okhttp: Is there an order of precedence for response caching headers? e.g. I'm getting both `ETag` ...

# squarelibraries

[Colton Idle]

okhttp: Is there an order of precedence for response caching headers?

e.g. I'm getting both ETag and Last-Modified: but it seems like the follow up request uses ETAG in the response. Whereas my server team really wants us to use Last-Modified.

[yschimke]

The tests are here https://github.com/square/okhttp/blob/master/okhttp%2Fsrc%2Ftest%2Fjava%2Fokhttp3%2FCacheTest.kt#L1202

[yschimke]

Are you expecting if-modified-since in request?

[yschimke]

Yep, looks like etag is preferred https://github.com/square/okhttp/blob/master/okhttp%2Fsrc%2Fmain%2Fkotlin%2Fokhttp3%2Finternal%2Fcache%2FCacheStrategy.kt#L200

[jessewilson]

Don’t send an etag if you don’t want the client to use it!

[glureau]

@jessewilson how to handle retro-compat for some clients expecting etag then? Any solution but a new endpoint?

[yschimke]

You can add an interceptor to change the response headers.

[jessewilson]

Why does the server send an etag if they don’t want you to use it?

[yschimke]

https://www.ietf.org/rfc/rfc2616.html#section-13.3.4

[Colton Idle]

Thank you for the rfc link and for finding where the etag is preferred Yuri, and thank you for confirming jesse.

I will screenshot that message and let them know that the library author suggests that... which is funny... because thats what i told them yesterday. "don't send the header if you don't want the client to use it"

😂

[yschimke]

I think the RFC says we SHOULD send both. I need to confirm if we do

[Colton Idle]

It does not seem like we send both. Here is the original 200'ing response

HTTP/1.1 200 OK
Date: Wed, 20 Mar 2024 17:25:11 GMT
Server: Apache/2.4.41 (Ubuntu)
X-Powered-By: Express
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
Last-Modified: 2023-09-12T15:48:48.000Z
Content-Type: application/json; charset=utf-8
Content-Length: 455
ETag: FOO
Keep-Alive: timeout=5, max=99
Connection: Keep-Alive

and here is the subsequent request

GET /api/blah
HTTP/1.1
Authorize: BLAH
Host: <http://blah.com|blah.com>
Connection: Keep-Alive
Accept-Encoding: gzip
User-Agent: okhttp/4.11.0
If-None-Match: FOO

[yschimke]

Ill file a ticket to track. But I don't think it's breaking anything.

[Colton Idle]

Maybe a dumb question, but is there a good list of all cache headers?

on mdn, it kinda makes it seem like etag is not really a header for caching since its not under the cache-control page. unless im misunderstanding something (completely possible) lol

[jessewilson]

The ones OkHttp use are all here: https://github.com/square/okhttp/blob/master/okhttp/src/main/kotlin/okhttp3/internal/cache/CacheStrategy.kt

[jessewilson]

Age
Cache-Control
Date
ETag
Expires
Last-Modified
If-None-Match
If-Modified-Since