6
One of the most effective way of increasing your hit ratio is to
7
increase the time-to-live (ttl) of your objects. But, as you're aware
8
of, in this twitterific day of age serving content that is outdated is
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The solution is to notify Varnish when there is fresh content
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available. This can be done through two mechanisms. HTTP purging and
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bans. First, let me explain the HTTP purges.
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An HTTP purge is similar to a HTTP GET request, except that the
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*method* is PURGE. Actually you can call the method whatever you'd
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like, but most people refer to this as purging. Squid supports the
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same mechanism. In order to support purging in Varnish you need the
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following VCL in place:::
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# allow PURGE from localhost and 192.168.55...
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if (req.request == "PURGE") {
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if (!client.ip ~ purge) {
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error 405 "Not allowed.";
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if (req.request == "PURGE") {
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# Note that setting ttl to 0 is magical.
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# the object is zapped from cache.
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if (req.request == "PURGE") {
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error 404 "Not in cache.";
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As you can see we have used to new VCL subroutines, vcl_hit and
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vcl_miss. When we call lookup Varnish will try to lookup the object in
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its cache. It will either hit an object or miss it and so the
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corresponding subroutine is called. In vcl_hit the object that is
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stored in cache is available and we can set the TTL.
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So for vg.no to invalidate their front page they would call out to
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And Varnish would then discard the front page. If there are several
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variants of the same URL in the cache however, only the matching
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variant will be purged. To purge a gzip variant of the same page the
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request would have to look like this:::
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There is another way to invalidate content. Bans. You can think of
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bans as a sort of a filter. You *ban* certain content from being
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served from your cache. You can ban content based on any metadata we
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Support for bans is built into Varnish and available in the CLI
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interface. For VG to ban every png object belonging on vg.no they could
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purge req.http.host == "vg.no" && req.http.url ~ "\.png$"
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Quite powerful, really.
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Bans are checked when we hit an object in the cache, but before we
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deliver it. An object is only checked against newer bans. If you have
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a lot of objects with long TTL in your cache you should be aware of a
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potential performance impact of having many bans.
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You can also add bans to Varnish via HTTP. Doing so requires a bit of VCL.::
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if (req.request == "BAN") {
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# Same ACL check as above:
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if (!client.ip ~ purge) {
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error 405 "Not allowed.";
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purge("req.http.host == " req.http.host
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"&& req.url == " req.url);
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# Throw a synthetic page so the
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# request wont go to the backend.
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error 200 "Ban added"
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This VCL sniplet enables Varnish to handle a HTTP BAN method. Adding a
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ban on the URL, including the host part.
added
removed
doc/sphinx/tutorial/purging.rst
