LV sessions are merged data between what is in the plug/http session store and what is provided by the :session option on live_session. The merged data is put on the html tags to be sent back by LV. It’s a bit unfortunate that LV calls that data in the html tag session given it’s technically completely unrelated to plug/http level session data.
This basically affects any data you’d get in mount/3s second parameter. So e.g. (not different to http level caching) needing to deal with logged in users would likely require the usage of the session.
Unless I’m terribly wrong, this is not the case: only data set via the :session attribute of a live_session is stored in the HTML, and not the session data from the cookie. I have the feeling you assume the websocket connection doesn’t include the cookie, which is wrong - it does. Hence the CSWH issue discussed in the blog post.
Let’s check it out after setting 1) session cookie data 2) setting live_session :default, session: %{"titi" => "toto"} do:
Seems like I got the actual implementation mixed up with how it works conceptually and what you eventually get in mount/3. But at least in the current implementation of socket you still need a csrf token to get access to the cookie. Iirc earlier versions didn’t allow that at all.
Nice. I was thinking of a static site generator for the static part so the initial pages can be on a CDN (and socket activated on first interaction?). Seems this lib and the enhancement it brought to the core makes a good base.
I guess if you can tell your CDN to browse your sitemap then you can cache your pages this way. I was thinking of warming locally the cache by simulating user requests using something like Invoke Phoenix endpoint programmatically at runtime. Will probably write a blog post about it.
By the way, if you’re using a cloud application platform (Gigalixir, render, fly.io…) and your users are always near one of your servers, then maybe you no longer need a CDN to cache pages. It’d be interesting to make measures and have feedback about it.
Thanks to the Phoenix team 
The new option will be available as of Phoenix 1.7.15