hulla.dev

Serverless

Choose a web runtime

Connect a Fetch handler to Bun, Deno, or Vercel.

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Bun, Deno, and Vercel can all run a handler that accepts a Request and returns a Response. Create that handler with fetchAdapter().mount(implementation), then connect it to your host using the examples below.

HostRegister the mounted handlerChoose it when
BunPass handler to Bun.serve()Your application owns the listener and startup code.
DenoExport { fetch: handler } for deno serveDeno should own the listener; use Deno.serve() when your application must own startup.
VercelExport { fetch: handler } from a root api/*.ts fileVercel should discover and deploy a standalone Web Handler.

The Bun and Deno rows start a long-running server process. The Vercel row exports a function that Vercel invokes from its file-based api directory. Use a framework or host-specific adapter when the host should own a native router, parsed body, response writer, or request context instead.

Once you have a contract implementation, mounting it is the same on every Fetch-compatible host:

TypeScript
// src/api/handler.ts
import { fetchAdapter } from '@hulla/api/fetch'
import { implementation } from './server'

export const handler = fetchAdapter().mount(implementation)

handler receives the complete Web Request, matches its method and pathname against the contract, decodes and validates declared inputs, runs the selected implementation, and returns a Web Response. The host then decides how that function is reached and where the response is sent.

@hulla/api owns the portable request lifecycle:

  • contract route selection, including normal 404 and 405 responses;
  • declared parameter, query, header, and body decoding and validation;
  • server context and handler execution;
  • declared response encoding and headers;
  • propagation of the Web request’s cancellation signal to portable handler code.

The host owns everything around that handler:

  • listener or function discovery and public path mapping;
  • ports, TLS, deployment, authentication, and platform logging;
  • body-size, duration, concurrency, streaming, and cancellation behavior;
  • host-native values that are not part of a Web Request.

Read the Fetch transport guide for adapter options such as maxBodyBytes and preserveRequestBody. Then choose a host-specific setup:

  • Bun mounts the handler in Bun.serve and verifies it with a local HTTP request.
  • Deno exports the handler for deno serve and keeps additional runtime permissions explicit.
  • Vercel Functions exports the handler from a root api file and aligns file routing with the contract path.