varnishlog

Over the past few months, I've been reading about BigPipe, Cache Tags, Dynamic Page Cache, and all the other amazing-sounding new features for performance in Drupal 8. I'm working on a blog post that more comprehensively compares and contrasts Drupal 8's performance with Drupal 7, but that's a topic for another day. In this post, I'll focus on cache tags in Drupal 8, and particularly their use with Varnish to make cached content expiration much easier than it ever was in Drupal 7.

If you're a Drupal or PHP developer used to debugging or troubleshooting some code by adding a print $variable; or dpm($object); to your PHP, and then refreshing the page to see the debug message (or using XDebug, or using watchdog logging...), debugging Varnish's VCL language can be intimidating.

VCL uses C-like syntax, and is compiled when varnish starts, so you can't just modify a .vcl file and refresh to see changes or debug something. And there are only a few places where you can simply stick a debug statement. So, I'll explain four different ways I use to debug VCLs in this post (note: don't do this on a production server!):

Simple Error statements (like print in PHP)

Sometimes, all you need to do is see the output of a variable, like req.http.Cookie, inside vcl_recv(). In these cases, you can just add an error statement to throw an error in Varnish and output the contents of a string, like the Cookie: