Caching a Drupal site with session-specific data on every page

Question

We have a site written in Drupal 6.

We want to use Drupal's caching mechanism to improve performance, but when we turned it on, we found problems because our site has session data displayed on every page. Drupal's caching system only works for static content page, and our session data effectively means that none of our pages are static.

We've come up with two solutions to this:

1. Rewrite the block that shows the session data such that it loads the non-static data using Ajax, so that the main HTML content is then static. The down-sides of this are that it will mean quite a lot of work, and also that it will reduce functionality for non-Javascript-enabled users and speciality browsers (ie screen-readers).

2. Move the session data to an iframe or similar. This just sounds wrong and I don't want to do it, but it would fix the problem, I think.

3. or find an alternative caching system that will allow us to cache the site in a way that can deal with a there being a small block of dynamic data on every page. This is our preferred option, but we haven't been able to make any progress on finding anything, hence the question here.

So if anyone has any suggestions that would help us with option 3, please let us know. Thank you.

(ps - I know Drupal 7 does have an improved caching system which could help with this, but we're not ready to move to D7 yet)

[EDIT] Haven't made much progress on this, so I'll open it up to a bounty.

I've had a look into how Ubercart deals with a similar situation (since it's one of the most popular Drupal modules and has basically the same problem to solve as we do). They get around it by having the cart info block display static text when the user is not logged in (ie "Click here to view your basket"), and only showing the dynamic cart data on every page when the user is logged in. We don't feel this actually solves the problem; it just evades it, and that's not what we want. But I'm worried that if Ubercart has resorted to doing it this way, it may turn out to be the only option.

Answer 1

Ajax Blocks.

A simpler Solution than Authcache is Ajax Blocks.

http://drupal.org/project/ajaxblocks

Works well, and from what I remember it handles users without js fairly well.

Answer 2

Have you looked at the Authenticated User Page Caching (Authcache) module? It works similarly to what you describe in option 1 above.

Answer 3

Another solution would be to use an ESI gate to render your pages as an assembly of blocks. There are a few projects around this, I'm no expert but searching google could give you some ideas.

PS: You will have the same problem with D7, caching is more or less the same.

Answer 4

You might want to check out this module: http://drupal.org/project/blockcache_alter. I'm not sure if that will accomplish what you are looking for or not, but it might help give you insight into block caching.

Also, there are a lot of great performance resources here that could lead to something helpful: http://drupal.org/node/326504

Answer 5

my problem is very similar. Depending on a session setting, the body background changes. I could add a class to the body tag with jQuery, but don't know if this performance will look fine.

I'll test it and report back.