests per second on an AMD Ryzen 5 1600, an 8-year-old mid-range processor, using only half of its cores (wrk uses the other half). It sets a new standard for HTTP parsing, leaving the top performers far behind. On a modern server-grade CPU, Kourier beats Lithium, the fastest C++ server on TechEmpower benchmarks and whose parser only scans for CRLFs without enforcing any HTTP syntax rule.

Kourier uses OpenSSL for TLS encryption. Although OpenSSL is battle-tested, integrating it properly can be a challenging task. Almost all users of OpenSSL employ file descriptor-based BIOs, which notoriously consume vast amounts of memory beyond being slow.

Kourier's implementation excels at TLS performance because it provides custom memory BIOs to OpenSSL, restricting it to TLS computations while keeping all other responsibilities under Kourier's control.

Signals and slots promote loosely coupled designs that propagate events through signals. Signals and slots unify frontend and backend programming into a single paradigm, as events can represent either user interaction or asynchronous I/O.

Kourier implements a modern signals and slots mechanism built upon C++'s powerful template metaprogramming capabilities. It is an order of magnitude faster than Qt's signals and slots and consumes four times less memory.

Writing a reliable server requires timers. Malicious users intentionally send data slowly on multiple connections to attack a server. Timers help to prevent that abuse from happening. However, userspace timer implementations are generally not designed with the requirements of high-performance servers as a use case.

For example, every time a server starts to process a request, it resets a request timer, and whenever the server responds to a request, it starts an idle timer. Typical timer implementations are not optimized for this use case, which involves multiple resets without timeouts.

Although deadlines prevent slow senders, they are useless against rogue senders that keep the connection open indefinitely after sending only part of a request.

Kourier provides a timer implementation that allows timers to be reset millions of times per second without incurring system calls or memory allocations. Kourier's implementation provides timeout-based timers and can be viewed as an ultra-lightweight version of Qt's coarse timers.

Kourier also provides one of the best implementations for using epoll, a high-performance Linux IO event notification interface, to monitor file descriptors.

On the Internet, the debate over level-triggered vs. edge-triggered is often limited to the difference in how the state is reset. However, the implications are more far-reaching than many developers think. As always, the truth lies in the source code. With level-triggering, epoll keeps all file descriptors that have become ready on its ready list even after their state is reset.

Implementing a server with unprecedented performance requires sharp attention to detail. How the system interacts with the low-level IO readiness model provided by the Kernel is crucial for its performance and reliability.

Kourier integrates epoll into Qt's event system and implements socket classes that utilize the signals and slots mechanism to abstract epoll-based I/O operations while providing all the benefits of having a Qt event loop running on worker threads.

Kourier exports TcpSocket and TlsSocket classes, which you can use instead of Qt's socket classes. Both are significantly faster and lighter than their Qt-based counterparts.

Kourier is developed with strict and demanding requirements, where all possible behaviors are comprehensively verified in specifications written in the Gherkin style. Test files use Spectator, a test framework that is part of Kourier and is included in Kourier's repository. You can check all files ending in .spec.cpp in the Kourier repository to see how meticulously tested Kourier is. There is a stark difference in testing rigor between Kourier and other publicly available servers.

Kourier is open-source. Thus, anyone can study, build, validate, and benchmark Kourier.

You can contact me at [email protected] if your Business is incompatible with the AGPL and you want to license Kourier under alternative terms.