Precompute

This page shows how to implement the precompute pattern in Next.js to keep pages static, even when multiple feature flags are used on a single page, or even when feature flags are used across multiple pages. Ensure you've read about the general precompute concept to understand the pattern and benefits:

Manual approach

It's possible to manually create variants of page by creating two versions of the same page. For example, app/home-a/page.tsx and app/home-b/page.tsx. Then, use Edge Middleware to rewrite the request either to /home-a or /home-b.

import { flag } from 'flags/next';
 
export const homeFlag = flag<boolean>({
  key: 'home',
  decide: () => Math.random() > 0.5,
});

import { NextResponse, type NextRequest } from 'next/server';
import { homeFlag } from './flags.ts';
 
export const config = { matcher: ['/'] };
 
export async function middleware(request: NextRequest) {
  const home = await homeFlag();
 
  // Determine which version to show based on the feature flag
  const version = home ? '/home-b' : '/home-a';
 
  // Rewrite the request to the appropriate version
  const nextUrl = new URL(version, request.url);
  return NextResponse.rewrite(nextUrl);
}

This approach works well for simple cases, but has a few downsides:

• It can be cumbersome having to maintain both /home-a/page.tsx and /home-b/page.tsx.
• It doesn't scale well when a feature flag is used on more than one page, or when multiple feature flags are used on a single page.

Precompute pattern

Use the precompute functionality of the Flags SDK to work around the limitations of the manual approach. Use the precompute pattern to keep pages static, even when multiple feature flags are used on a single page, or even when feature flags are used across multiple pages.

Prerequisites

Ensure you have a FLAGS_SECRET environment variable set. This variable is used by precompute to encrypt the precomputation result into the URL. The FLAGS_SECRET environment variable is required for precompute to work. It must contain a random string of 32 characters, encoded as Base64.

Generate a random value using the following command:

node -e "console.log(crypto.randomBytes(32).toString('base64url'))"

Create the environment variable on Vercel, and fill with the generated random value:

vc env add FLAGS_SECRET

Pull the environment variable to your local project:

vc env pull

Alternatively, set the FLAGS_SECRET environment variable locally in .env.local:

FLAGS_SECRET=<your-secret>

1. Create flags to be precomputed

Export one or multiple flags as an array to be precomputed.

import { flag } from 'flags/next';
 
export const showSummerSale = flag({
  key: 'summer-sale',
  decide: () => false,
});
 
export const showBanner = flag({
  key: 'banner',
  decide: () => false,
});
 
// a group of feature flags to be precomputed
export const marketingFlags = [showSummerSale, showBanner] as const;

2. Precompute flags in middleware

Import and pass the group of flags to the precompute function in middleware. Then, forward the precomputation result (code) to the underlying page using an URL rewrite:

import { type NextRequest, NextResponse } from 'next/server';
import { precompute } from 'flags/next';
import { marketingFlags } from './flags';
 
// Note that we're running this middleware for / only, but
// you could extend it to further pages you're experimenting on
export const config = { matcher: ['/'] };
 
export async function middleware(request: NextRequest) {
  // precompute returns a string encoding each flag's returned value
  const code = await precompute(marketingFlags);
 
  // rewrites the request to include the precomputed code for this flag combination
  const nextUrl = new URL(
    `/${code}${request.nextUrl.pathname}${request.nextUrl.search}`,
    request.url,
  );
 
  return NextResponse.rewrite(nextUrl, { request });
}

3. Access the precomputation result from a page

Next, import the feature flags you created earlier, such as showBanner, while providing the code from the URL and the marketingFlags list of flags used in the precomputation.

When the showBanner flag is called within this component it reads the result from the precomputation, and it does not invoke the flag's decide function again:

import { marketingFlags, showSummerSale, showBanner } from '../../flags';
type Params = Promise<{ code: string }>;
 
export default async function Page({ params }: { params: Params }) {
  const { code } = await params;
  // access the precomputed result by passing params.code and the group of
  // flags used during precomputation of this route segment
  const summerSale = await showSummerSale(code, marketingFlags);
  const banner = await showBanner(code, marketingFlags);
 
  return (
    <div>
      {banner ? <p>welcome</p> : null}
 
      {summerSale ? (
        <p>summer sale live now</p>
      ) : (
        <p>summer sale starting soon</p>
      )}
    </div>
  );
}

This approach allows middleware to decide the value of feature flags and to pass the precomputation result down to the page. This approach also works with API Routes.

Enabling ISR (optional)

You can enable Incremental Static Regeneration (ISR) to cache generated pages after their initial render:

import type { ReactNode } from 'react';
 
export async function generateStaticParams() {
  // returning an empty array is enough to enable ISR
  return [];
}
 
export default async function Layout({ children }: { children: ReactNode }) {
  return children;
}

In the example above, we used generateStaticParams on the layout. You can also specify it on the page instead. It depends on whether a single page needs the flag or all pages within that layout need the flag.

Opting into build-time rendering (optional)

The flags/next submodule exposes the generatePermutations helper function for generating pages for different combinations of flags at build time. This function is called and takes a list of flags and returns an array of strings representing each combination of flags:

import type { ReactNode } from 'react';
import { generatePermutations } from 'flags/next';
 
export async function generateStaticParams() {
  const codes = await generatePermutations(marketingFlags);
  return codes.map((code) => ({ code }));
}
 
export default function Page() {
  /* ... */
}

You can further customize which specific combinations you want render by passing a filter function as the second argument of generatePermutations. And just like in the example above, you can also control whether you specify these permutations on the individual pages or on a layout.

Pages Router

If you're using the Pages Router, you need to pass a flag to generatePermutations which accepts the code from context and the group of flags.

You also need to specify a getStaticPaths function which can return the permutations to generate at build time or an empty array to use ISR.

import { generatePermutations } from 'flags/next';
import { marketingFlags, exampleFlag } from '../flags';
 
export const getStaticPaths = (async () => {
  const codes = await generatePermutations(marketingFlags);
 
  return {
    paths: codes.map((code) => ({ params: { code } })),
    fallback: 'blocking',
  };
}) satisfies GetStaticPaths;
 
export const getStaticProps = (async (context) => {
  if (typeof context.params?.code !== 'string') return { notFound: true };
 
  const example = await exampleFlag(context.params.code, marketingFlags);
  return { props: { example } };
}) satisfies GetStaticProps<{ example: boolean }>;`}

Declaring available options (optional)

Options are the possible values that a flag can take. You can declare the available options for a flag by passing an options array to the flag function:

export const greetingFlag = flag<string>({
  key: 'greeting',
  options: ['Hello world', 'Hi', 'Hola'],
  decide: () => 'Hello world',
});

Instead of passing the values directly you can also pass an object containing a label and value property:

export const greetingFlag = flag<string>({
  key: 'greeting',
  options: [
    { label: 'Hello world', value: 'Hello world' },
    { label: 'Hi', value: 'Hi' },
    { label: 'Hola', value: 'Hola' },
  ],
  decide: () => 'Hello world',
});

To pass objects you must specify a label and value property:

export const greetingFlag = flag<string>({
  key: 'greeting',
  options: [
    {
      label: 'Hello world',
      value: {
        /* your object here */
      },
    },
  ],
});

The Flags SDK uses the declared options for multiple purposes:

1. Efficiently encode the flag values into the URL

   The precompute function generates a short code which your appliciation then transports through the URL. The URLs must remain fairly short for the system to stay efficient. When a feature flag's decide function returns a value not explicitly declared in options the whole value needs to be inlined into the code which can quickly exceed the URL size limits. Especially for ISR the URL length needs to stay below 1024 characters.

2. Generate the possible permutations flags

   The generatePermutations function generates all possible combinations of flags for prerendering at build time. The function needs to know the available options for each flag to generate the possible permutations. It can only generate and prerender options decalred by the flag.

3. Show the available options in the Flags Explorer

   All options declared for a flag are shown in the Flags Explorer. If present, the label is used as the option name.

Adjusting the setup

This section shows how to adapt the precompute pattern to different scenarios.

Precomputing a single page only

The examples above use a single top-level group of flags, which will opt all pages nested under app/[code] into precomputation. Instead of opting the whole application into precomputation, you can also precompute a single page only.

For example, if you want to precompute the /pricing page only:

• Move your pricing page from app/pricing/page.tsx to app/pricing/[pricingCode]/page.tsx
• Export a pricingFlags array of flags from your flags.ts file, containing all flags used by the pricing page
• Run Edge Middleware for requests to /pricing, and pass pricingFlags to the precompute function
• Adjust the rewrite in Edge Middleware to rewrite requests from /pricing to /pricing/[pricingCode]
• Use the pricingFlags array to access the precomputed result in app/pricing/[pricingCode]/page.tsx when using Flags

import type { ReactNode } from 'react';
import { generatePermutations } from 'flags/next';
import { pricingFlags, discountFlag } from '../../../flags';
 
export async function generateStaticParams() {
  const codes = await generatePermutations(pricingFlags);
  return codes.map((code) => ({ pricingCode: code }));
}
 
export default async function Page(props: { params: Promise<{ pricingCode: string }> }) {
  const { pricingCode } = await props.params;
  const example = await discountFlag(pricingCode, pricingFlags);
 
  // ...
}

Precomputing a subset of pages

This section describes an alternative folder structure where only a part of the page tree makes use of precomputation.

So far the examples have used a single top-level group of flags under app/[code].

Instead, you can nest precomputed flags under a folder like app/precomputed/[code]. This makes it clear that only those pages will have access to the precomputed flags.

app
├─page.tsx
└─precomputed
   └─[rootCode]
      └─page.tsx

Adjust the rewrite in Edge Middleware to include the precomputed segment.

Note that you will need to manually maintain the paths in Edge Middleware for which the rewrite should run.

Multiple groups

Define multiple groups of flags to avoid unnecessarily generating permutations for flags which are not used by all pages. This control allows you to only generate permutations of the precise flags used by eacha subset of your pages.

For example, you can have a root group of flags which apply to all pages, and a nested group of flags which only apply to a single page or subset of pages. Create a root [rootCode] at the root of your application for the common flags, and a [pricingCode] for the flags used by the pricing page.

// all available flags
export const navigationFlag = flag(/* ... */)
export const bannerFlag = flag(/* ... */)
export const discountFlag = flag(/* ... */)
 
// two groups of flags
export const rootFlags = [navigationFlag, bannerFlag];
export const pricingFlags = [discountFlag];

The file tree would look like this:

app
└─[rootCode]
   ├─ page.tsx
   └─ pricing
       └─ [pricingCode]
            └─ page.tsx

To use this pattern, you need to adjust Edge Middleware:

• Precompute both groups of flags by calling precompute for each group
• Adjust the rewrite to add the precomputed codes at the right segments for each group of flags

When calling the feature flag, ensure you specify the code and group of flags it was precomputed for. For example, use await discountFlag(pricingCode, pricingFlags) to access the precomputed value of the discountFlag that is part of the pricingFlags group.

Combinatory explosion

Using the precompute pattern for pages which consist of many feature flags, or feature flags which have many possible values will lead to an exponential increase in the number of permutations that exist of given page.

• Build time will increase if you rely on build time rendering.
• Cache hit rates will decrease if you rely on lazily generating the permutations.

Be mindful of the flags you precompute, and whether you pregenerate permutations at build time or on demand.

You can manually specify which permutations you generate at build time by passing a second argument to generatePermutations. See the API Reference for more information. All remaining permutations will be generated on demand the first time they are requested.

Next steps