Varnish: Purging the Cache
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Join For FreeWe’re using varnish to cache all the requests that come through our web servers and especially in our pre-production environments we deploy quite frequently and want to see the changes that we’ve made.
This means that we need to purge the pages we’re accessing from varnish so that it will actually pass the request through to the application server and serve up the latest version of the page.
For some reason my google-fu when trying to remember/work out how to do this has always been weak but my colleague Shodhan helped me understand how to do this today so I thought I better record it so I don’t forget!
The way we’ve configured varnish to allow us to purge entries is similar to that described on the purging and banning page.
We have the following code in our /etc/varnish/default.vcl:
acl purge_acl {
"localhost";
}
# purge individual URLs from the cache
if(req.request == "PURGE") {
if(!client.ip ~ purge_acl) {
error 405 "Not allowed";
} else {
purge("req.url == " req.url);
error 200 "Purged";
}
}We only allow purging to be done if you’re on the machine i.e. your IP needs to be localhost and if you pass a request method of ‘PURGE’ that will remove that specific URL from varnish.
Any requests made will go first through nginx before being passed onto varnish.
If we start off with a URL which is already cached:
$ curl --insecure -I -H "Host: www.realhostgoeshere.com" https://localhost HTTP/1.1 200 OK Server: nginx Date: Wed, 10 Oct 2012 23:21:36 GMT Content-Type: text/html; charset=utf-8 Content-Length: 12929 Connection: keep-alive Vary: Accept-Encoding Vary: Accept-Encoding Vary: Accept-Encoding Status: 200 OK Cache-Control: max-age=1800, public X-UA-Compatible: IE=Edge,chrome=1 ETag: "9b2569322b89a71aca8c67f5f2b3a873" X-Request-Id: 9c783ff4386fa9a7937f31d22854dcc8 X-Varnish: 1045982044 1045982042 Via: 1.1 varnish age: 0 X-Cache: HIT Strict-Transport-Security: max-age=2419200
We can see that we got a varnish cache hit based on the ‘X-Cache’ header returned on the second last line.
To purge the cache for the root URL we’d do this:
$ curl --insecure -I -H "Host: www.realhostgoeshere.com" https://localhost -X PURGE HTTP/1.1 200 Purged Server: nginx Date: Wed, 10 Oct 2012 23:22:46 GMT Content-Type: text/html; charset=utf-8 Content-Length: 449 Connection: keep-alive Vary: Accept-Encoding Retry-After: 0 X-Varnish: 1045982047 Age: 0 Via: 1.1 varnish X-Cache: MISS Strict-Transport-Security: max-age=2419200
The 200 response code tells us that we were successful with our purging although from my brief experiments it seems like you would get that response code even if you purged a URL that didn’t exist!
If we hit that URL again:
curl --insecure -I -H "Host: www.realhostgoeshere.com" https://localhost HTTP/1.1 200 OK Server: nginx Date: Wed, 10 Oct 2012 23:23:17 GMT Content-Type: text/html; charset=utf-8 Content-Length: 12929 Connection: keep-alive Vary: Accept-Encoding Vary: Accept-Encoding Vary: Accept-Encoding Status: 200 OK Cache-Control: max-age=1800, public X-UA-Compatible: IE=Edge,chrome=1 ETag: "9b2569322b89a71aca8c67f5f2b3a873" X-Request-Id: bfcf1568382ce42ee0f9d4245b23f5cd X-Varnish: 1045982048 Via: 1.1 varnish age: 0 X-Cache: MISS Strict-Transport-Security: max-age=2419200
We can see that it’s been purge and varnish therefore had to go and request the page from the application server. If we hit it one more time we can see that it’s now cached again!
curl --insecure -I -H "Host: www.realhostgoeshere.com" https://localhost HTTP/1.1 200 OK Server: nginx Date: Wed, 10 Oct 2012 23:26:43 GMT Content-Type: text/html; charset=utf-8 Content-Length: 12929 Connection: keep-alive Vary: Accept-Encoding Vary: Accept-Encoding Vary: Accept-Encoding Status: 200 OK Cache-Control: max-age=1800, public X-UA-Compatible: IE=Edge,chrome=1 ETag: "9b2569322b89a71aca8c67f5f2b3a873" X-Request-Id: bfcf1568382ce42ee0f9d4245b23f5cd X-Varnish: 1045982050 1045982048 Via: 1.1 varnish age: 0 X-Cache: HIT Strict-Transport-Security: max-age=2419200
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