Heroku & Rails - Varnish is only caching very occasionally

Question

I have an issue similar to Heroku & Rails - Varnish HTTP Cache Not Working , but the solution (wait for a while, then everything works) doesn't seem to apply - I've had the setup below for several days.

This thread on the Heroku Google group has some users with the same problem. They mention that it takes a while for everything to be cached, but my understanding is that after a while, everything should get cached, no? Or does that only apply if there is a Lot of traffic?

I need some advice on where I should be looking/what I can try changing in order to get caching working properly.

My setup:

I have http://www.swingoutlondon.co.uk running on Heroku (Rails 3.0.3, Ruby 1.9.2, bamboo-mri-1.9.2) and the main index page performs a lot of database queries to return what is essentially a static page - usually taking about 2-3 seconds (yes, that's something I really do need to address, but I figure varnish caching is a quick win) .

I've set the Cache-Control response header as described here , and indeed that does seem to have been set on the page:

>> curl -I http://swingoutlondon.co.uk

HTTP/1.1 200 OK
Server: nginx
Date: Sun, 13 May 2012 00:01:05 GMT
Content-Type: text/html; charset=utf-8
Connection: keep-alive
Cache-Control: public, max-age=300
Etag: "2565201f3ae39c6a9a1f6b1fb8bbbe0a"
X-Ua-Compatible: IE=Edge,chrome=1
X-Runtime: 1.699667
Content-Length: 44224
Accept-Ranges: bytes
X-Varnish: 681634826
Age: 0
Via: 1.1 varnish

Note: Cache-Control: public, max-age=300

I assume that Age: 0 indicates that it hasn't retrieved a cached copy, and indeed the command returns in the normal slow 2-3 seconds.

If keep repeatedly trying that curl, I can occasionally a cached copy (the page loads in under half a second and Age is greater than 0).

I must confess to not fully understanding HTTP headers, but one clue might be: when Age is greater than 0, I get two lots of digits in X-Varnish (in all other cases I only get one set):

X-Varnish: 848670407 848650521

Here's what I've checked:

• the source of is identical each time.
• I have one before_filter on that page, which sets the time the page was last updated as an instance variable.
• there are a number of cookies - as far as I can see they are all set by either Google Analytics or the Twitter or Facebook buttons.

For good measure, here are my Request headers:

Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-GB,en-US;q=0.8,en;q=0.6
Cache-Control:max-age=0
Connection:keep-alive
Cookie:__utma=264326157.189257391.1336869624.1336869624.1336869624.1; __utmb=264326157.2.10.1336869624; __utmc=264326157; __utmz=264326157.1336869624.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none)
Host:www.swingoutlondon.co.uk
If-None-Match:"2565201f3ae39c6a9a1f6b1fb8bbbe0a"
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.168 Safari/535.19

Answer 1

Ah well - turns out that because Heroku uses multiple independent Varnish servers, and because traffic to Swing Out London is relatively low, I shouldn't expect to have many pages served by the caches if my max-age is only 5 minutes. Setting it to 20 or 30 minutes results in much more caching.

I've written a detailed blog post collecting my learnings. Thanks to Garry Shulter for helping me out with this one.