This document provides an overview of how to install, configure, and use Varnish as a caching reverse proxy and load balancer. It discusses key Varnish concepts like backends, directors, health checks, caching and purging strategies. Varnish can be used to accelerate websites by caching content, balance loads across multiple servers, and provide high availability through features like grace mode and saint mode.

1. Varnish

2. What is a reverse proxy?
Forward Proxy
Reverse Proxy
http://en.wikipedia.org/wiki/Proxy_server

3. Install Varnish
curl http://repo.varnish-cache.org/debian/GPG-key.txt | apt-key add -
echo "deb http://repo.varnish-cache.org/ubuntu/ $(lsb_release -s -c)
varnish-3.0" >> /etc/apt/sources.list
apt-get update
apt-get install varnish
https://www.varnish-cache.org/installation/ubuntu

4. Concept of Backend Servers
# /etc/varnish/default.vcl Backend Servers provide the
backend default { content to varnish
.host = "127.0.0.1"
.port = "80"
}

5. Start Varnish
varnishd -F -f /etc/varnish/default.vcl -a 0.0.0.0:8080
-F
Run in the foregroud
-f /etc/varnish/default.vcl
Use our default config file
-a 0.0.0.0:8080
Listen for requests on port 8080

6. VCL
Varnish Configuration Language
Used to configure how varnish handles requests
Can be used to manipulate headers

7. How Varnish knows what to cache?
Varnish will not cache:
An object coming from the backend server with a Set-Cookie
header.
A request coming from the client with a cookie header

8. Purging the cache
# /etc/varnish/default.vcl
acl purge { Simple script that will allow only
"localhost"; requests from localhost to purge the
}
sub vcl_recv { cache
if (req.request == "PURGE") {
if (!client.ip ~ purge) {
error 405 "Not allowed."; PURGE is a request method, just like
}
return (lookup); GET or POST
}
}
sub vcl_hit {
if (req.request == "PURGE") {
purge;
error 200 "Purged.";
}
}
sub vcl_miss {
if (req.request == "PURGE") {
purge;
error 200 "Purged.";
}
}

9. Not just a HTTP Accelerator
Varnish can be setup as a load balancer to distribute the
requests across multiple servers

10. Multiple Backends
# /etc/varnish/default.vcl You can define as many backends
backend server01 { as needed
.host = "127.0.0.1";
.port = "8080"; Each can be different hosts and
} different ports
backend server02 {
.host = "127.0.0.1";
.port = "8080";
}

11. Directors
# /etc/varnish/default.vcl Directors are groups of
director default round-robin { backends
{
.backend = "server01";
}
{
.backend = "server02";
}
}

12. Health Checks
# /etc/varnish/varnish. url
vcl URL to check
backend server01 { interval
.host = "127.0.0.1"; Poll every X
.port = "8080"; timeout
.probe { Timeout after X
.url = "/"; window
.interval = 5s; The past X requests
.timeout = 1s; threshold
.window = 5; How many good probes
.threshold = 3;
}
}

13. High Availability
● Grace Mode
○ Instructs Varnish to keep objects past TTL
● Saint Mode
○ Try another server or serve stale content
● God Mode
○ Not yet implemented

14. Grace Mode Example
# /etc/varnish/default.vcl set beresp.grace
sub vcl_fetch { Instructs Varnish to store
set beresp.grace = 30m; objects for n minutes
}
sub vcl_recv { set req.grace
if (!req.backend.healthy) { Instructs Varnish to serve
set req.grace = 5m; objects that are n seconds old
} else {
set req.grace = 15s;
}
}

15. Saint Mode Example
# /etc/varnish/default.vcl set beresp.saintmode
sub vcl_fetch { How long before
if (beresp.status == 500) { Varnish can ask that
set beresp.saintmode = 10s; server for the URL again.
restart;
} set beresp.grace
set beresp.grace = 5m; Instructs Varnish to
} store objects for n minutes

16. Questions?
Documentation TOC
https://www.varnish-cache.org/docs/3.0/index.html
Reference Manual
https://www.varnish-cache.org/docs/3.0/reference/index.html
Random Outbursts
https://www.varnish-cache.org/docs/3.0/phk/index.html