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v5-beta

On-demand Rendering

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Your Astro project code must be rendered to HTML in order to be displayed on the web.

By default, Astro pages, routes, and API endpoints will be pre-rendered at build time as static pages. However, you can choose to render some or all of your routes on demand by a server when a route is requested.

On-demand rendered pages and routes are generated per visit, and can be customized for each viewer. For example, a page rendered on demand can show a logged-in user their account information or display freshly updated data without requiring a full-site rebuild. On-demand rendering on the server at request time is also known as server-side rendering (SSR).

Enabling on-demand rendering

Section titled Enabling on-demand rendering

By default, your entire Astro site will be prerendered, and static HTML pages will be sent to the browser. However, you may opt out of prerendering on any routes that require server rendering, for example, a page that checks for cookies and displays personalized content.

First, add an adapter integration for your server runtime to enable on-demand server rendering in your Astro project.

Then, add export const prerender = false at the top of the individual page or endpoint you want to render on demand. The rest of your site will remain a static site:

src/pages/page-rendered-on-demand.astro
---
export const prerender = false
---
<html>
<!--
This content will be server-rendered on demand!
Just add an adapter integration for a server runtime!
All other pages are statically-generated at build time!
-->
<html>

The following example shows opting out of prerendering in order to display a random number each time the endpoint is hit:

src/pages/randomnumber.js
export const prerender = false;


export async function GET() {
  let number = Math.random();
  return new Response(
    JSON.stringify({
      number,
      message: `Here's a random number: ${number}`,
    }),
  );
}

'server' mode

Section titled &#39;server&#39; mode

For a highly dynamic app, after adding an adapter, set your build output configuration to output: 'server' to server-render all your pages by default. This is the equivalent of opting out of prerendering on every page.

Then, if needed, you can choose to prerender any individual pages that do not require a server to execute, such as a privacy policy or about page.

src/pages/about-my-app.astro
---
export const prerender = true
---
<html>
<!--
`output: 'server'` is configured, but this page is static!
The rest of my site is rendered on demand!
-->
<html>

Add export const prerender = true to any page or route to prerender a static page or endpoint:

src/pages/myendpoint.js
export const prerender = true;


export async function GET() {
  return new Response(
    JSON.stringify({
      message: `This is my static endpoint`,
    }),
  );
}
See more about the output setting in the configuration reference.

Add an Adapter

Section titled Add an Adapter

To render any of your pages on demand, for any output mode, you need to add an adapter.

Astro maintains official adapters for Node.js, Netlify, Vercel, and Cloudflare. You can find both official and community adapters in our integrations directory. Choose the one that corresponds to your deployment environment.

Each adapter allows Astro to output a script that runs your project on a specific runtime: the environment that runs code on the server to generate pages when they are requested (e.g. Netlify, Cloudflare).

You can add any of the official adapter integrations maintained by Astro with the following astro add command. This will install the adapter and make the appropriate changes to your astro.config.mjs file in one step.

For example, to install the Netlify adapter, run:

Terminal window
npx astro add netlify

You can also add an adapter manually by installing the NPM package (e.g. @astrojs/netlify) and updating astro.config.mjs yourself.

Note that different adapters may have different configuration settings. Read each adapter’s documentation, and apply any necessary config options to your chosen adapter in astro.config.mjs

On-demand rendering features

Section titled On-demand rendering features

HTML streaming

Section titled HTML streaming

With HTML streaming, a document is broken up into chunks, sent over the network in order, and rendered on the page in that order. Astro uses HTML streaming in on-demand rendering to send each component to the browser as it renders them. This makes sure the user sees your HTML as fast as possible, although network conditions can cause large documents to be downloaded slowly, and waiting for data fetches can block page rendering.

Méthode associée : Using streaming to improve page performance (EN)

Cookies

Section titled Cookies

A page or API endpoint rendered on demand can check, set, get, and delete cookies.

The example below updates the value of a cookie for a page view counter:

src/pages/index.astro
---
export const prerender = false; // Not needed in 'server' mode


let counter = 0


if (Astro.cookies.has('counter')) {
  const cookie = Astro.cookies.get('counter')
  const value = cookie?.number()
  if (value !== undefined && !isNaN(value)) counter = value + 1
}


Astro.cookies.set('counter', String(counter))
---
<html>
  <h1>Counter = {counter}</h1>
</html>

See more details about Astro.cookies and the AstroCookie type in the API reference.

Response

Section titled Response

Astro.response is a standard ResponseInit object. It can be used to set the response status and headers.

The example below sets a response status and status text for a product page when the product does not exist:

src/pages/product/[id].astro
---
export const prerender = false; // Not needed in 'server' mode


import { getProduct } from '../api';


const product = await getProduct(Astro.params.id);


// No product found
if (!product) {
  Astro.response.status = 404;
  Astro.response.statusText = 'Not found';
}
---
<html>
  <!-- Page here... -->
</html>

Astro.response.headers

Section titled Astro.response.headers

You can set headers using the Astro.response.headers object:

src/pages/index.astro
---
export const prerender = false; // Not needed in 'server' mode


Astro.response.headers.set('Cache-Control', 'public, max-age=3600');
---
<html>
  <!-- Page here... -->
</html>

Return a Response object

Section titled Return a Response object

You can also return a Response object directly from any page using on-demand rendering either manually or with Astro.redirect.

The example below looks up an ID in the database on a dynamic page and either it returns a 404 if the product does not exist, or it redirects the user to another page if the product is no longer available, or it displays the product:

src/pages/product/[id].astro
---
export const prerender = false; // Not needed in 'server' mode


import { getProduct } from '../api';


const product = await getProduct(Astro.params.id);


// No product found
if (!product) {
  return new Response(null, {
    status: 404,
    statusText: 'Not found'
  });
}


// The product is no longer available
if (!product.isAvailable) {
  return Astro.redirect("/products", 301);
}
---
<html>
  <!-- Page here... -->
</html>

Request

Section titled Request

Astro.request is a standard Request object. It can be used to get the url, headers, method, and even the body of the request.

You can access additional information from this object for pages that are not statically generated.

Astro.request.headers

Section titled Astro.request.headers

The headers for the request are available on Astro.request.headers. This works like the browser’s Request.headers. It is a Headers object where you can retrieve headers such as the cookie.

src/pages/index.astro
---
export const prerender = false; // Not needed in 'server' mode


const cookie = Astro.request.headers.get('cookie');
// ...
---
<html>
  <!-- Page here... -->
</html>

Astro.request.method

Section titled Astro.request.method

The HTTP method used in the request is available as Astro.request.method. This works like the browser’s Request.method. It returns the string representation of the HTTP method used in the request.

src/pages/index.astro
---
export const prerender = false; // Not needed in 'server' mode


console.log(Astro.request.method) // GET (when navigated to in the browser)
---

See more details about Astro.request in the API reference.

Server Endpoints

Section titled Server Endpoints

A server endpoint, also known as an API route, is a special function exported from a .js or .ts file within the src/pages/ folder. A powerful feature of server-side rendering on demand, API routes are able to securely execute code on the server.

The function takes an endpoint context and returns a Response.

To learn more, see our Endpoints Guide.

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