Flags SDK

Precompute

Using feature flags on static pages.

Precomputing describes a pattern where Edge Middleware uses feature flags to decide which variant of a page to show. This allows to keep the page itself static, which leads to incredibly low latency globally as the page can be served by an Edge Network.

Manual approach

In its most simple form this pattern is implemented creating two versions of the home page in app/home-a/page.tsx and app/home-b/page.tsx. Then, use Edge Middleare to rewrite the request either to /home-a or /home-b.

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

// middleware.ts
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 or /home-b. The approach also 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.

Precomputing

This is an extension to the previously described pattern. It allows combining multiple feature flags on a single, static page.

This pattern is useful for experimentation on static pages, as it allows middleware to make routing decisions, while being able to keep the different variants of the underlying flags static.

It further allows generating a page for each combination of feature flags either at build time or lazily the first time it is accessed. It can then be cached using ISR so it does not need to be regenerated.

Technically this works by using dynamic route segments to transport an encoded version of the feature flags computed within Edge Middleware. Encoding the values within the URL allows the page itself to access the precomputed values, and also ensures there is a unique URL for each combination of feature flags on a page. Because the system works using rewrites, the visitor will never see the URL containing the flags. They will only see the clean, original URL.

Export flags to be precomputed

You can export one or multiple arrays of flags to be precomputed. This by itself does not do anything yet, but you will use the exported array in the next step:

// flags.ts
import { flag } from '@vercel/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;

Precompute flags in middleware

In this step, import marketingFlags from the flags file that you created in the previous step. Then, call precompute with the list of flags to be precomputed. You'll then forward the precomputation result to the underlying page using an URL rewrite:

// middleware.ts
import { type NextRequest, NextResponse } from 'next/server';
import { precompute } from '@vercel/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 });
}

Accessing 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:

// app/[code]/page.tsx
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)

So far you've set up middleware to decide the value of each feature flag to be precomputed and to pass the value down. In this step you can enable ISR to cache generated pages after their initial render:

// app/[code]/layout.tsx
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;
}

Opting into build-time rendering (optional)

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

// app/[code]/page.tsx
import type { ReactNode } from 'react';
import { generatePermutations } from '@vercel/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.

Example

See the Marketing Pages example which implements this pattern.