Be able to vary the isr cache based on headers

[jean-pasqualini]

Goals

1. This would help to vary the isr content by country for instance using a http header provided by cloudflare, exactly like the vary header does with varnish.

Non-Goals

1. This feature will be limited to the possibility to vary the cache based on a header without any transformation.

Background

Discuss prior art, why do you think this feature is needed?
I think this is really needed to handle geolocation content not related to the users' language.

Are there current alternatives?

• that could be handled this entirely on client side
  • pro: works with the current next.js
  • con: with content that comes from a cms that change depends on the country for regulatory reason, handle it on the client side needs to add all the logic there
• that could be handled with a specific url
  • pro: easy to implement and handle the content for different country without adding complexity
  • con: this become complex for use case where we don't want to add this information

Question:

• What can be used today to discriminate the cached content when using isr ?

Proposal

We could have a configuration to manage the cache key based on the request.

cacheKey(req) => "cache key customized"