Container APIs

proposals/0048-container-api.md

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+- Start Date: 2024-04-26
+- Reference Issues: https://github.com/withastro/roadmap/issues/533
+- Implementation PR: 
+
+# Summary
+
+Astro components are tightly coupled to `Astro` (the metaframework). This proposal introduces a possible server-side API for rendering `.astro` files in isolation, outside the full `astro` build pipeline.
+
+# Example
+
+```js
+import { AstroContainer } from 'astro/container';
+import Component from './components/Component.astro';
+
+const container = await AstroContainer.create();
+console.log(await container.renderToString(Component, { props, slots }))
+```
+
+The container can be optionally constructed with some settings that are typically reserved for Astro configuration.
+```js
+const container = await AstroContainer.create({
+    mode: "production",
+    streaming: true,
+    astroConfig: {
+        site: "https://example.com",
+        trailingSlash: false
+    }
+})
+```
+
+The second argument to `renderToString` optionally provides render-specific data that may be exposed through the Astro global, like `props`, `slots`, `request`, and/or `params`.
+```js
+await astro.renderToString(Component, { props, slots, request, params })
+```
+
+# Background & Motivation
+
+Some of our own proposals, as well as many third-party tool integrations, are blocked until we expose a proper server-side rendering API for `.astro` components. Other frameworks have tools like `preact-render-to-string` or `react-dom/server` that make rendering components on the server straight-forward.
+
+We've avoided doing this because... it's very hard! Part of the appeal of `.astro` components is that they have immediate access to a number of powerful APIs, context about your site, and a rich set of framework renderers. In the full `astro` build process, we are able to wire up all this context invisibly. Wiring up the context manually as a third-party is prohibitively complex.
+
+
+# Goals
+
+- Provide a **low-level** API for rendering `.astro` components in isolation
+- Expose a familiar, user-friendly API
+- Surface enough control for full `astro` parity, but abstract away internal APIs
+- Enable third-party tools to consume `.astro` files on the server
+- Enable unit testing of `.astro` component output
+- Possibly unblock `.mdx` compiledContent/html output?
+- Support Astro framework renderers (`@astrojs/*`) if possible
+
+# Non-Goals
+
+- Provide a way to **import** `.astro` components. Users will be responsible to provide a compiled component.
+- Provide a way to **import** `astro.config.(mjs|mts)` out of the box.
+
+# Detailed Design
+
+## Preface
+
+**This** RFC will have a smaller scoped compared to what users have envisioned. The reason why the scope shrunk is that 
+I want to ship a smaller feature on the surface, which will allow us, later to enhance it based on user's feedback and use cases.
+
+That's why the first iteration of the APIs won't provide a built-in way to compile Astro components.
+
+## Internal concepts
+
+The API took a lot of time to land because it required some internal refactors. Thanks to these refactors, we will be able
+to land this API by using the **very same engine** that Astro uses under the hood.
+
+Eventually, we landed to a common and abstract concept called **Pipeline**. A pipeline is simply an abstract
+class that is responsible to render any kind of Astro route: page, endpoint, redirect, virtual.
+
+Each pipeline inside the Astro codebase (dev, build, SSR and now test) is responsible to collect the important information in different way,
+but eventually each route must be rendered using the same data, which are:
+- **Manifest**: this is a serializable/deserializable version of our "configuration", however it also contains more information, such as renderers, client directives, styles.
+- **Request**: a standard https://developer.mozilla.org/en-US/docs/Web/API/Request object.
+- **RouteData**: a type that identifies a generic Astro route. It contains information such as the type of route, the associated component, etc.
+- **ComponentInstance**: The instance of an Astro component. This is the **compiled** component. Internally, an Astro component undergoes various transformations via Vite, and it eventually gets consumed by our rendering engine (if it's a page).
+
+## Public APIs
+
+- `AstroContainer::create`: creates a new instance of the container.
+- `AstroContainer::renderToString`: renders a component and return a string.
+- `AstroContainer::renderToResponse`: renders a component and returns the `Response` emitted by the rendering phase.
+
+### `create` function
+
+A container is class exposed via the `astro/container` specifier. Users **shouldn't** create a container using the `new` instance, but they should use he static function `create`:
+
+```js
+const container = await AstroContainer.create()
+```
+
+The reason why `create` returns a promise is because internally we will validate and parse the user configuration. As a developer, you will be able to pass the same configuration that Astro uses:
+
+```js
+import astroConfig from "../src/astro.config.mjs";
+
+const container = await AstroContainer.create({
+    astroConfig
+})
+```
+
+The function `create` will throw an error if there's a validation error.
+
+> [!NOTE]
+> If you use a TypeScript file for your configuration, you are responsible for loading and transforming it to JavaScript.
+
+
+The `container` binding will expose a `renderToString` that accepts a Astro component and will return a `string`:
+
+```js
+import astroConfig from "../src/astro.config.mjs";
+
+const container = await AstroContainer.create({
+    astroConfig
+})
+
+const content = await container.renderToString(AstroComponent);
+```
+
+#### Options
+
+It will be possible to tweak the container APIs with options and such. The `.create` function will accept the following interface:
+
+```ts
+type AstroContainerOptions = {
+    streaming: boolean;
+    renderers: AstroRenderer[];
+    astroConfig: AstroUserConfig;
+}
+```
+
+The `astroConfig` object is literally the same object exposed by the `defineConfig`, inside the `astro.config.mjs` file. This very configuration 
+will go under the same schema validation that Astro uses internally. This means that an invalid schema will result in an error of the `create` function.
+
+
+### `renderToString` and `renderToResponse` options
+
+This function can accept an optional object as a second parameter:
+
+```js
+import { onRequest } from "../src/middleware.js"
+
+const response = await container.renderToString(Card, {
+    slots: {
+        default: await container.renderToString(CardItem).text() 
+    },
+    request: new Request("https://example.com/blog/blog-slug", {
+        headers: {}
+    }),
+    params: {
+        "slug": "blog-slug"
+    }, // in case your route is `pages/blog/[slug].astro`
+    locals: {
+        someFn() {
+
+        },
+        someString: "string",
+        someNumber: 100
+    },
+    status: 400
+});
+```
+
+- `slots`: required in case your component is designed to render some slots inside of it. It supposed named slots too.
+- `request`: required in case your component/page access to some information such as `Astro.url` or `Astro.request`.
+- `params`: the `Astro.params` to provide to the components 
+- `locals`: initial value of the `Astro.locals`.
+- `status`: useful in case you're rendering an error page.
+
+# Testing Strategy
+
+- Integration tests to cover the majority of the options.
+- Example folders to show how to integrate inside an application
+
+# Drawbacks
+
+I have considered a more high-level API, however with a high-level API it gets more difficult to integrate it inside other testing frameworks.
+
+While a low-level API requires more maintenance and work to provide more work, the main idea is to provide a bare-metal API so users can tweak it as they see fit. 
+
+# Alternatives
+
+I have considered the idea of exposing the compiler itself, with a thin layer of APIs, however users won't be able to compile the majority of formats like Markdown, MDX, etc.
+
+# Adoption strategy
+
+Considering the fact that this API doesn't involve any configuration or virtual module, the API
+will be released with a `experimental_` prefix, e.g. `experimental_AstroContainer`. 
+
+This implies that the public APIs of the class will be deemed unstable, and they can change anytime, in `patch` releases too. 
+
+Once the API is deemed stable, the `experimental_` prefix will be removed.