Dedupe

Prevent duplicate work.

Any function wrapped in dedupe will only ever run once for the same request within the same runtime and given the same arguments.

The dedupe function is an integral piece when working with the Flags SDK.

Example

import { dedupe } from '@vercel/flags/next';

const dedupeExample = dedupe(() => {
  return Math.random();
});

export default async function Page() {
  const random1 = await dedupeExample();
  const random2 = await dedupeExample();
  const random3 = await dedupeExample();

  // these will all be the same random number
  return <div>{random1} {random2} {random3}</div>;
}

Example of output:

0.874725 0.874725 0.874725

Use case: Avoiding duplicate work

This helper is extremly useful in combination with the identify function, as it allows the identification to only happen once per request. This is useful in preventing overhead when passing the same identify function to multiple feature flags.

Use case: Generating consistent random IDs

When experimenting on anonymous visitors it is common to set a cookie containing a random id from Edge Middleware. This random id is later used to consistently assign users to specific groups of A/B tests.

For this use case the function generating the random id can be wrapped in dedupe. The deduplicated function is then called in Edge Middleware to produce the random id, and from a flag's identify function to identify the user even on the first page visit when no cookie is present yet.

As the function is guaranteed to generate the same id the Edge Middleware can set a cookie containing the generated id in a response, and the feature flag can already use the generated id even if the original request did not contain the id.

Learn more about this approach in the Marketing Pages example.