99-writing-adapters.md

title
Writing adapters

If an adapter for your preferred environment doesn't yet exist, you can build your own. We recommend looking at the source for an adapter to a platform similar to yours and copying it as a starting point.

Adapter packages implement the following API, which creates an Adapter:

// @errors: 2322
// @filename: ambient.d.ts
type AdapterSpecificOptions = any;

// @filename: index.js
// ---cut---
/** @param {AdapterSpecificOptions} options */
export default function (options) {
	/** @type {import('@sveltejs/kit').Adapter} */
	const adapter = {
		name: 'adapter-package-name',
		async adapt(builder) {
			// adapter implementation
		},
		async emulate() {
			return {
				async platform({ config, prerender }) {
					// the returned object becomes `event.platform` during dev, build and
					// preview. Its shape is that of `App.Platform`
				}
			}
		},
		supports: {
			read: ({ config, route }) => {
				// Return `true` if the route with the given `config` can use `read`
				// from `$app/server` in production, return `false` if it can't.
				// Or throw a descriptive error describing how to configure the deployment
			},
			instrumentation: () => {
				// Return `true` if this adapter supports loading `instrumentation.server.js`.
				// Return `false if it can't, or throw a descriptive error.
			}
		}
	};

	return adapter;
}

Of these, name and adapt are required. emulate and supports are optional.

Within the adapt method, there are a number of things that an adapter should do:

• Clear out the build directory
• Write SvelteKit output with builder.writeClient, builder.writeServer, and builder.writePrerendered
• Output code that:
  • Imports Server from ${builder.getServerDirectory()}/index.js
  • Instantiates the app with a manifest generated with builder.generateManifest({ relativePath })
  • Listens for requests from the platform, converts them to a standard Request if necessary, calls the server.respond(request, { getClientAddress }) function to generate a Response and responds with it
  • expose any platform-specific information to SvelteKit via the platform option passed to server.respond
• Bundle the output to avoid needing to install dependencies on the target platform, if necessary
• Put the user's static files and the generated JS/CSS in the correct location for the target platform

Where possible, we recommend putting the adapter output under the build/ directory with any intermediate output placed under .svelte-kit/[adapter-name].