We are using Phoenix with Plug.Static to serve static files (e.g. images and js files).

Some images were not cached in the borwser, and it turned out we were not using Routes.static_path(@conn, "images/bla.png") instead, we just added the path like this: <img src="images/bla.png" />.

I see in the browser, that the server replies with the following response headers:

cache-control: public
Connection: keep-alive
Date: Mon, 16 Jan 2023 13:15:03 GMT
etag: “52BBD1A”

This is correct, since public is the default value for cache_control_for_etags. I tried to understand what the default (without explicit expiration date) exactly means, but the online resources are a bit sparse.

This SO answers recommends not to use “Cache-control: public” at all, while this article recommends not to put a max-age at all:

97.3% of responses containing it [Cach-control: public] have max-age or s-maxage too, making it redundant

So far, what I understood is that by omitting the expiration date, the browser (or the CDN in between) guesses the expiration value using some heuristics (source).

My question is the one stated in the topic, why does “public” without expiration makes sense as the default value?

I am not sure if this is the correct place to ask. Thank you in advance for answering.

Kiru