Varnish and WordPress, it is possible real caching without external plugin? - varnish

Question

Maybe it sounds a novice question in Varnish Cache world, but why in WordPress it seems that is a need to install a external cache plugin, to working fully cached?
Websites are correctly loaded via Varnish, a curl -I command:
HTTP/1.1 200 OK
Server: nginx/1.11.12
Date: Thu, 11 Oct 2018 09:39:07 GMT
Content-Type: text/html; charset=UTF-8
Connection: keep-alive
Vary: Accept-Encoding
Cache-Control: max-age=0, public
Expires: Thu, 11 Oct 2018 09:39:07 GMT
Vary: Accept-Encoding
X-Varnish: 19575855
Age: 0
Via: 1.1 varnish-v4
X-Cache: MISS
Accept-Ranges: bytes
Pragma: public
Cache-Control: public
Vary: Accept-Encoding
With this configuration, by default WordPress installations are not being cached.
After test multiple cache plugins -some not working, or not working without complex configuration- i found the Swift Performance, in their Lite version, simply activating the Cache option, here really takes all advantages and here i can see varnish is working fully with very good results in stress test.
This could be ok for a single site on a single environment, but in shared hosting terms, when every customer can have their own WP (or other CMS) installation could be a problem.
So the key is there are no way to take full caching advantage from Varnish without installing 3rd party caching (and complex) plugins? Why not caching all by default?
Any kind of suggestions and help will be high welcome, thanks in advance.

Answer 1

With this configuration, by default WordPress installations are not being cached
By default, if you don't change anything in neither Wordpress or Varnish configuration, things would work together in a way that Wordpress pages are cached for 120 seconds. So real caching is possible, but it will be a short lived cache and highly ineffective one.
Your specific headers indicate that no caching should happen. They are either sent by Varnish itself (we're all guilty of copy pasting stuff without thinking what it does), or a Wordpress plugin (more often bad ones, than good). Without knowing your specific configuration, it's hard to decipher anything.
Varnish is a transparent HTTP caching proxy. Which means it’s just going to, by default, use HTTP headers, which are sent by backend (Wordpress), like Cache-Control, to make a decision on whether resource can be cached and for how long.
Wordpress, in fact, does not send cache related headers other than in a few specific areas (error pages, login POST submission, etc).
The standard approach outlined here is configuring Varnish with the highest TTL. With that:
Varnish has no idea when you update an article contents, or change theme. Typical solution to this lies in using cache invalidation plugin like Varnish HTTP Purge.
A plugin requirement comes from necessity to purge cache, when content is changed.
Suppose that you update a Wordpress page's text. You had that same page previously visited and it went into Varnish cache for storage. What happens upon the next visit, is that Varnish will serve the same, now stale content to all the next visitors.
The Wordpress plugins for Varnish, like Varnish HTTP Purge, will hook into Wordpress in a way that they will instruct Varnish to clear cache when pages are updated. This is their primary purpose.
That kind of approach (high TTL and cache purging) is de-facto standard with Varnish. As Varnish has no information about when you update content, the inner workings of purging cache is with the application itself. The cache purging feature is either bundled into CMS code itself (Magento 2, for example has it out of the box, without any extra plugins), or a Wordpress plugin.