PlusWeb

An Express-style HTTP framework for C++, built on a libuv event loop, the llhttp parser, and a routing trie.

Try the router in your browser — the real trie compiled to WebAssembly. Or read why I wrote this, including the std::map that was making every response 339x slower than it needed to be.

#include <PlusWeb/HttpServer.h>

int main() {
    HttpServer app(3000);

    app.GET("/users/:id", [](HttpRequest& req, HttpResponse& res) {
        res.status(200).send(nlohmann::json{
            {"id", req.params["id"]},
            {"name", "ada"},
        });
    });

    app.serve([] { std::cout << "listening on :3000\n"; });
}

Work in progress

Routing, middleware, JSON and HTTP parsing work and are covered by tests. The core is fast and the parser handles malformed input safely. But there is no TLS, no body size limit, no idle timeout, no cookie parsing, and no static file serving. Handlers run on the event loop thread, so a handler that blocks stops the server.

Use it to learn how a web framework works, or to serve something you control. Do not put it on the open internet yet. The roadmap lists what is missing.

Speed

PlusWeb is a lot faster than Express. How much depends on how many routes you have, because Express matches routes by walking its layer list in registration order while PlusWeb walks a trie.

Both servers running single threaded on one pinned core, identical routes and handlers, 32 keep-alive connections:

PlusWeb stays flat at roughly 90,000 requests per second no matter how big the route table gets. Express drops by a factor of 50 between its first registered route and its last once you have 10,000 of them.

The gap is widest on things a real app does. Requests that match nothing (404s, scanners, bad links) run at 76,568 rps against Express's 393 once 10,000 routes are registered.

Under load the difference shows up as reliability rather than throughput. Driving both servers with 512 concurrent connections against a 1,000 route table, Express left 264 of those connections with no response at all across a 5 second run, and its slowest request took 4,988 ms. PlusWeb answered every connection in every run at every concurrency level tested, worst case 20 ms.

It is also smaller: 4.6 MB resident against Express's 92 MB, and it starts fast enough that boot time does not register.

Express wins on breadth, not speed. It has sessions, cookies, static files, body parsers, template engines and TLS. PlusWeb has a fast core and a list of things it cannot do yet.

Where the gap narrows: 100 KB response bodies bring it down to 4.5x, and connections that close after one request bring it down to 4.1x, because PlusWeb allocates a parser per connection. Full numbers, including the cases where it does worst, are in PROFILING_REPORT.md.

Why

I wanted to understand what a web framework actually does between the socket and the handler, so I wrote one. The interesting parts turned out to be the router and the parser, and most of the bugs were in the parser.

Requirements

• A C++17 compiler
• CMake 3.14+
• libuv 1.0+ (libuv-dev on Debian, libuv on Arch and Homebrew)
• libcurl, for the tests only

llhttp, nlohmann/json and GoogleTest are fetched by CMake if they are not already installed.

Build

cmake -B build
cmake --build build
ctest --test-dir build

Builds are Release unless you ask for something else, for example cmake -B build -DCMAKE_BUILD_TYPE=Debug.

Run an example from the repo root, so the static file path resolves:

./build/examples/hello_world     # :3000
./build/examples/rest_api        # :8084, shows routing, middleware, routers, JSON

Use it in your project

However you get it, the consumer side is the same:

find_package(PlusWeb CONFIG REQUIRED)
target_link_libraries(my_app PRIVATE PlusWeb::PlusWeb)

Install it from source with cmake --install build, or from a package manager. PlusWeb is not in the vcpkg or Conan registries yet, so both come from this repo for now:

# vcpkg, as an overlay port
vcpkg install plusweb --overlay-ports=PlusWeb/packaging/vcpkg

# Conan, from the recipe in the repo
conan create PlusWeb/packaging/conan --build=missing

packaging/ has the details, including what changes when these go upstream. Windows is not supported yet: nothing in the library is POSIX-bound, but it has never been built there.

Routing

GET, POST, PUT, DELETE, PATCH, OPTIONS, HEAD and ALL are available, each with a lowercase alias like app.get(...). Path parameters use :name and land in req.params:

app.GET("/users/:id/posts/:postId", [](HttpRequest& req, HttpResponse& res) {
    res.send("user " + req.params["id"] + " post " + req.params["postId"]);
});

Literal segments beat parameters, so /users/new wins over /users/:id when both are registered. If the literal branch has no handler for the path being matched, the router backtracks to the parameter branch. Registering /users/new/edit therefore still leaves /users/new matching /users/:id.

A GET route also answers HEAD with the same headers and no body.

Request

Response

status() and setHeader() chain. send() picks the Content-Type from the type you hand it: nlohmann::json becomes application/json, a std::string becomes text/html, a std::vector<uint8_t> becomes application/octet-stream.

res.status(201).setHeader("X-Powered-By", "PlusWeb").send(json{{"ok", true}});

Unmatched routes get a JSON 404: {"error": "Not Found", "path": ..., "method": ...}.

Middleware

Middleware runs before routing, so it can short-circuit a request. Call next() to continue, or respond without calling it to stop the chain:

app.use([](HttpRequest& req, HttpResponse& res, NextFunction next) {
    if (req.headers["Authorization"].empty()) {
        res.status(401).send(json{{"error", "Unauthorized"}});
        return;   // chain stops here, the route never runs
    }
    next();
});

app.use("/api", authMiddleware);   // scoped to a path prefix

Routers

Group routes and mount them under a prefix:

Router api;
api.GET("/health", [](HttpRequest& req, HttpResponse& res) {
    res.send(json{{"status", "ok"}});
});

app.use("/v1", api);   // serves GET /v1/health

Middleware has to be registered on a router before you mount it.

Serving files

There is no express.static() equivalent, so you wire the route yourself:

app.GET("/logo.png", [](HttpRequest& req, HttpResponse& res) {
    std::vector<uint8_t> bytes;
    if (!Utils::readFile("public/logo.png", bytes)) {
        res.status(404).send(json{{"error", "Not Found"}});
        return;
    }
    res.status(200).send(bytes).setHeader("Content-Type", Utils::mimeTypeFor("logo.png"));
});

Shutdown

serve() blocks until stop() is called. stop() is safe to call from another thread, which is how you unblock it:

std::thread t([&] { app.serve(); });
app.stop();
t.join();

How it works

Routes live in a trie keyed by METHOD:/path/segments. A lookup costs one step per path segment instead of a scan over every registered route, and that holds whether the path matches or not. Parameter nodes get their own pointer rather than being found by scanning a node's children, which is what keeps 404s cheap.

Requests are parsed by llhttp, the parser Node uses. It does framing as well as parsing, so pipelined requests, chunked bodies and requests split across packets all work, and malformed input gets a 400 instead of being trusted. Request lines and headers are capped at 16 KiB, past which the request gets a 431.

One libuv event loop handles every connection. Idle keep-alive clients cost nothing and the number of concurrent clients is not capped by the CPU count. Handlers run on the loop thread, so a slow handler blocks everything, the same rule that applies in Node.

Source layout: include/PlusWeb/ has the public headers, src/ the implementation, tests/ the unit and integration tests, examples/ runnable programs, bench/ the benchmark harness, and wasm/ a WebAssembly build of the router and parser.

Tests

17 tests. Unit tests for path splitting, plus an integration suite that boots a real server and drives it over loopback with libcurl.

ctest --test-dir build --output-on-failure

CI runs them on Linux and macOS, then again under AddressSanitizer, UndefinedBehaviorSanitizer and LeakSanitizer.

The benchmark and profiling harness lives in bench/ and builds with -DPLUSWEB_BUILD_BENCH=ON. It includes a load generator that reports how many connections got no response at all, which is the number that matters when a server is saturated. That generator and the reference server are written against epoll, so those two are Linux-only; the microbenchmarks and the server under test build anywhere. PROFILING_REPORT.md has the full numbers and the methodology, including the cases where PlusWeb does worst.

Roadmap

• TLS
• Request body size limits and idle connection timeouts
• Moving slow handlers off the loop thread with uv_queue_work
• express.static() style directory serving
• Cookie parsing, since req.cookies exists but is never populated
• Route-specific middleware, as in app.get(path, mw, handler)
• Pooling connection and parser objects, which is what costs PlusWeb most on connections that do not keep alive
• Structured logging instead of std::cerr

License

MIT