Varnish 101

An introduction to caching, Varnish, and .vcl files.

James Fuller
Web Application Developer
University of Iowa - ITS
d.o: fullerja

Follow along at: http://rawgit.com/jwfuller/varnish101/master/index.html#/

What is Varnish?

• “A web application accelerator also known as a caching HTTP reverse proxy.”
• A piece of software that makes dynamic websites really really really fast.
• It sits between the client and the webserver and caches webpages.

Drupal with out any caching

~ 3 mph

Drupal with core caching

~ 80 mph

Drupal with Varnish

~ 357.2 mph

Loading a webpage from Drupal

Plain ol' Drupal

1. User requests a URL.
2. Web server (Apache/Nginx/IIS) sends it to Drupal.
3. Drupal bootstraps and initializes the database, sessions etc.
4. Maps the path to a callback function, which gets the primary content.
5. Modules can hook into the process and extend functionality and alter the content.
6. The Theme System generates the HTML and applies styles it to.
7. Drupal returns a fully formed HTML page to the browser.
8. The browser renders the HTML page for the user.

Drupal with core caching

1. User requests a URL.
2. Web server (Apache/Nginx/IIS) sends it to Drupal.
3. Drupal bootstraps and initializes the database, sessions etc.
Maps the path to a callback function, which gets the primary content.
Modules can hook into the process and extend functionality and alter the content.
The Theme System generates the HTML and applies styles it to.
4. Drupal returns a fully formed HTML page to the browser.
5. The browser renders the HTML page for the user.

Drupal (Now with Varnish!)

1. User requests a URL
Web server (Apache/Nginx/IIS) sends it to Drupal.
Drupal bootstraps and initializes the database, sessions etc.
Maps the path to a callback function, which gets the primary content.
Modules can hook into the process and extend functionality and alter the content.
The Theme System generates the HTML and applies styles it to.
Drupal returns a fully formed HTML page to the browser.
2. The browser renders the HTML page for the user.

How does Varnish do this?

1. A request is recieved
2. Varnish determines if it should Lookup, Pass, Pipe or Error.
   • Lookup = Request that could be cached
   • Pass = Request cannot be cached (cookies) or should not be cached (configuration)
   • Pipe = Request should be mainlined to the server (POST or similar transmition of data that should not be interupted)
   • Error = Backend is down or varnish is configured to error
3. Assuming a Lookup, Varnish sends the request to Drupal.
4. Drupal does the long look up and responds.
5. Varnish caches a copy of that response.
   • This can be from RAM or disk, RAM is highly recommended.
6. Request is made for the same thing
7. Varnish does a Lookup
   • Checks to make sure the cache lifetime has not expired
8. Responds with its copy from RAM. (Skipping Drupal entirely)

Installation

(20,000 ft view)

1. Download and install Varnish
   • (apt-get, yum, make/install)
2. Swap Apache off port 80 and put Varnish on
3. Adjust your configuration in the .vcl file
4. Install the Drupal Varnish module
   • (including editing your settings.php file)
5. Adjust your Performance settings

Anatomy of a VCL

Backend Probe

# Define the health check for Drupal Servers.
probe drupal_healthcheck {
   .url = "/health_check.php";
   .interval = 5s;
   .timeout = 1s;
   .window = 5;
   .threshold = 3;
   .expected_response = 200;
}
					

Backend Definitions

# Define the list of backends (web servers).
# Port 80 Backend Servers
backend web1 {
	.host = "192.10.0.1”;
	.probe = drupal_healthcheck;
	}

backend web2 {
	.host = "192.10.0.2”;
	.probe = drupal_healthcheck;
	}

# Port 443 Backend Servers for SSL
backend web1_ssl {
	.host = "192.10.0.1”;
	.port = "443”;
	.probe = drupal_healthcheck;
	}

backend web2_ssl {
	.host = "192.10.0.2”;
	.port = "443”;
	.probe = drupal_healthcheck;
	}
					

Directors

# Define the director that determines how to distribute incoming requests.
director default_director round-robin {
  { .backend = web1; }
  { .backend = web2; }
}

director ssl_director round-robin {
  { .backend = web1_ssl; }
  { .backend = web2_ssl; }
}
					

Basic Receiver Sub Routine

# Respond to incoming requests.
sub vcl_recv {
  # Set the director to cycle between the web servers.
  if (server.port == 443) {
    set req.backend = ssl_director;
  }
  else {
   set req.backend = default_director;
  }
}
					

Basic Fetch Sub Routine

sub vcl_fetch {
  # Don’t allow static files to set cookies.
  if (req.url ~ "(?i)\.(png|gif|jpeg|jpg|ico|swf|css|js)(\?[a-z0-9]+)?$") {
    unset beresp.http.set-cookie;
  }

# Allow items to be stale if needed.
  set beresp.grace = 1h;
}

					

Deliver Sub Routine

# Set a header to track a cache HIT/MISS.
sub vcl_deliver {
  if (obj.hits > 0) {
    set resp.http.X-Varnish-Cache = "HIT";
  }
  else {
    set resp.http.X-Varnish-Cache = "MISS";
  }
}
					

Varnish can’t do everything

Varnish is not great with

• Forms
• Authenticated Users
• Things that really need cookies

But you can do some stuff to help

Checkout ESI (Edge Side Includes)

Metrics

What should you measure?

• Time to First Byte
• Number of requests per second

Metrics

How can you measure it?

• Time to First Byte
  • For publically accessible web servers - www.webpagetest.org
  • For local development or web servers – Chrome Dev Tools
• Number of Request per second
  • Load testing tools – JMeter, BlazeMeter, Siege, Apache Benchmark

How about some Data

• Drupal
  • 8130 ms
  • ~10 concurrent requests
• Drupal w/ core caching
  • 2090 ms
  • ~100 concurrent requests
• Drupal w/ Varnish
  • 68 ms
  • 2000+ concurrent requests

Additional Resources

• VCL files
  • 4Kitchens
    https://fourkitchens.atlassian.net/wiki/display/TECH/Configure+Varnish+3+for+Drupal+7
  • Lullabot
    https://www.lullabot.com/sites/lullabot.com/files/default_varnish3.vcl_.txt
• Caching Deep Drive – DrupalCon Austin
  https://austin2014.drupal.org/session/caching-deep-dive
• Sweet VM for playing with courtesy of:
  https://ohthehugemanatee.org/downloads/drupal/varnish101.zip
  • Campbell Vertesi - Technical Architect
  • Earnest Berry - Senior Developer at The Economist
  • Jason Ford - CTO of BlackMesh Managed Hosting
• http://www.isvarnishworking.com

Modules to help with caching and cache clearing

• https://www.drupal.org/project/varnish
• https://www.drupal.org/project/expire
• https://www.drupal.org/project/entitycache
• https://www.drupal.org/project/advagg
• https://www.drupal.org/project/views_cache_bully
• https://www.drupal.org/project/panels_hash_cache
• https://www.drupal.org/project/authcache