How it works

Nest talks to its HTTP platform through the HttpServer interface, implemented by AbstractHttpAdapter. The Express and Fastify adapters implement it on top of Node’s http module. BunAdapter implements it on top of Bun.serve(). There is no node:http anywhere in the request path.

One request, end to end

1. Bun.serve calls the adapter’s fetch(request, server) with a Web Request.
2. The adapter wraps it in a BunRequest and creates a BunResponse.
3. The Router walks its layer stack in registration order: hooks, body parsers, CORS, Nest middleware, then the route handlers Nest registered, then Nest’s not-found and error handlers.
4. The matched Nest handler runs guards, pipes, interceptors and the controller method, then calls adapter.reply(res, body).
5. reply ends the response. BunResponse resolves its Promise<Response>, and Bun.serve sends it.

BunRequest

Express-shaped: method, url, path, query, params, headers, body, ip, hostname, protocol, get(), is(). It extends the node:stream Readable so stream-based middleware can consume the body. Everything not needed on the hot path is a lazy getter (headers, query, socket, ip), and the abort listener is only attached when something asks for disconnect events. That laziness is worth about 1.3µs per request, which is most of the gap to raw Bun.serve.

The Web Request is req.native.

BunResponse

Implements the node:stream Writable interface (Bun’s native implementation; no Node runtime involved) and resolves into a Web Response. Two paths:

• Buffered. end(body) with no prior write() collects the body and builds one fixed-length Response. This is every normal JSON reply.
• Streaming. The first write(), writeHead() or flushHeaders() commits status and headers and hands Bun a ReadableStream; later writes are enqueued. Nest’s SSE stream and StreamableFile call stream.pipe(response), which is why the response must speak the Writable interface at all.

On top sits the Express surface: status, set, json, send, redirect, cookie, sendFile, sendBunFile, render.

Router

An Express-style layer stack on path-to-regexp v8, the same route syntax Express 5 and Nest use. use() mounts prefix-matched middleware, verb methods register exact-match routes, first match wins, next() advances, next(err) jumps to the next 4-arity handler. HEAD requests fall back to GET routes.

Lifecycle

getHttpServer() returns a BunHttpServer, an EventEmitter that looks enough like a Node net.Server for Nest’s listen() and getUrl(): listen, close, address(), 'error' and 'listening' events. Graceful close() waits for server.pendingRequests to drain (up to shutdownTimeout) and then stops the server, dropping idle keep-alive connections, because Bun’s stop(false) cannot be followed by stop(true).

Two Bun details that mattered

• request.body.getReader() is much slower than request.arrayBuffer(). The body parsers use the fast path whenever Content-Length is present and stream only for chunked bodies.
• Bun 1.4 compiles TC39 decorators unless it finds experimentalDecorators in a tsconfig resolved from the working directory. Keep one at your repo root.