Next: centralize preferredRegion configuration

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1

Goal

I’d like to have all of our edge functions run only from EU edge nodes in order for our application to comply with GDPR privacy requirements.

Relevant Documentation

Configuring Regions for Vercel Functions says

You can set one or more preferred regions using the route segment config preferredRegion or specify a regions key within a config object to set one or more regions for you functions to execute in

What I Tried

// lib/constants.js
export const PREFERRED_VERCEL_REGIONS = ['fra1', 'arn1', 'dub1'];

// app/api/edge-handler-one/route.js
import { PREFERRED_VERCEL_REGIONS } from '@/lib/constants';
export const preferredRegion = PREFERRED_VERCEL_REGIONS;

// app/api/edge-handler-two/route.js
import { PREFERRED_VERCEL_REGIONS } from '@/lib/constants';
export const preferredRegion = PREFERRED_VERCEL_REGIONS;

// app/api/edge-handler-three/route.js
import { PREFERRED_VERCEL_REGIONS } from '@/lib/constants';
export const preferredRegion = PREFERRED_VERCEL_REGIONS;

This results in the following error during build:

Error: Invalid request: regions[0] should be equal to one of the allowed values “cdg1, arn1, dub1, lhr1, iad1, sfo1, pdx1, cle1, gru1, hkg1, hnd1, icn1, kix1, sin1, bom1, syd1, fra1, cpt1”.

What Works

// app/api/edge-handler-one/route.js
export const preferredRegion = ['fra1', 'arn1', 'dub1'];

// app/api/edge-handler-two/route.js
export const preferredRegion = ['fra1', 'arn1', 'dub1'];

// app/api/edge-handler-three/route.js
export const preferredRegion = ['fra1', 'arn1', 'dub1'];

What I Would Like

Having to repeat the same literal in every edge file makes it harder to manage this list. Imagine this very likely scenario:

  1. Engineer A opens a pull request to modify the list of regions
  2. Engineer B opens a pull request that adds a new edge handler, copying the list of regions from an existing handler

Those pull requests don’t have any merge conflicts, so nothing stops the team from merging both, leaving the app in an inconsistent state.

But even centralizing the configuration in a constant wouldn’t solve the problem where an engineer adds a new handler and omits the preferredRegion altogether.

Ideally, I’d be able to configure this list centrally and have it apply to all edge execution, both middleware and route handlers. Perhaps something like

// vercel.json
{
  "edgeRegions" ['fra1', 'arn1', 'dub1']
}
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