Varnish is a web accelerator written with performance and flexibility in mind. It's modern architecture gives it a significantly better performance than many of it's competing products. Varnish store web pages in memory so the web servers don't have to create the same web page over and over again. The web server only recreates a page when it is changed. Additionally Varnish can serve web pages much faster then any application server is capable of - giving the website a significant speed up.

What is stopping Varnish and Drupal Pressflow from caching anonymous users page views.

Every so often with Pressflow and Varnish you might find that your anonymous users aren't being cached via Varnish.

A quick way to see whether your anonymous pages are being served is to add some debugging headers to Varnish by adding some code like the snippet here to vcl_deliver()

sub vcl_deliver {
if (obj.hits > 0) {
set resp.http.X-Varnish-Cache = "HIT";
else {
set resp.http.X-Varnish-Cache = "MISS";

and hit them with

curl -I

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