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Single Fetch
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Single Fetch

Remix introduced support for "Single Fetch" (RFC) behind the future.unstable_singleFetch flag in v2.9.0 which allows you to opt-into this behavior which will become the default in Remix v3.

Overview

When you enable Single Fetch, Remix will make a single HTTP call to your server on client-side transitions, instead of multiple HTTP calls in parallel (one per loader). If you are currently returning Response instances from your loaders (i.e., json/defer) then you shouldn't need to make many changes to your app code, but please read through the "breaking" changes below to be aware of some of the underlying behavior changes - specifically around serialization and status/header behavior.

Breaking Changes

  • Single fetch uses a new streaming format under the hood via turbo-stream, which means that we can stream down more complex data than just JSON
  • Naked objects returned from loader and action functions are no longer automatically converted into a JSON Response and are serialized as-is over the wire
  • To get the most accurate type inference, you should do two things:
    • Add @remix-run/react/future/single-fetch.d.ts to the end of your tsconfig.json's compilerOptions.types array
    • Begin using unstable_defineLoader/unstable_defineAction in your routes
      • This can be done incrementally - you should have mostly accurate type inference in your current state
  • Revalidation after an action 4xx/5xx Response is now opt-in, versus opt-out
  • The headers function is no longer used when Single Fetch is enabled, in favor of the new response stub passed to your loader/action functions
  • The old installGlobals() polyfill doesn't work for Single Fetch, you must either use the native Node 20 fetch API or call installGlobals({ nativeFetch: true }) in your custom server to get the undici-based polyfill

Details

Streaming Data Format

Previously, Remix used JSON.stringify to serialize your loader/action data over the wire, and needed to implement a custom streaming format to support defer responses.

With Single Fetch, Remix now uses turbo-stream under the hood which provides first class support for streaming and allows you to automatically serialize/deserialize more complex data than JSON. The following data types can be streamed down directly via turbo-stream: BigInt, Date, Error, Map, Promise, RegExp, Set, Symbol, and URL. Subtypes of Error are also supported as long as they have a globally available constructor on the client (SyntaxError, TypeError, etc.).

This may or may not require any immediate changes to your code once enabling Single Fetch:

  • ✅ json responses returned from loader/action functions will still be serialized via JSON.stringify so if you return a Date, you'll receive a string from useLoaderData/useActionData
  • ⚠️ If you're returning a defer instance or a naked object, it will now be serialized via turbo-stream, so if you return a Date, you'll receive a Date from useLoaderData/useActionData
    • If you wish to maintain current behavior (excluding streaming defer responses), you may just wrap any existing naked object returns in json

This also means that you no longer need to use the defer utility to send Promise instances over the wire! You can include a Promise anywhere in a naked object and pick it up on useLoaderData().whatever. You can also nest Promise's if needed - but beware of potential UX implications.

Once adopting Single Fetch, it is recommended that you incrementally remove the usage of json/defer throughout your application in favor of returning raw objects.

React Rendering APIs

In order to maintain consistency between document and data requests, turbo-stream is also used as the format for sending down data in initial document requests. This means that once opted-into Single Fetch, your application can no longer use renderToString and must use a React streaming renderer API such as renderToPipeableStream or renderToReadableStream) in entry.server.tsx.

This does not mean you have to stream down your HTTP response, you can still send the full document at once by leveraging the onAllReady option in renderToPipeableStream, or the allReady promise in renderToReadableStream.

On the client side, this also means that your need to wrap your client-side hydrateRoot call in a startTransition call because the streamed data will be coming down wrapped in a Suspense boundary.

Streaming Timeout

Previously, Remix has a concept of an ABORT_TIMEOUT built-into the default entry.server.tsx files which would terminate the React renderer, but it didn't do anything in particular to clean up any pending deferred promises.

Now that Remix is streaming internally, we can cancel the turbo-stream processing and automatically reject any pending promises and stream up those errors to the client. By default, this happens after 4950ms - a value that was chosen to be just under the current 5000ms ABORT_DELAY in most entry.server.tsx files - since we need to cancel the promises and let the rejections stream up through the React renderer prior to aborting the React side of things.

You can control this by exporting a streamTimeout numeric value from your entry.server.tsx and Remix will use that as the number of milliseconds after which to reject any outstanding Promises from loader/action's. It's recommended to decouple this value from the timeout in which you abort the React renderer - and you should always set the React timeout to a higher value so it has time to stream down the underlying rejections from your streamTimeout.

// Reject all pending promises from handler functions after 5 seconds
export const streamTimeout = 5000;

// ...

function handleBrowserRequest(
  request: Request,
  responseStatusCode: number,
  responseHeaders: Headers,
  remixContext: EntryContext
) {
  return new Promise((resolve, reject) => {
    const { pipe, abort } = renderToPipeableStream(
      <RemixServer
        context={remixContext}
        url={request.url}
        abortDelay={ABORT_DELAY}
      />,
      {
        onShellReady() {
          /* ... */
        },
        onShellError(error: unknown) {
          /* ... */
        },
        onError(error: unknown) {
          /* ... */
        },
      }
    );

    // Automatically timeout the react renderer after 10 seconds
    setTimeout(abort, 10000);
  });
}

Type Inference

Without Single Fetch, any plain Javascript object returned from a loader or action is automatically serialized into a JSON response (as if you returned it via json). The type inference assumes this is the case and infer naked object returns as if they were JSON serialized.

With Single Fetch, naked objects will be streamed directly, so the built-in type inference is no longer accurate once you have opted-into Single Fetch. For example, they would assume that a Date would be serialized to a string on the client 😕.

In order to ensure you get the proper types when using Single Fetch, we've included a set of type overrides that you can include in your tsconfig.json's compilerOptions.types array which aligns the types with the Single Fetch behavior:

{
  "compilerOptions": {
    //...
    "types": [
      // ...
      "@remix-run/react/future/single-fetch.d.ts"
    ]
  }
}

🚨 Make sure the single-fetch types come after any other Remix packages in types so that they override those existing types.

Loader/Action Definition Utilities

To enhance type-safety when defining loaders and actions with single fetch, you can use the new unstable_defineLoader and unstable_defineAction utilities:

import { unstable_defineLoader as defineLoader } from "@remix-run/node";

export const loader = defineLoader(({ request }) => {
  //                                  ^? Request
});

Not only does this give you types for arguments (and deprecates LoaderFunctionArgs), but it also ensures you are returning single-fetch compatible types:

export const loader = defineLoader(() => {
  return { hello: "world", badData: () => 1 };
  //                       ^^^^^^^ Type error: `badData` is not serializable
});

export const action = defineAction(() => {
  return { hello: "world", badData: new CustomType() };
  //                       ^^^^^^^ Type error: `badData` is not serializable
});

Single-fetch supports the following return types:

type Serializable =
  | undefined
  | null
  | boolean
  | string
  | symbol
  | number
  | bigint
  | Date
  | URL
  | RegExp
  | Error
  | Array<Serializable>
  | { [key: PropertyKey]: Serializable } // objects with serializable values
  | Map<Serializable, Serializable>
  | Set<Serializable>
  | Promise<Serializable>;

There are also client-side equivalents un defineClientLoader/defineClientAction that don't have the same return value restrictions because data returned from clientLoader/clientAction does not need to be serialized over the wire:

import { unstable_defineLoader as defineLoader } from "@remix-run/node";
import { unstable_defineClientLoader as defineClientLoader } from "@remix-run/react";

export const loader = defineLoader(() => {
  return { msg: "Hello!", date: new Date() };
});

export const clientLoader = defineClientLoader(
  async ({ serverLoader }) => {
    const data = await serverLoader<typeof loader>();
    //    ^? { msg: string, date: Date }
    return {
      ...data,
      client: "World!",
    };
  }
);

export default function Component() {
  const data = useLoaderData<typeof clientLoader>();
  //    ^? { msg: string, date: Date, client: string }
}

These utilities are primarily for type inference on useLoaderData and it's equivalents. If you have a resource route that returns a Response and is not consumed by Remix APIs (such as useFetcher) than you can just stick with your normal loader/action definitions. Converting those routes to use defineLoader/defineAction would cause type errors because turbo-stream cannot serialize a Response instance.

useLoaderData, useActionData, useRouteLoaderData, useFetcher

These methods do not require any code changes on your part - adding the single fetch types will cause their generics to deserialize correctly:

export const loader = defineLoader(async () => {
  const data = await fetchSomeData();
  return {
    message: data.message, // <- string
    date: data.date, // <- Date
  };
});

export default function Component() {
  // ❌ Before single fetch, types were serialized via JSON.stringify
  const data = useLoaderData<typeof loader>();
  //    ^? { message: string, date: string }

  // ✅ With single fetch, types are serialized via turbo-stream
  const data = useLoaderData<typeof loader>();
  //    ^? { message: string, date: Date }
}

useMatches

useMatches requires a manual cast to specify the loader type in order to get proper type inference on a given match.data. When using Single Fetch, you will need to replace the UIMatch type with UIMatch_SingleFetch:

  let matches = useMatches();
- let rootMatch = matches[0] as UIMatch<typeof loader>;
+ let rootMatch = matches[0] as UIMatch_SingleFetch<typeof loader>;

meta Function

meta functions also require a generic to indicate the current and ancestor route loader types in order to properly type the data and matches parameters. When using Single Fetch, you will need to replace the MetaArgs type with MetaArgs_SingleFetch:

  export function meta({
    data,
    matches,
- }: MetaArgs<typeof loader, { root: typeof rootLoader }>) {
+ }: MetaArgs_SingleFetch<typeof loader, { root: typeof rootLoader }>) {
    // ...
  }

Revalidations

Previously, Remix would always revalidate all active loaders after any action submission, regardless of the result of the action. You could opt-out of revalidation on a per-route basis via shouldRevalidate.

With Single Fetch, if an action returns or throws a Response with a 4xx/5xx status code, Remix will not revalidate loaders by default. If an action returns or throws anything that is not a 4xx/5xx Response, then the revalidation behavior is unchanged. The reasoning here is that in most cases, if you return a 4xx/5xx Response, you didn't actually mutate any data so there is no need to reload data.

If you want to continue revalidating one or more loaders after a 4xx/5xx action response, you can opt-into revalidation on a per-route basis by returning true from your shouldRevalidate function. There is also a new unstable_actionStatus parameter passed to the function that you can use if you need to decide based on the action status code.

Revalidation is handled via a ?_routes query string parameter on the single fetch HTTP call which limits the loaders being called. This means that when you are doing fine-grained revalidation, you will have cache enumerations based on the routes being requested - but all of the information is in the URL so you should not need any special CDN configurations (as opposed to if this was done via a custom header that required your CDN to respect the Vary header).

Headers

The headers function is no longer used when Single Fetch is enabled. Instead, your loader/action functions now receive a mutable ResponseStub unique to that execution:

  • To alter the status of your HTTP Response, set the status field directly:
    • response.status = 201
  • To set the headers on your HTTP Response, use the standard Headers APIs:
    • response.headers.set(name, value)
    • response.headers.append(name, value)
    • response.headers.delete(name)
export const action = defineAction(
  async ({ request, response }) => {
    if (!loggedIn(request)) {
      response.status = 401;
      response.headers.append("Set-Cookie", "foo=bar");
      return { message: "Invalid Submission! " };
    }
    await addItemToDb(request);
    return null;
  }
);

You can also throw these response stubs to short circuit the flow of your loaders and actions:

export const loader = defineLoader(
  ({ request, response }) => {
    if (shouldRedirectToHome(request)) {
      response.status = 302;
      response.headers.set("Location", "/");
      throw response;
    }
    // ...
  }
);

Each loader/action receives it's own unique response instance so you cannot see what other loader/action functions have set (which would be subject to race conditions). The resulting HTTP Response status and headers are determined as follows:

  • Status Code
    • If all status codes are unset or have values <300, the deepest status code will be used for the HTTP response
    • If any status codes are set to a value >=300, the shallowest >=300 value will be used for the HTTP Response
  • Headers
    • Remix tracks header operations and will replay them on a fresh Headers instance after all handlers have completed
    • These are replayed in order - action first (if present) followed by loaders in top-down order
    • headers.set on any child handler will overwrite values from parent handlers
    • headers.append can be used to set the same header from both a parent and child handler
    • headers.delete can be used to delete a value set by a parent handler, but not a value set from a child handler

Because single fetch supports naked object returns, and you no longer need to return a Response instance to set status/headers, the json/redirect/redirectDocument/defer utilities are considered deprecated when using Single Fetch. You may still continue returning normal Response instances and they'll apply status codes in the same way as the response stub, and will apply all headers via headers.set - overwriting any same-named header values from parents. If you need to append a header, you will need to switch from returning a Response instance to using the new response parameter.

Client Loaders

If your app has routes using clientLoader functions, it's important to note that the behavior of Single Fetch will change slightly. Because clientLoader is intended to give you a way to opt-out of calling the server loader function - it would be incorrect for the Single Fetch call to execute that server loader. But we run all loaders in parallel and we don't want to wait to make the call until we know which clientLoader's are actually asking for server data.

For example, consider the following /a/b/c routes:

// routes/a.tsx
export function loader() {
  return { data: "A" };
}

// routes/a.b.tsx
export function loader() {
  return { data: "B" };
}

// routes/a.b.c.tsx
export function loader() {
  return { data: "C" };
}

export function clientLoader({ serverLoader }) {
  await doSomeStuff();
  const data = await serverLoader();
  return { data };
}

If a user navigates from / -> /a/b/c, then we need to run the server loaders for a and b, and the clientLoader for c - which may eventually (or may not) call it's own server loader. We can't decide to include the c server loader in a single fetch call when we want to fetch the a/b loader's, nor can we delay until c actually makes the serverLoader call (or returns) without introducing a waterfall.

Therefore, when you export a clientLoader that route opts-out of Single Fetch and when you call serverLoader it will make a single fetch to get only it's route server loader. All routes that do not export a clientLoader will be fetched in a singular HTTP request.

So, on the above route setup a navigation from / -> /a/b/c will result in a singular single-fetch call up front for routes a and b:

GET /a/b/c.data?_routes=routes/a,routes/b

And then when c calls serverLoader, it'll make it's own call for just the c server loader:

GET /a/b/c.data?_routes=routes/c

Resource Routes

Because of the new streaming format used by Single Fetch, raw JavaScript objects returned from loader and action functions are no longer automatically converted to Response instances via the json() utility. Instead, in navigational data loads they're combined with the other loader data and streamed down in a turbo-stream response.

This poses an interesting conundrum for resource routes which are unique because they're intended to be hit individually -- and not always via Remix APIs. They can also be accessed via any other HTTP client (fetch, cURL, etc.).

If a resource route is intended for consumption by internal Remix APIs, we want to be able to leverage the turbo-stream encoding to unlock the ability to stream down more complex structures such as Date and Promise instances. However, when accessed externally, we'd probably prefer to return the more easily consumable JSON structure. Thus, the behavior is slightly ambiguous if you return a raw object in v2 - should it be serialized via turbo-stream or json()?

To ease backwards-compatibility and ease the adoption of the single fetch future flag, Remix v2 will handle this based on whether it's accessed from a Remix API or externally. In the future Remix will require you to return your own JSON response if you do not want raw objects to be streamed down for external consumption.

The Remix v2 behavior wih single fetch enabled is as follows:

  • When accessing from a Remix API such as useFetcher, raw Javascript objects will be returned as turbo-stream responses, just like normal loaders and actions (this is because useFetcher will append the .data suffix to the request)

  • When accessing from an external tool such as fetch or cURL, we will continue this automatic conversion to json() for backwards-compatibility in v2:

    • Remix will log a deprecation warning when this situation is encountered
    • At your convenience, you can add the json() call to impacted resource route handlers
    • Addressing these deprecation warnings will better prepare you for the eventual Remix v3 upgrade
    export function loader() {
      return {
        message: "My externally-accessed resource route",
      };
    }
    
    import { json } from "@remix-run/react";
    
    export function loader() {
      return json({
        message: "My externally-accessed resource route",
      });
    }
    

Note: It is not recommended to use defineLoader/defineAction for externally-accessed resource routes that need to return specific Response instances. It's best to just stick with loader/LoaderFunctionArgs for these cases.

Response Stub and Resource Routes

Ad discussed above, the headers export is deprecated in favor of a new response stub passed to your loader and action functions. This is somewhat confusing in resource routes, though, because you get to return the actual Response - there's no real need for a "stub" concept because there's no merging results from multiple loaders into a single Response:

// Using your own Response is the most straightforward approach
export async function loader() {
  const data = await getData();
  return json(data, {
    status: 200,
    headers: {
      "X-Custom": "whatever",
    },
  });
}

To keep things consistent, resource route loader/action functions will still receive a response stub and you can use it if you need to (maybe to share code amongst non-resource-route handlers):

// But you can still set values on the response stubstraightforward approach
export async function loader({
  response,
}: LoaderFunctionArgs) {
  const data = await getData();
  response?.status = 200;
  response?.headers.set("X-Custom", "whatever");
  return json(data);
}

It's best to try to avoid using the response stub and also returning a Response with custom status/headers, but if you do, the following logic will apply":

  • The Response instance status will take priority over any response stub status
  • Headers operations on the response stub headers will be re-played on the returned Response headers instance

Fetch Polyfill

Single Fetch requires using undici as your fetch polyfill, or using the built-in fetch on Node 20+, because it relies on APIs available there but not in the @remix-run/web-fetch polyfill. Please refer to the Undici section in the 2.9.0 release notes below for more details.

  • If you are managing your own server and calling installGlobals(), you will need to call installGlobals({ nativeFetch: true }) to avoid runtime errors when using Single Fetch
  • If you are using remix-serve, it will use undici automatically if Single Fetch is enabled.

Inline Scripts

The <RemixServer> component renders inline scripts that handle the streaming data on the client side. If you have a content security policy for scripts with nonce-sources, you can use <RemixServer nonce> to pass through the nonce to these <script> tags.

Docs and examples licensed under MIT