I have a page /data.txt, which is cached in the client's browser. Based on data which might be known only to the server, I now know that this page is out of date and should be refreshed. However, since it is cached, they will not re-request it for a long time (until the cache expires).
The client is now requesting a different page /foo.html. How can I make the client's browser re-request /data.txt and update its cache?
This should be done using HTTP or HTML (not all clients have JS).
(I specifically want to avoid the "cache-busting" pattern of appending version numbers to the /data.txt URL, like /data.txt?v=2. This fills the cache with useless entries rather than replacing expired ones.)
Edit for clarity: I specifically want to cache /data.txt for a long time, so telling the client not to cache it is unfortunately not what I'm looking for (for this question). I want /data.txt to be cached forever until the server chooses to invalidate it. But since the user never re-requests /data.txt, I need to invalidate it as a side effect of another request (for /foo.html).
To expand my comment:
You can use IF-Modified-Since and Etag, and to invalidate the resource that has been already downloaded you may take a look at the different approaches suggested in Clear the cache in JavaScript and fetch(), how do you make a non-cached request?, most of the suggestions there mentioned fetching the resource from JavaScript with no-cache header fetch(url, {cache: "no-store"}).
Or, if you can try sending a Clear-Site-Data header if your clients' browsers are supported.
Or maybe, give up this time only for the cache-busting solution. And if it's possible for you, rename the file to something else rather than adding a querystring as suggested in Revving Filenames: don’t use querystring.
Update after clarification:
If you are not maintaining a legacy implementation with users that already have /data.txt cached, the use of Etag And IF-Modified-Since headers should help.
And for the users with the cached versions, you may redirect to: /newFile.txt or /data.txt?v=1 from /foo.html. The new requests will have the newly added headers.