Achieving a high hitrate

Now that Varnish is up and running you can access your web application through Varnish. Unless your application is specifically written to work behind a web accelerator you’ll probably need to do some changes to either the configuration or the application in order to get a high hitrate in Varnish.

Varnish will not cache your data unless it’s absolutely sure it is safe to do so. So, for you to understand how Varnish decides if and how to cache a page, we’ll guide you through a couple of tools that you should find useful to understand what is happening in your Varnish setup.

Note that you need a tool to see the HTTP headers that fly between Varnish and the backend. On the Varnish server, the easiest way to do this is to use varnishlog and varnishtop but sometimes a client-side tool makes sense. Here are the ones we commonly use.

Tool: varnishtop

You can use varnishtop to identify what URLs are hitting the backend the most. varnishtop -i BereqURL is an essential command, showing you the top requests Varnish is sending to the backend. You can see some other examples of varnishtop usage in Statistics.

Tool: varnishlog

When you have identified an URL which is frequently sent to the backend you can use varnishlog to have a look at the request. varnishlog -q 'ReqURL ~ "^/foo/bar"' will show you the requests coming from the client matching /foo/bar.

For more information on how varnishlog works please see Logging in Varnish or then man page.

For extended diagnostics headers, see https://www.varnish-cache.org/trac/wiki/VCLExampleHitMissHeader

Tool: lwp-request

lwp-request is tool that is a part of The World-Wide Web library for Perl. It’s a couple of really basic programs that can execute an HTTP request and show you the result. We mostly use the two programs, GET and HEAD.

vg.no was the first site to use Varnish and the people running Varnish there are quite clueful. So it’s interesting to look at their HTTP Headers. Let’s send a GET request for their home page:

$ GET -H 'Host: www.vg.no' -Used http://vg.no/
GET http://vg.no/
Host: www.vg.no
User-Agent: lwp-request/5.834 libwww-perl/5.834

200 OK
Cache-Control: must-revalidate
Refresh: 600
Title: VG Nett - Forsiden - VG Nett
X-Age: 463
X-Cache: HIT
X-Rick-Would-Never: Let you down
X-VG-Jobb: http://www.finn.no/finn/job/fulltime/result?keyword=vg+multimedia Merk:HeaderNinja
X-VG-Korken: http://www.youtube.com/watch?v=Fcj8CnD5188
X-VG-WebCache: joanie
X-VG-WebServer: leon

OK. Lets look at what GET does. GET usually sends off HTTP 0.9 requests, which lack the ‘Host’ header. So we add a ‘Host’ header with the ‘-H’ option. ‘-U’ print request headers, ‘-s’ prints response status, ‘-e’ prints response headers and ‘-d’ discards the actual content. We don’t really care about the content, only the headers.

As you can see, VG adds quite a bit of information in their headers. Some of the headers, like the ‘X-Rick-Would-Never’ are specific to vg.no and their somewhat odd sense of humour. Others, like the ‘X-VG-Webcache’ are for debugging purposes.

So, to check whether a site sets cookies for a specific URL, just do:

GET -Used http://example.com/ |grep ^Set-Cookie

Tool: Live HTTP Headers

There is also a plugin for Firefox called Live HTTP Headers. This plugin can show you what headers are being sent and received. Live HTTP Headers can be found at https://addons.mozilla.org/en-US/firefox/addon/3829/ or by googling “Live HTTP Headers”.

The role of HTTP Headers

Along with each HTTP request and response comes a bunch of headers carrying metadata. Varnish will look at these headers to determine if it is appropriate to cache the contents and how long Varnish can keep the content cached.

Please note that when Varnish considers these headers Varnish actually considers itself part of the actual webserver. The rationale being that both are under your control.

The term surrogate origin cache is not really well defined by the IETF or RFC 2616 so the various ways Varnish works might differ from your expectations.

Let’s take a look at the important headers you should be aware of:

Cookies

Varnish will, in the default configuration, not cache an object coming from the backend with a ‘Set-Cookie’ header present. Also, if the client sends a Cookie header, Varnish will bypass the cache and go directly to the backend.

This can be overly conservative. A lot of sites use Google Analytics (GA) to analyze their traffic. GA sets a cookie to track you. This cookie is used by the client side javascript and is therefore of no interest to the server.

Cookies from the client

For a lot of web applications it makes sense to completely disregard the cookies unless you are accessing a special part of the web site. This VCL snippet in vcl_recv will disregard cookies unless you are accessing /admin/:

if (!(req.url ~ "^/admin/")) {
    unset req.http.Cookie;
}

Quite simple. If, however, you need to do something more complicated, like removing one out of several cookies, things get difficult. Unfortunately Varnish doesn’t have good tools for manipulating the Cookies. We have to use regular expressions to do the work. If you are familiar with regular expressions you’ll understand whats going on. If you aren’t we recommend that you either pick up a book on the subject, read through the pcrepattern man page, or read through one of many online guides.

Lets use the Varnish Software (VS) web as an example here. Very simplified the setup VS uses can be described as a Drupal-based backend with a Varnish cache in front. VS uses some cookies for Google Analytics tracking and similar tools. The cookies are all set and used by Javascript. Varnish and Drupal doesn’t need to see those cookies and since Varnish will cease caching of pages when the client sends cookies Varnish will discard these unnecessary cookies in VCL.

In the following VCL we discard all cookies that start with an underscore:

# Remove has_js and Google Analytics __* cookies.
set req.http.Cookie = regsuball(req.http.Cookie, "(^|;\s*)(_[_a-z]+|has_js)=[^;]*", "");
# Remove a ";" prefix, if present.
set req.http.Cookie = regsub(req.http.Cookie, "^;\s*", "");

Lets look at an example where we remove everything except the cookies named “COOKIE1” and “COOKIE2” and you can marvel at the “beauty” of it:

sub vcl_recv {
    if (req.http.Cookie) {
        set req.http.Cookie = ";" + req.http.Cookie;
        set req.http.Cookie = regsuball(req.http.Cookie, "; +", ";");
        set req.http.Cookie = regsuball(req.http.Cookie, ";(COOKIE1|COOKIE2)=", "; \1=");
        set req.http.Cookie = regsuball(req.http.Cookie, ";[^ ][^;]*", "");
        set req.http.Cookie = regsuball(req.http.Cookie, "^[; ]+|[; ]+$", "");

        if (req.http.Cookie == "") {
            unset req.http.Cookie;
        }
    }
}

A somewhat simpler example that can accomplish almost the same functionality can be found below. Instead of filtering out “other” cookies it instead picks out “the one” cookie that is needed, copies it to another header and then copies it back to the request, deleting the original cookie header.

sub vcl_recv {
    # save the original cookie header so we can mangle it
    set req.http.X-Varnish-PHP_SID = req.http.Cookie;
    # using a capturing sub pattern, extract the continuous string of
    # alphanumerics that immediately follows "PHPSESSID="
    set req.http.X-Varnish-PHP_SID =
       regsuball(req.http.X-Varnish-PHP_SID, ";? ?PHPSESSID=([a-zA-Z0-9]+)( |;| ;).*","\1");
    set req.http.Cookie = req.X-Varnish-PHP_SID;
    unset req.X-Varnish-PHP_SID;
}

There are other scary examples of what can be done in VCL in the Varnish Cache Wiki.

Cookies coming from the backend

If your backend server sets a cookie using the ‘Set-Cookie’ header Varnish will not cache the page when using the default configuration. A hit-for-pass object (see actions) is created. So, if the backend server acts silly and sets unwanted cookies just unset the ‘Set-Cookie’ header and all should be fine.

Cache-Control

The ‘Cache-Control’ header instructs caches how to handle the content. Varnish cares about the max-age parameter and uses it to calculate the TTL for an object.

So make sure you issue a ‘Cache-Control’ header with a max-age header. You can have a look at what Varnish Software’s Drupal server issues:

$ GET -Used http://www.varnish-software.com/|grep ^Cache-Control
Cache-Control: public, max-age=600

Age

Varnish adds an ‘Age’ header to indicate how long the object has been kept inside Varnish. You can grep out ‘Age’ from varnishlog with varnishlog -I RespHeader:^Age.

Pragma

An HTTP 1.0 server might send the header Pragma: nocache. Varnish ignores this header. You could easily add support for this header in VCL.

In vcl_backend_response:

if (beresp.http.Pragma ~ "nocache") {
    set beresp.uncacheable = true;
    set beresp.ttl = 120s; # how long not to cache this url.
}

Authorization

If Varnish sees an ‘Authorization’ header it will pass the request. If this is not what you want you can unset the header.

Overriding the time-to-live (TTL)

Sometimes your backend will misbehave. It might, depending on your setup, be easier to override the TTL in Varnish then to fix your somewhat cumbersome backend.

You need VCL to identify the objects you want and then you set the ‘beresp.ttl’ to whatever you want:

sub vcl_backend_response {
    if (bereq.url ~ "^/legacy_broken_cms/") {
        set beresp.ttl = 5d;
    }
}

This example will set the TTL to 5 days for the old legacy stuff on your site.

Forcing caching for certain requests and certain responses

Since you still might have this cumbersome backend that isn’t very friendly to work with you might want to override more stuff in Varnish. We recommend that you rely as much as you can on the default caching rules. It is perfectly easy to force Varnish to lookup an object in the cache but it isn’t really recommended.

Normalizing your namespace

Some sites are accessed via lots of hostnames. http://www.varnish-software.com/, http://varnish-software.com/ and http://varnishsoftware.com/ all point at the same site. Since Varnish doesn’t know they are the same, Varnish will cache different versions of every page for every hostname. You can mitigate this in your web server configuration by setting up redirects or by using the following VCL:

if (req.http.host ~ "(?i)^(www.)?varnish-?software.com") {
    set req.http.host = "varnish-software.com";
}

HTTP Vary

HTTP Vary is not a trivial concept. It is by far the most misunderstood HTTP header.

A lot of the response headers tell the client something about the HTTP object being delivered. Clients can request different variants of a HTTP object, based on their preference. Their preferences might cover stuff like encoding or language. When a client prefers UK English this is indicated through Accept-Language: en-uk. Caches need to keep these different variants apart and this is done through the HTTP response header ‘Vary’.

When a backend server issues a Vary: Accept-Language it tells Varnish that its needs to cache a separate version for every different Accept-Language that is coming from the clients.

If two clients say they accept the languages “en-us, en-uk” and “da, de” respectively, Varnish will cache and serve two different versions of the page if the backend indicated that Varnish needs to vary on the ‘Accept-Language’ header.

Please note that the headers that ‘Vary’ refer to need to match exactly for there to be a match. So Varnish will keep two copies of a page if one of them was created for “en-us, en-uk” and the other for “en-us,en-uk”. Just the lack of a whitespace will force Varnish to cache another version.

To achieve a high hitrate whilst using Vary is there therefore crucial to normalize the headers the backends varies on. Remember, just a difference in casing can force different cache entries.

The following VCL code will normalize the ‘Accept-Language’ header to either “en”, “de” or “fr”, in this order of precedence:

if (req.http.Accept-Language) {
    if (req.http.Accept-Language ~ "en") {
        set req.http.Accept-Language = "en";
    } elsif (req.http.Accept-Language ~ "de") {
        set req.http.Accept-Language = "de";
    } elsif (req.http.Accept-Language ~ "fr") {
        set req.http.Accept-Language = "fr";
    } else {
        # unknown language. Remove the accept-language header and
        # use the backend default.
        unset req.http.Accept-Language
    }
}

Vary parse errors

Varnish will return a “503 internal server error” page when it fails to parse the ‘Vary’ header, or if any of the client headers listed in the Vary header exceeds the limit of 65k characters. An ‘SLT_Error’ log entry is added in these cases.

Pitfall - Vary: User-Agent

Some applications or application servers send Vary: User-Agent along with their content. This instructs Varnish to cache a separate copy for every variation of ‘User-Agent’ there is and there are plenty. Even a single patchlevel of the same browser will generate at least 10 different ‘User-Agent’ headers based just on what operating system they are running.

So if you really need to vary based on ‘User-Agent’ be sure to normalize the header or your hit rate will suffer badly. Use the above code as a template.