What happens when you use Cache-Control: max-age with ETags?

Question

There are several similar questions, but none of the ones I've found are clear or definitive.

Here's the behavior I want:

• As long as the network is available, the browser must check if there's a new version
• If there isn't a new version, the browser can use the cached version
• The cached version expires after X days

I think I can do this with Cache-Control: max-age and ETags. However, I can't find whether the max-age should be 0 or how long the content should be cached for.

Eg if I do Cache-Control: max-age=86400 (1 day) with an ETag, would it:

1. Make a server request every time, but the server will just return 304 if the ETag didn't change. After 1 day, discard the cached version, and refetch from the server (which should be the same as the discarded version).

2. Doesn't make any server requests for a day. Then, after 1 day, the server can still return 304. The cached version can stay indefinitely.

I'd like the browser to refetch after X days because in case there's a bug, I don't want users stuck with a broken cached version.

Answer 1

Here's the behavior I want:

• As long as the network is available, the browser must check if there's a new version
• If there isn't a new version, the browser can use the cached version

This is a common use case, and can be accomplished by using Cache-Control: no-cache (or max-age=0, must-revalidate ) and providing an ETag or Last-Modified header.

• The cached version expires after X days

This is not possible. It's not part of the design of HTTP caching because there's no use case for it.

I'd like the browser to refetch after X days because in case there's a bug, I don't want users stuck with a broken cached version.

If the browser is checking for a new version each time, how can the user ever get stuck with a "broken" cached version?

If I do Cache-Control: max-age=86400 (1 day) with an ETag, would it:

1. Make a server request every time, but the server will just return 304 if the ETag didn't change. After 1 day, discard the cached version, and refetch from the server (which should be the same as the discarded version).

2. Doesn't make any server requests for a day. Then, after 1 day, the server can still return 304. The cached version can stay indefinitely.

Number 2. The max-age tells the browser how long it can consider the resource to be fresh , meaning that the cached version can be used without checking with the server. When that time has expired, the resource is considered stale , and a new request has to be made. If the cached resource has an ETag or Last Modified header that request can be a conditional one to allow the server to avoid sending the entire resource in the response.