wiki:Introduction

An introduction to Varnish

For many years the old standby for caching was Squid, a caching proxy server that with the right magic could be turned into a fairly functional http accelerator. While Squid performed quite well for those willing to do the work to configure it properly, at its heart it's still a forward proxy and it could be somewhat difficult to get it to do exactly what you wanted it to.

Enter Varnish.

Varnish is a relative newcomer to the caching scene, having its 1.0 release in 2006. Unlike Squid, Varnish is designed from the ground up to be an http accelerator, a caching reverse proxy. It is designed specifically to solve the problem we face as web-application developers, namely how to improve performance on a site or application that has many moving parts and performance dependencies (databases, etc.).

There are many benefits to using Varnish, and while I don't want to start a laundry list, I will cover the two that I find to be most interesting. First, it has an extremely flexible config language that makes it possible to control nearly every aspect of how web requests are processed and how your app is cached. Second, it supports ESI, or Edge Side Includes, a small XML-based markup language for dynamically assembling web content.

Edge side includes allow you to break your pages into smaller pieces, and cache those pieces independently. ESI is a complicated topic, which I will cover in another article. Today, we are going to focus on getting Varnish up and running.

Getting Varnish up and running

As of this writing, the current Varnish release is 2.0.2. While there are Varnish binary packages available for many *nix distributions, before installing them, please ensure that they provide the 2.0.2 release. If they do not, I suggest strongly that you compile your own. The 2.0.2 release includes several improvements and fixes that you want to have. This article (and the next) assumes you are using the 2.0.2 version.

I will not cover the download and installation process, as that is very well covered on the Varnish Wiki. We will begin at the most important part, configuring varnish. Go ahead and get Varnish installed... I'll wait.

Configuring Varnish

Welcome back. Let's get to configuring Varnish. As I mentioned before, Varnish is specifically designed to solve the problem we have, and as such, comes with a default configuration built in that can be used out of the box to get http acceleration running. To use Varnish in its basic config, you simply have to start it up (don't do this just yet):

 varnishd -a :80 -b localhost:8080 -T localhost:6082

This will start varnish listening on port 80, and forwarding traffic to a web server listening on localhost port 8080. It also turns on the management interface on port 6082.

I can hear you thinking 'Ok... If it's that easy, why don't we just use the default configuration?' The answer is somewhat complicated, but the short version is that the Varnish developers know the problem we are dealing with and know what the correct behavior is in the majority of cases. In the majority of cases, though, the application Varnish is talking to is not designed to work with a cache, and believes it is talking to a web browser. As such, Varnish rightly behaves very conservatively about what it can cache and what it can't.

Since our application understands that it's talking to a cache, we can let Varnish do a bit more for us. You did read last year's article, right? If not, you should. Even if you haven't, though, you are OK. Our configuration is going to be a little more lax than Varnish's default, but not too much.

The Varnish Configuration Language

As you already saw, Varnish can receive its most critical information as command line arguments. However, if you want to really harness the power of Varnish, you need to provide it a more advanced configuration in the form of a VCL file.

A VCL file tells Varnish exactly how to handle each phase of request processing. VCL is very powerful and allows for evaluation and modification of nearly every aspect of an HTTP request and response. It allows you to examine headers, do regular expresion comparisons and substitutions, and generally muck with incoming web requests on the fly. It provides a robust programming language that will be somewhat familiar to anyone who has programmed in Perl or C.

While the Varnish Configuration Language is quite robust, it does have its limitations. If you happen to find yourself in the quite rare situation where you are running into those limitations, VCL allows you to do something almost unheard of... It lets you drop into C to perform your task. While we won't use this functionality in this article, it does give you a hint about just how powerful the Varnish cache really is.

How VCL works

Your VCL file is more than just a config file. Your Varnish config is in fact a mini program, a program that is actually compiled and linked in to Varnish at runtime. There are several steps to how Varnish handles each request and in your VCL file you have the option to customize the behavior of each one.

There are actually 8 subroutines that control how Varnish behaves, and that you can change in your Varnish config. They are:

vcl_recv()

Called after a request is received from the browser, but before it is processed.

vcl_pipe()

Called when a request must be forwarded directly to the backend with minimal handling by Varnish (think HTTP CONNECT)

vcl_hash()

Called to determine the hash key used to look up a request in the cache.

vcl_hit()

Called after a cache lookup when the object requested has been found in the cache.

vcl_miss()

Called after a cache lookup when the object requested was not found in the cache.

vcl_pass()

Called when the request is to be passed to the backend without looking it up in the cache.

vcl_fetch()

Called when the request has been sent to the backend and a response has been received from the backend.

vcl_deliver()

Called before a response object (from the cache or the web server) is sent to the requesting client.

That seems like a lot, but as I mentioned before Varnish has pretty reasonable defaults, so you only need to override a few of these.

Strictly speaking, Varnish doesn't actually let you replace its defaults. Your definitions of the above routines simply run before the builtin versions of those same routines. Fortunately, we can prevent Varnish from proceeding on to the builtin versions if we wish by returning the appropriate value within our version of the routine. If that seems confusing, don't worry. It will become clear when we start looking at our config.

The routines we are most interested in are vcl_recv, which handles the incoming request from the browser, and vcl_fetch, which is where we will decide whether the object just retrieved should go into the cache.

Our configuration

As I mentioned earlier, Varnish has some pretty sensible defaults, but it assumes that your app is unaware of the cache. Since we are in control of our application, we can make some assumptions varnish can't by default. By default, for example, Varnish will not cache any page when the request that started it included a cookie. While this makes sense if the web app is unknown, if the web app is known, it makes much more sense to let the web app decide what is the appropriate cache response.

So, since we know the system we are working with, we will adjust the config somewhat. When using our config file, Varnish will behave as follows. Varnish will cache all static content (within a particular directory). Varnish will not cache anything else unless the web app provides a max-age or s-max-age Cache-Control header. It will cache content if the above rules are met, even if a cookie is present. The assumption is that your web app is smart enough to know if the content can be cached whether a cookie is present or not. Again, see last years article to see how to set this up on the app side. The VCL file

Our VCL file in its entirety is available here. The file itself is well commented and is relatively easy to understand. If you are the kind of person who just jumps right in, you should have no trouble grabbing that config and hitting the ground running. If, however, you would like a more in-depth exploration of that config, please continue reading.

In order to create the config we described earlier, we only need to create two custom vcl subroutines, vcl_recv and vcl_fetch.

Set a backend

The first thing we need to do in our Varnish config is set up the web server that Varnish will be talking to. Varnish actually has some sophisticated backend selection, allowing you to use a Varnish server as both a cache and a load balancer, serving traffic to multiple backend servers. This capability includes niceties such as web-server health checking and Varnish's flexible VCL language allows you to use complex logic to decide how traffic is routed.

An entire article could be written describing your options when using the backend selection and load balancing capabilities of Varnish. For today, we will describe a simple single-server backend running on the same server we are running varnish on.

 backend catalystsite {
     .host = "localhost";
     .port = "81";
 }

That's about as simple as it gets. This tells Varnish that there is a web server called 'catalystsite' which is listening on port 81 on localhost. This does NOT tell Varnish to use it, however, that happens later.

The inbound request: vcl_recv

The vcl_recv subroutine handles the incoming request from the client. This is where you set basic config options and do any request setup and tweaking before varnish looks up the item or passes the request on to the backend. You can see the complete routine in the config file. Here, we will break it down and explore each piece of functionality one at a time.

The incoming request is available within vcl_recv as 'req', you can review the Varnish docs to see all the properties that are available. We'll cover the ones we use as we encounter them.

 set req.http.host = "mycatalystsite.com";

The req.http variable contains the headers supplied with the request. This sets the host header to pass on to the backend. This is not strictly necessary and can cause trouble if you are fronting several sites with a single varnish installation. However, without this header, requests to mycatalystsite.com and www.mycatalystsite.com would be treated as completely separate by Varnish, potentially doubling every entry in your cache. If you are fronting only a single site, normalizing the hostname is a good idea.

 set req.backend = catalystsite;

As you can probably guess, this simply tells Varnish which backend should process this request. If you had multiple backends, this is where you would do your backend selection.

 if (req.http.Cache-Control ~ "no-cache") {
     purge_url(req.url);
 }

What this block does is tell Varnish to purge the cache for a particular URL when the no-cache cache-control header is sent from the client. That header is provided whenever you hold shift while pressing reload in the Firefox browser. In other words, this forces Varnish to refresh the cache for the page you are looking at when you hit shift-reload. Note that this will happen if anyone does a shift-reload, so you may not want this in a production environment, but it is definitely useful in debugging your initial deployment.

 if (req.request == "GET" && req.url ~ "^/static/") {
     unset req.http.cookie;
     unset req.http.Authorization;
     lookup;
 }

This tells Varnish if the request is for anything that starts with /static that it should remove any cookie header or authorization information, and then look up the request in the cache. The lookup line is important. This tells Varnish that it should stop executing the vcl_recv routine and look up the item in the cache. This is an example of a keyword.

Keywords in Varnish can be though of as a 'return' for the subroutine combined with the value it is returning. If you do not use a keyword somewhere to terminate your subroutine, control will fall through to the default Varnish subs. There is a knack to figuring out when to return and when to fall through, and you will get the hang of it after working with Varnish for a short while. In the meantime, you can rely on the fact that the config presented here does the right thing. You can read up on the default behavior in the Varnish docs and wiki also. For now, let's move on to the next part of our vcl file.

 if (req.request == "POST") {
     pass;
 }

This tells Varnish that if the request method is POST, that it should NOT look it up in the cache and should instead pass it directly to the backend. This is most likely what you want, as POST data is generally form submission and the result will vary from user to user and request to request.

This snippit also introduces our next keyword, pass. Pass tells Varnish that it should pass the request through to the backend without looking it up in the cache. This is a subtle but critical detail because even if you put something into the cache in vcl_fetch, if you pass when receiving a request for that item, it will never actually be served from the cache.

 if (req.request != "GET" && req.request != "HEAD" &&
     req.request != "PUT" && req.request != "POST" &&
     req.request != "TRACE" && req.request != "OPTIONS" &&
     req.request != "DELETE") {

     # Non-RFC2616 or CONNECT which is weird. #
     pass;
 }

Again we have an exclusion rule. This says that if the request method isn't one of the 'normal' web site and web service methods, Varnish should send it right through to the backend.

 if (req.http.Authorization) {
     # Not cacheable by default #
     pass;
 }

And our last check. If the client is providing an Authorization header that indicates some sort of access control is in place and we want to pass it directly through to the backend. Chances are, if an Authorization header is being provided, the data coming back is going to be tailored to the user in question, so we don't bother looking it up.

Finally... our last line of vcl_recv:

 lookup;

If we haven't explicitly handled it already somewhere along the way, we look it up in the cache. Note that since our vcl_recv ends with a keyword, Varnish's builtin vcl_recv never gets a chance to execute. That's OK in this case, because we have handled the different scenarios that we are interested in.

Handling the web-server response: vcl_fetch

When Varnish enters the vcl_fetch subroutine, it has already requested data from the backend web server and has received a response. It has not, however, inserted anything into the cache yet. In fact, most of what you are likely to be doing in vcl_fetch is determining whether the response should be cached or not.

In many ways the rules you place in vcl_fetch will directly correlate to the rules you placed in vcl_recv earlier. The difference is that in vcl_recv you were deciding whether to look up the item in the cache, whereas in vcl_fetch you are deciding whether you should insert the response into the cache in the first place.

The difference between the lookup check and the insert check is a subtle one, as cache control can really be done in either place. It is, however, important to use vcl_fetch effectively, because limiting what goes into the cache is the only real control you have over the the size of your cache. It's also all too easy to accidentally let a request pass into lookup that you didn't want to. The best way to avoid serving up invalid cached data is to make sure it's never placed in the cache in the first place. Ultimately, it's best to use vcl_recv and vcl_fetch in tandem to make sure that what goes into and comes out of the cache is exactly what you want.

All that said, let's begin exploring our vcl_fetch routine.

 if (req.request == "GET" && req.url ~ "^/static" ) {

     unset obj.http.Set-Cookie;
     set obj.ttl = 30m;
     deliver;
 }

We start out with a very important piece of our cache control. This block tells Varnish that if the request is GETing something from /static/* it should be placed in the cache. The line that begins with unset ensures that if, for some reason, a cookie is being set by the server, the cookie header is removed before it is placed in the cache. We also set obj.ttl to 30 minutes, forcing static content to expire in 30 minutes regardless of what we get from the server. This makes sense as we may not have direct control over the cache control headers on static files.

Notice that we are working with two objects now, the req object, which is the original request, and the 'obj' object, which is the response received from the backend server. We also encounter our first vcl_fetch keyword here, deliver. When deliver is encountered it tells Varnish that the object it currently is working with should be inserted into the cache.

 if (obj.status >= 300) {
     pass;
 }

This is another important piece of cache control. This tells Varnish that if the object it got from the web server has a status that is not in the 200s, that it should not cache it. This is important because if you have an error or other exceptional condition on the server, you do not want to be serving that error over and over to your site visitors. Sometimes 30x responses, such as redirects, could be cached in order to relieve the backend server of work, but in this case, we play it safe and assume that if it's not an 'OK' response it's not cacheable. Notice we have re-encountered the 'pass' keyword. In vcl_fetch, pass tells Varnish to send the response to the client, but don't save the response in the cache.

 if (!obj.cacheable) {
     pass;
 }

Glancing at this block it's pretty obvious what is going on. The cacheable property on the response is set by Varnish, and it is basically Varnish's opinion as to whether the object could be cached or not. Varnish is pretty smart about this, so if it thinks the object is not cacheable, we should trust its judgement and avoid placing it in the cache. Note that we don't automatically assume that if Varnish thinks it is cacheable that we should cache it. We make that decision ourselves.

 if (obj.http.Set-Cookie) {
     pass;
 }

By now you are probably getting the hang of it. If the response is setting a cookie, it's a safe bet that that cookie is intended for someone in particular. We should return the request to the client, but don't cache it.

 if(obj.http.Pragma ~ "no-cache" ||
    obj.http.Cache-Control ~ "no-cache" ||
    obj.http.Cache-Control ~ "private") {
         pass;
 }

Now we are getting into the meat of our cache control. This is the first point where what happens on the server directly controls what happens within Varnish. This block says that if any of the cache-control headers are set to 'no cache', obey them. This allows you, within your web app, to directly tell Varnish Do NOT cache this. Without this block Varnish would attempt to guess whether the response was cacheable, and it would likely be wrong as Varnish does not obey no-cache instructions in its default configuration.

 if (obj.http.Cache-Control ~ "max-age") {
     unset obj.http.Set-Cookie;
     deliver;
 }
 pass;

And here is perhaps the most important rule in our cache control policy and where we depart significantly from Varnish's default behavior. This looks at the Cache-Control header in the response object to see whether the string 'max-age' appears within it. If it does, then it clears out any cookie related headers and places the response object in the cache. If it doesn't, we tell Varnish not to cache it, no matter what it thinks about the response's cacheability.

As we touched on before, anything containing any cookie related headers is immediately deemed uncacheable by Varnish by default. Varnish's behavior is exactly right in most cases. What we are doing with this block is explicitly saying 'Trust the backend server, if it says it's cacheable, cache it.' Since we are running the backend server and in an app like Catalyst we have complete and reliable control over the headers provided, this is relatively safe to do.

By default, Varnish also interprets the Expires header to determine cacheability. We eschew Varnish's logic here and say 'if we didn't set max-age on the web-server, then you are not to cache it.' This gives us one clear method for controlling the cacheability of data in our application... and ensures that if we don't explicitly set the 'max-age' property in our app, the item will not be cached. A quick side note: Is it a hit?

When you are first working with the cache in place, you will at some point want to know if a piece of content you are looking at came from the cache or from the backend server. Yes, you could go to the backend server, make the request and watch the access logs, but there is an easier way.

If you look at the headers returned on the item in question, you will see a header called 'X-Varnish.' That header will contain either one or two numbers (separated by a space.) If the 'X-Varnish' header contains two numbers, the data came from the cache. If it contains only one, it did not come out of the cache. Knowing this piece of information can make your debugging much simpler.

Basically what we have done with this configuration is require an explicit cache 'allow' and what it means is that nothing in our application will be cached until we say it should be cached. This is, in my opinion, the only safe way to operate a cache in front of a web application.

In anything but the smallest web apps, there are always pieces that you don't think of or that interact in a way you don't recognize until you are presented with a trouble ticket. The best thing to do in my experience is to turn caching on as you work through specific paths in your web app. The configuration presented here allows you to do that and be sure that only those pieces you have explored are cached.

Starting Varnish

Now that we have configured Varnish with a sensible set of rules, we need to turn it on. We've seen once how simple it is to start Varnish, but we have also seen that Varnish's defaults are not always what we want.

Varnish is a modern application, engineered to 21st century standards and it figures out a lot on its own. This is a very good thing. However, there is one detail that often is at a odds with how I wind up deploying caching.

Varnish in its default configuration basically expects it has run of the server it is running on and can use as much virtual memory as it wants. This is exactly what you want if you have a dedicated cache server. Unfortunately for me, I have often found it a hard sell, at least initially, to get a dedicated cache machine. So most of the time my caching runs on the same server that is doing the web serving.

In this situation, you need to manually limit the memory that Varnish can grab. It sounds difficult, but it is quite simple. To start Varnish with the config we just reviewed, and a cache limited to 512 megabytes, you would use the following command line:

 varnishd -a :80 -T localhost:6082 -f catalyst.vcl -s file,/var/cache/varnish.cache,512M

Note that there are startup scripts included with the Varnish packages on most distributions, and often they provide a lot of other OS specific tweaks to the startup environment. It's a good idea to use the startup scripts provided, if available for your OS of choice, and simply customize the options as appropriate. Summary

Congratulations, you have completed a pretty in-depth look into the Varnish caching software and how to deploy it with your web application. Though we have covered a lot of ground, we have only scratched the surface of what Varnish can do.

Don't let that fool you, though. Even the straightforward configuration described here can make a HUGE difference in the performance of your site or application. I have seen sites that could barely handle their traffic become speed demons using caching configurations nearly identical to the one we explored today.

But don't take my word for it. This configuration is specifically designed to be non-intrusive. Take it, install it, try it yourself. You won't be disappointed.

AUTHOR jayk - Jay Kuri <jayk@…>

This article was written by Jay Kuri and published 2008-12-14  here in the advent calendar on  http://www.catalystframework.org/. Jay graciously allowed us to nick it and put it here.