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SvelteKitBuild and deploy

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:

/** @param {AdapterSpecificOptions} options */ export default function (options: any
@paramoptions 
options) { /** @type {import('@sveltejs/kit').Adapter} */ const const adapter: Adapter
@type{import('@sveltejs/kit').Adapter}
adapter = { Adapter.name: string
The name of the adapter, using for logging. Will typically correspond to the package name.
 
name: 'adapter-package-name', async Adapter.adapt: (builder: Builder) => MaybePromise<void>
This function is called after SvelteKit has built your app.
 
@parambuilder An object provided by SvelteKit that contains methods for adapting the app
adapt(builder: Builderbuilder) { // adapter implementation }, async Adapter.emulate?: (() => MaybePromise<Emulator>) | undefined
Creates an Emulator, which allows the adapter to influence the environment during dev, build and prerendering.
 
emulate() { return { async 
Emulator.platform?(details: {  config: any;  prerender: PrerenderOption; }): MaybePromise<App.Platform>
A function that is called with the current route config and prerender option and returns an App.Platform object
 
platform({ config: anyconfig, prerender: PrerenderOptionprerender }) { // the returned object becomes `event.platform` during dev, build and // preview. Its shape is that of `App.Platform` } } }, 
Adapter.supports?: {  read?: (details: {  config: any;  route: {  id: string;  };  }) => boolean;  instrumentation?: () => boolean; } | undefined
Checks called during dev and build to determine whether specific features will work in production with this adapter.
 
supports: { read: ({ config: anyconfig, 
route: {  id: string; }
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 }, tracing: () => voidtracing: () => { // Return `true` if this adapter supports loading `tracing.server.js`. // Return `false if it can't, or throw a descriptive error. } } };  return const adapter: Adapter
@type{import('@sveltejs/kit').Adapter}
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
    • Globally shims fetch to work on the target platform, if necessary. SvelteKit provides a @sveltejs/kit/node/polyfills helper for platforms that can use undici
  • 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].

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