Centralize region configuration

[jamesarosen]

Goals

1. Converge serverless and edge region configuration
2. Allow application developers to define region(s) for edge centrally

Non-Goals

• This proposal does not seek to support multi-region serverless functions

Background

Problem 1: Inconsistency

Per the Configuring Regions for Vercel Functions docs,

[for serverless functions, you] can define the region in your vercel.json using the regions setting

and

[for edge functions, 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

These are configured separately, in separate ways. It's great to have a way to configure things centrally and a separate way to override those configurations on a case-by-case basis. It's not great for each of those to only apply to half of an application.

Problem 2: No Central config for Edge Functions

I can't define the region for edge functions centrally. I can't even extract it into a shared module.

Per the Route Segment Config,

The values of the config options currently need be statically analyzable. For example revalidate = 600 is valid, but revalidate = 60 * 10 is not.

That is, I can't do this:

// constants.ts
export preferredRegion = ['cdg1']

// app/my/route.ts
export { preferredRegion } from 'constants`

Use-Case

We run an application that has strict privacy constraints to support GDPR. We absolutely must not run the app in any region outside of the EU.

In order to support that, I need to

1. set regions: ['cdg1'] in my vercel.json for any serverless functions
2. export preferredRegion = 'cdg1' in every single edge route and layout

If one developer forgets to do this, we've just broken our contract with our customers.

Proposal

Option 1: functions using the edge runtime automatically inherit preferredRegion from regions from vercel.json. Since this might be considered a breaking change, I offer

Option 2: functions using the edge runtime automatically inherit preferredRegion from a new preferredRegions option in vercel.json.