Astro Adapter API

Astro is designed to make it easy to deploy to any cloud provider for SSR (server-side rendering). This ability is provided by adapters, which are integrations. See the SSR guide to learn how to use an existing adapter.

What is an adapter

An adapter is a special kind of integration that provides an entrypoint for server-side rendering. An adapter does two things:

• Implements host-specific APIs for handling requests.
• Configures the build according to host conventions.

Building an Adapter

An adapter is an integration and can do anything that an integration can do.

An adapter must call the setAdapter API in the astro:config:done hook like so:

my-adapter.mjs

export default function createIntegration() {
  return {
    name: '@matthewp/my-adapter',
    hooks: {
      'astro:config:done': ({ setAdapter }) => {
        setAdapter({
          name: '@matthewp/my-adapter',
          serverEntrypoint: '@matthewp/my-adapter/server.js',
          supportedAstroFeatures: {
              staticOutput: 'stable'
          }
        });
      },
    },
  };
}

The object passed into setAdapter is defined as:

interface AstroAdapter {
  name: string;
  serverEntrypoint?: string;
  exports?: string[];
  adapterFeatures: AstroAdapterFeatures;
  supportedAstroFeatures: AstroFeatureMap;

}

export interface AstroAdapterFeatures {
  /**
   * Creates an edge function that will communicate with the Astro middleware.
   */
  edgeMiddleware: boolean;
  /**
   * SSR only. Each route becomes its own function/file.
   */
  functionPerRoute: boolean;
}

export type SupportsKind = 'unsupported' | 'stable' | 'experimental' | 'deprecated';

export type AstroFeatureMap = {
  /**
   * The adapter is able to serve static pages
   */
  staticOutput?: SupportsKind;
  /**
   * The adapter is able to serve pages that are static or rendered via server
   */
  hybridOutput?: SupportsKind;
  /**
   * The adapter is able to serve SSR pages
   */
  serverOutput?: SupportsKind;
  /**
   * The adapter can emit static assets
   */
  assets?: AstroAssetsFeature;
};

export interface AstroAssetsFeature {
  supportKind?: SupportsKind;
  /**
   * Whether this adapter deploys files in an environment that is compatible with the library `sharp`
   */
  isSharpCompatible?: boolean;
  /**
   * Whether this adapter deploys files in an environment that is compatible with the library `squoosh`
   */
  isSquooshCompatible?: boolean;
}

The properties are:

• name: A unique name for your adapter, used for logging.
• serverEntrypoint: The entrypoint for server-side rendering.
• exports: An array of named exports when used in conjunction with createExports (explained below).
• adapterFeatures: An object that enables specific features that must be supported by the adapter. These features will change the built output, and the adapter must implement the proper logic to handle the different output.
• supportedAstroFeatures: A map of Astro built-in features. This allows Astro to determine which features an adapter is unable or unwilling to support so appropriate error messages can be provided.

Server Entrypoint

Astro’s adapter API attempts to work with any type of host, and gives a flexible way to conform to the host APIs.

Exports

Some serverless hosts expect you to export a function, such as handler:

...

export function handler(event, context) {
}

With the adapter API you achieve this by implementing createExports in your serverEntrypoint:

import { App } from 'astro/app';

export function createExports(manifest) {
  const app = new App(manifest);

  const handler = (event, context) => {
    // ...
  };

  return { handler };
}

And then in your integration, where you call setAdapter, provide this name in exports:

my-adapter.mjs

export default function createIntegration() {
  return {
    name: '@matthewp/my-adapter',
    hooks: {
      'astro:config:done': ({ setAdapter }) => {
        setAdapter({
          name: '@matthewp/my-adapter',
          serverEntrypoint: '@matthewp/my-adapter/server.js',
          exports: ['handler'],
        });
      },
    },
  };
}

Start

Some hosts expect you to start the server yourselves, for example by listening to a port. For these types of hosts, the adapter API allows you to export a start function which will be called when the bundle script is run.

import { App } from 'astro/app';

export function start(manifest) {
  const app = new App(manifest);

  addEventListener('fetch', event => {
    // ...
  });
}

astro/app

This module is used for rendering pages that have been prebuilt through astro build. Astro uses the standard Request and Response objects. Hosts that have a different API for request/response should convert to these types in their adapter.

import { App } from 'astro/app';
import http from 'http';

export function start(manifest) {
  const app = new App(manifest);

  addEventListener('fetch', event => {
    event.respondWith(
      app.render(event.request)
    );
  });
}

The following methods are provided:

app.render(request, routeData, locals)

This method calls the Astro page that matches the request, renders it, and returns a Promise to a Response object. This also works for API routes that do not render pages.

const response = await app.render(request);

The method accepts a mandatory request argument, and two other optional arguments: routeData and locals.

Provide a value for routeData if you already know the route to render. Doing so will bypass the internal call to app.match to determine the route to render.

When used, locals must be the third argument passed. You can pass undefined for routeData if you are not targeting a specific route.

The example below reads a header named x-private-header, attempts to parse it as an object and pass it to locals, which can then be passed to any middleware function.

const privateHeader = request.headers.get("x-private-header");
let locals = {};
try {
    if (privateHeader) {
        locals = JSON.parse(privateHeader);
    }
} finally {
    const response = await app.render(request, undefined, locals);
}

app.match(request)

This method is used to determine if a request is matched by the Astro app’s routing rules.

if(app.match(request)) {
  const response = await app.render(request);
}

You can usually call app.render(request) without using .match because Astro handles 404s if you provide a 404.astro file. Use app.match(request) if you want to handle 404s in a different way.

Allow installation via astro add

The astro add command allows users to easily add integrations and adapters to their project. If you want your adapter to be installable with this tool, add astro-adapter to the keywords field in your package.json:

{
  "name": "example",
  "keywords": ["astro-adapter"],
}

Once you publish your adapter to npm, running astro add example will install your package with any peer dependencies specified in your package.json. We will also instruct users to update their project config manually.

Astro features

Added in: astro@3.0.0

Astro features are a way for an adapter to tell Astro whether they are able to support a feature, and also the adapter’s level of support.

When using these properties, Astro will:

• run specific validation;
• emit contextual to the logs;

These operations are run based on the features supported or not supported, their level of support, and the configuration that the user uses.

The following configuration tells Astro that this adapter has experimental support for assets, but the adapter is not compatible with the built-in services Sharp and Squoosh:

my-adapter.mjs

export default function createIntegration() {
  return {
    name: '@matthewp/my-adapter',
    hooks: {
      'astro:config:done': ({ setAdapter }) => {
        setAdapter({
          name: '@matthewp/my-adapter',
          serverEntrypoint: '@matthewp/my-adapter/server.js',
          supportedAstroFeatures: {
            assets: {
              supportKind: "experimental",
              isSharpCompatible: false,
              isSquooshCompatible: false
            }
          }
        });
      },
    },
  };
}

Astro will log a warning to the terminal:

[@matthewp/my-adapter] The feature is experimental and subject to issues or changes.

and an error if the service used for assets is not compatible with the adapter:

[@matthewp/my-adapter] The currently selected adapter `@matthewp/my-adapter` is not compatible with the service "Sharp". Your project will NOT be able to build.

Adapter features

A set of features that changes the output of the emitted files. When an adapter opts in to these features, they will get additional information inside specific hooks.

functionPerRoute

This is a feature that is enabled when using SSR only. By default, Astro emits a single entry.mjs file, which is responsible for emitting the rendered page on each request.

When functionPerRoute is true, Astro will instead create a separate file for each route defined in the project.

Each file emitted will only render one page. The pages will be emitted inside a dist/pages/ directory (or under a /pages/ directory in the directory specified for outDir), and the emitted files will keep the same file paths of the src/pages/ directory.

The files inside the pages/ directory of the build will mirror the directory structure of your page files in src/pages/, for example:

• 디렉터리dist/

  • 디렉터리pages/

    • 디렉터리blog/

      • entry._slug_.astro.mjs
      • entry.about.astro.mjs
    • entry.index.astro.mjs

Enable the feature by passing true to the adapter.

my-adapter.mjs

export default function createIntegration() {
  return {
    name: '@matthewp/my-adapter',
    hooks: {
      'astro:config:done': ({ setAdapter }) => {
        setAdapter({
          name: '@matthewp/my-adapter',
          serverEntrypoint: '@matthewp/my-adapter/server.js',
          adapterFeatures: {
              functionPerRoute: true
          }
        });
      },
    },
  };
}

Then, consume the hook astro:build:ssr, which will give you an entryPoints object that maps a page route to the physical file emitted after the build.

my-adapter.mjs

export default function createIntegration() {
  return {
    name: '@matthewp/my-adapter',
    hooks: {
      'astro:config:done': ({ setAdapter }) => {
        setAdapter({
          name: '@matthewp/my-adapter',
          serverEntrypoint: '@matthewp/my-adapter/server.js',
          adapterFeatures: {
              functionPerRoute: true
          }
        });
      },

      'astro:build:ssr': ({ entryPoints }) => {
          for (const [route, entryFile] of entryPoints) {
              // do something with route and entryFile
          }
      }
    },
  };
}

Caution

The entryFile is of type URL and represents the physical path of the file in the file system. This means that the paths change based on the OS where the code runs.

Serverless environments

Setting functionPerRoute: true in a serverless environment creates a JavaScript file (handler) for each route. A handler might have different names based on your hosting platform: lambda, function, page, etc.

Each of these routes is subject to a cold start when the handler runs, which may cause some delay. This delay is influenced by different factors.

With functionPerRoute: false, there is only one single handler in charge of rendering all your routes. When this handler is first triggered, you will be subject to a cold start. Then, all other routes should function without delay. However, you will lose the benefit of code splitting that functionPerRoute: true provides.

Note

It’s important that you understand your hosting platform, and how it works, in order to choose the appropriate functionPerRoute configuration for your project.

edgeMiddleware

Defines whether any SSR middleware code will be bundled when built.

When enabled, this prevents middleware code from being bundled and imported by all pages during the build:

my-adapter.mjs

export default function createIntegration() {
  return {
    name: '@matthewp/my-adapter',
    hooks: {
      'astro:config:done': ({ setAdapter }) => {
        setAdapter({
          name: '@matthewp/my-adapter',
          serverEntrypoint: '@matthewp/my-adapter/server.js',
          adapterFeatures: {
              edgeMiddleware: true
          }
        });
      },
    },
  };
}

Then, consume the hook astro:build:ssr, which will give you a middlewareEntryPoint, an URL to the physical file on the file system.

my-adapter.mjs

export default function createIntegration() {
  return {
    name: '@matthewp/my-adapter',
    hooks: {
      'astro:config:done': ({ setAdapter }) => {
        setAdapter({
          name: '@matthewp/my-adapter',
          serverEntrypoint: '@matthewp/my-adapter/server.js',
          adapterFeatures: {
              edgeMiddleware: true
          }
        });
      },

      'astro:build:ssr': ({ middlewareEntryPoint }) => {
          // remember to check if this property exits, it will be `undefined` if the adapter doesn't opt in to the feature
          if (middlewareEntryPoint) {
             createEdgeMiddleware(middlewareEntryPoint)
          }
      }
    },
  };
}

function createEdgeMiddleware(middlewareEntryPoint) {
    // emit a new physical file using your bundler
}