A flag to clear the fetch cache before the dev server starts

[dwjohnston]

Goals

1. Developers should be able to run next dev with a flag like --clearCache and not have the cached data from the previous sessions exist. Fetch caching behaviour should still continue to occur.
2. Possibly an extension of this functionality that allows clearing the fetch cache without restarting the server.

Non-Goals

1. Anything to do with disabling fetch cache globally, or updates to the fetch caching behaviour.
2. Anything to with caching of other resources (eg. module/file caching)

Background

This has come up as part of an investigation of how next's caching behaviour works, and generally debugging how an application is behaving from a performance standpoint, comparing various tools/techniques as it relates to reducing API requests.

In the experiment I'm conducting I have an intentionally slow upstream API, and I want to demonstrate 'first load is slow, but subsequent ones are instant'.

Currently, cached requests from previous development sessions are persisted to disk, which means that we don't see the 'slow first load' functionality, which negates the scenario we're trying to demonstrate.

Our current work around is to remove the persisted files from disk before starting the server - we update our package script to:

    "dev": "rm -rf .next/cache/fetch-cache && next dev"

Proposal

The very low hanging fruit is to just reproduce the workaround described above (or similar) in a flag for the dev command.

The problem with this solution is that it does require the developer to kill and restart the server whenever they need to clear the cache.

Potentially a smarter solution would clear the cache at sensible times, perhaps at file change (although that's probably too often?) or allow the developer to imperatively remove the cache (but how? With a CLI command?).