Purging and banning

One of the most effective ways of increasing your hit ratio is to increase the time-to-live (ttl) of your objects. But, as you're aware of, in this twitterific day of age, serving content that is outdated is bad for business.

The solution is to notify Varnish when there is fresh content available. This can be done through three mechanisms. HTTP purging, banning and forced cache misses. First, lets look at HTTP purging.

HTTP Purging

A purge is what happens when you pick out an object from the cache and discard it along with its variants. Usually a purge is invoked through HTTP with the method PURGE.

An HTTP purge is similar to an HTTP GET request, except that the method is PURGE. Actually you can call the method whatever you'd like, but most people refer to this as purging. Squid, for example, supports the same mechanism. In order to support purging in Varnish you need the following VCL in place:

acl purge {
        "localhost";
        "192.168.55.0"/24;
}

sub vcl_recv {
        # allow PURGE from localhost and 192.168.55...

        if (req.method == "PURGE") {
                if (!client.ip ~ purge) {
                        return(synth(405,"Not allowed.");
                }
                # jump to hit/miss
                return (purge);
        }
}

As you can see we have used a new action - return(purge). This ends execution of vcl_recv and jumps to vcl_hash. This is just like we handle a regular request. When vcl_hash calls return(lookup) varnish will purge the object and then call vcl_purge. Here you have the option of adding any particular actions you want Varnish to take once it has purge the object.

So for example.com to invalidate their front page they would call out to Varnish like this:

PURGE / HTTP/1.0
Host: example.com

And Varnish would then discard the front page. This will remove all variants as defined by Vary.

Bans

There is another way to invalidate content: Bans. You can think of bans as a sort of a filter on objects already in the cache. You ban certain content from being served from your cache. You can ban content based on any metadata we have. A ban will only work on objects already in the cache, it does not prevent new content from entering the cache or being served.

Support for bans is built into Varnish and available in the CLI interface. To ban every png object belonging on example.com, issue the following command:

ban req.http.host == "example.com" && req.url ~ "\.png$"

Quite powerful, really.

Bans are checked when we hit an object in the cache, but before we deliver it. An object is only checked against newer bans.

Bans that only match against obj.* are also processed by a background worker threads called the ban lurker. The ban lurker will walk the heap and try to match objects and will evict the matching objects. How aggressive the ban lurker is can be controlled by the parameter 'ban_lurker_sleep'. The ban lurker can be disabled by setting 'ban_lurker_sleep' to 0.

Bans that are older than the oldest objects in the cache are discarded without evaluation. If you have a lot of objects with long TTL, that are seldom accessed, you might accumulate a lot of bans. This might impact CPU usage and thereby performance.

You can also add bans to Varnish via HTTP. Doing so requires a bit of VCL:

sub vcl_recv {
        if (req.method == "BAN") {
                # Same ACL check as above:
                if (!client.ip ~ purge) {
                        return(synth(403, "Not allowed."));
                }
                ban("req.http.host == " + req.http.host +
                      "&& req.url == " + req.url);

                # Throw a synthetic page so the
                # request won't go to the backend.
                return(synth(200, "Ban added"));
        }
}

This VCL stanza enables Varnish to handle a HTTP BAN method, adding a ban on the URL, including the host part.

The ban lurker can help you keep the ban list at a manageable size, so we recommend that you avoid using req.* in your bans, as the request object is not available in the ban lurker thread.

You can use the following template to write ban lurker friendly bans:

sub vcl_backend_response {
  set beresp.http.x-url = bereq.url;
}

sub vcl_deliver {
  unset resp.http.x-url; # Optional
}

sub vcl_recv {
  if (req.method == "PURGE") {
    if (client.ip !~ purge) {
      return(synth(403, "Not allowed"));
    }
    ban("obj.http.x-url ~ " + req.url); # Assumes req.url is a regex. This might be a bit too simple
  }
}

To inspect the current ban list, issue the ban.list command in the CLI. This will produce a status of all current bans:

0xb75096d0 1318329475.377475    10      obj.http.x-url ~ test
0xb7509610 1318329470.785875    20G     obj.http.x-url ~ test

The ban list contains the ID of the ban, the timestamp when the ban entered the ban list. A count of the objects that has reached this point in the ban list, optionally postfixed with a 'G' for "Gone", if the ban is no longer valid. Finally, the ban expression is listed. The ban can be marked as "Gone" if it is a duplicate ban, but is still kept in the list for optimization purposes.

Forcing a cache miss

The final way to invalidate an object is a method that allows you to refresh an object by forcing a hash miss for a single request. If you set 'req.hash_always_miss' to true, Varnish will miss the current object in the cache, thus forcing a fetch from the backend. This can in turn add the freshly fetched object to the cache, thus overriding the current one. The old object will stay in the cache until ttl expires or it is evicted by some other means.

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