Haproxy in front of varnish or the other way round?

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I built a similar setup a few years back for a busy web application (only I did it with Squid instead of Varnish), and it worked out well.

I would recommend using your first setup (HAProxy -> Varnish) with two modifications:

  1. Add a secondary HAProxy server using keepalived and a shared virtual IP
  2. Use the balance uri load balancing algorithm to optimize cache hits

Pros:

  • Peace of mind with HAProxy (x2) and Varnish (x3) redundancy
  • Better hit rate efficiency on Varnish with HAProxy URI load balancing option
  • Better performance from the cache servers as they don’t need to keep as much in memory
  • Invalidating cache is easier since the same URI will go to the same server every time

Cons:

  • URI balancing works well, but if a cache server goes down, your backend servers will get hit as the other cache server(s) that pick up the slack from the updated URI balancing hash will need to re-retrieve the cached data. Maybe not a big con, but I did have to keep that in mind for my system.

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