Varnish is an HTTP accelerator that acts as a caching reverse proxy, load balancer, and failover system. It sits in front of web servers to cache content and improve page load speeds by up to 1000x. Varnish uses Varnish Configuration Language (VCL) scripts to control caching, load balancing, and other behaviors. It supports Edge Side Includes (ESI) to cache page fragments with different expiration times. Popular sites using Varnish include Facebook, Twitter, and BBC.

1. Caching Solutions:
Varnish
with Bogdan Hadadea

2. 1. General Description: Varnish
• HTTP Accelerator
• Caching reverse proxy
o Sits in front of any HTTP server
• Load Balancer
• Fail Over System
• Perfect Browser caching solution
o Prevents hard refresh calls
• Typical speedup: 300x-1000x
• VCL – compiled to C
• ESI

3. 1. General Description: Varnish
ESI – Edge Side Includes
• Allows page fragmentation
• Fragments with different TTL
• ESI are serialized
• Configurable depth of ESI calls
• Doesn’t support alt and noerror attributes
• ESI calls can’t set cookies

4. 2. Usage: Varnish
• Originally built for Norwegian newspaper
• Current users:
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Facebook
Twitter
Hulu
BBC
Wired

5. 3. Architecture: Varnish - Overview

6. 3. Architecture: Varnish
Main methods:
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vcl_recv – beginning of request
vcl_fetch – after response from backend
vcl_hash – how to create a hash key
vcl_deliver – before a cached object is delivered
vcl_error – when there is an error

7. 3. Architecture: Varnish
VCL_RECV:
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Manipulate/remove cookies
Manipulate/add/remove headers
Request standardization
Cache bypassing

8. 3. Architecture: Varnish
VCL_FETCH
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Enable ESI
Manipulate headers
Manipulate cookies
Vary
Set/manipulate cache times

9. 4. Comparison: Squid
Advantages of Squid over Varnish:
• Built in SSL support
• Better support for streaming delivery of objects
• Support for antivirus plugins
Advantages of Varnish over Squid:
• Amazing configuration system – VCL
• Better performance and scalability
• Better invalidation support
• Better module support

10. 5. Conclusion
• Simple and great performance increase
• VCL is powerful but sometimes simple is better
• Make sure you can still handle a cold start