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Introduction to Caching With Varnish

Introduces Varnish and discusses what it's for, how it works, and how to install, configure, and manage it. Presented to the NYC Varnish Cache meetup group on 6/26/2012.

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Introduction to Caching With Varnish

  1. 1. NYC Varnish CacheIntroduction to Caching With Varnish by Pax Dickinson6/23/2012
  2. 2. Our Sponsors✤ Business Insider✤ Varnish Software✤ Your Company Here?
  3. 3. Varnish Resources✤ Varnish Cache Website✤ The Varnish Book Designed as a classroom-led official training manual. It’s new and I haven’t read much of it, probably good tho.
  4. 4. Introduction toCaching With Varnish
  5. 5. What Varnish Is For✤ Scaling to thousands of reqs/s✤ Backend performance✤ Scaling to thousands of reqs/s✤ Fault tolerance, pages serve from cache if backend is down✤ Did I mention scaling? photo by flickr user puuikibeach✤ S-C-A-L-I-N-G
  6. 6. What Varnish Is NOT For✤ Front-end Performance✤ Page Load Speed✤ HTTPS connections✤ Running on 32 bit servers image courtesy Steve Souders
  7. 7. So What Is Varnish? ✤ It’s not a floor polish, or a dessert topping ✤ It’s a *front-end caching reverse proxy* ✤ It sits between your webservers and your clients and caches full web pagesphoto by flickr user roadsidepictures photo by flickr user djwtwo
  8. 8. How Does Varnish Work?
  9. 9. Hits & Misses, Passes & Pipes✤ A hit happens when a request comes in and the hash matches a response in the cache. The response is sent to the client and the backend never knows about it.✤ A miss happens when a request is not present in the cache or is present but expired or banned. The request is sent to the backend and its response is saved in the cache.✤ A pass happens when varnish is configured to bypass certain requests. They are never cached and don’t figure in hit rates.✤ A pipe grants a direct passthrough to the backend. Used for media streams.
  10. 10. The Ban List✤ If you submit a ban to varnish, it will not serve any content matching that ban.✤ It’s possible to use this to actively purge content from the cache when it’s modified by regex matching on URLs or headers✤ The ban list is checked after a response is found but before it’s returned, so content remains in the cache until it’s requested again.
  11. 11. History of Varnish✤ In the beginning there was Squid.✤ But it was not good, for Squid was a forward proxy.✤ One could make it work as a reverse proxy, but there was much gnashing of teeth.✤ In 2006, Varnish 1.0 was released, designed from the start as a reverse proxy.✤ Currently it’s used by 5% of the top 10,0000 websites and climbing.
  12. 12. Varnish Adoption Ratessource:
  13. 13. Basic Configuration✤ Install from yum or apt or source✤ Add backend to default.vcl✤ Start Varnish on port 8080
  14. 14. Basic varnishd Options✤ -f config_filename Specifies the VCL file to use✤ -s storage_spec Tells Varnish where to store the cache and its size✤ -T ip_port Interface and port for the admin interface✤ -a ip_port Interface and port for clients
  15. 15. Specifying Storage✤ Using RAM: ✤ -s malloc,size✤ Using File: ✤ -s file,size,filename✤ Using disk has a slight performance impact but is usable in production. Use RAM if you have enough, otherwise disk it.
  16. 16. Threads & Tuning✤ In production you’ll need to adjust the amount of threads Varnish makes available based on your number of cores.✤ You definitely want to raise the defaults on threads, most other Varnish defaults are sensible.
  17. 17. Operating System Considerations✤ Varnish works a server hard and opens lots of files. So increase the outgoing ports and total number of file descriptors by adding these settings in your /etc/sysctl.conf:✤ Also ulimit -n 1000000 should be run to increase the number of file descriptors available to Varnish if it isn’t being run as root.✤ Further OS tuning may be needed depending on usage but the above is a bare minimum.
  18. 18. Using varnishadm✤ Use varnishadm to connect to a running varnish instance✤ Use this to set params and change VCL on a running Varnish without restarting and emptying the cache.
  19. 19. VCL In Brief✤ Varnish is configured and all caching is performed based on rules written in Varnish Configuration Language.✤ VCL looks like C and compiles into C when Varnish runs it.✤ It has system variables, system functions, and if statements.✤ It has no user defined vars or functions, and no looping structures.✤ Each request proceeds through predefined but configurable subroutines.
  20. 20. VCL Example✤ Tilde is a regular expression operator in VCL.✤ Different VCL subroutines have varying scope to read and alter the request and response HTTP headers.✤ A request’s progress through the various VCL subroutines determines whether it becomes a hit, miss, pass, or pipe.
  21. 21. VCL - Cache Miss Example ✤ Receive request from client ✤ Hash request ✤ Lookup object & either not found or banned ✤ Fetch response from backend ✤ Deliver response to client Image via 90kts on Slideshare
  22. 22. VCL - Cache Hit Example ✤ Receive request from client ✤ Hash request ✤ Lookup object & find it ✤ Deliver response to client Image via 90kts on Slideshare
  23. 23. VCL - Cache Pass Example ✤ Receive request from client ✤ Pass request to backend ✤ Fetch response from backend ✤ Deliver response to client Image via 90kts on Slideshare
  24. 24. VCL - Cache Pipe Example ✤ Receive request from client ✤ Pipe connection to backend Image via 90kts on Slideshare
  25. 25. VCL Configuration Gotchas✤ BY DEFAULT: Varnish will not cache in the presence of cookies!✤ BY DEFAULT: Varnish will not cache in the presence of HTTP auth headers!✤ Best approach: First pass the traffic through, then decide what to cache.
  26. 26. A Quick Tour of Varnishstat✤ Displays running totals of realtime activity in your Varnish install✤ Shows full totals, per second realtime and per second since boot image via Kristian Lyngstol’s blog✤ Stats that are all zeroes are not displayed to save space.
  27. 27. Interesting Varnishstat Numbers✤ Connections accepted / Client requests should be about a 1/10 ratio. If not there may be a keep-alive issue.✤ Backend Conn. Failures should be 0 or very close to 0. If not indicates your backend is timing out or failing.✤ N LRU Nuked Objects is the number of cached objects that Varnish has deleted for lack of space. If this is more than zero, allocate more space.✤ N overflowed work requests should be low and mostly static. Requests shouldn’t need to be queued if you have enough threads.
  28. 28. Trend Analysis With varnishtop✤ Ranks commonly occurring log entries✤ The log is voluminous and fast-moving, varnishtop helps make sense of it✤ -b and -c are used to limit output to backend or client requests.✤ -i and -x l are used to include or exclude by log line type✤ -I and -X are used to include or exclude by regex✤ Varnish 3.0+ has a -m tag:regex syntax that combines -i and -I.
  29. 29. Examples of varnishtop✤ varnishtop -i RxHeader -C -I ^User-Agent Display the most common user agents.✤ varnishtop -i VCL_call Display VCL subroutines called. Useful to see how much you’re passing, which isn’t reflected in hit rate.✤ varnishtop -i RxURL Show which URLs are most commonly incoming from users.✤ varnishtop -i TxURL -X track.gif Display the most common URLs sent to the backend, excluding those matching a regex of URLs we know we can’t cache. This is how you find out what to cache.
  30. 30. Troubleshooting With varnishlog✤ Same parameters as with varnishtop, except this shows log line excerpts as they happen rather than sorted by frequency.✤ Log lines start with a number that IDs the request. The -o parameter lets you see the full request that owns the matched tag or regex. Varnish 3.0 supports the -m option and implies -o unless -O is passed to negate it.✤ varnishlog -i Backend_health is useful to see that your backends are configured properly and varnish sees them.✤ varnishlog -c -o RxURL /some/url displays the full client requests belonging to a specific URL.
  31. 31. Varnish Facts✤ Varnish is for scaling, like beer is for drinking and Windows is for solitaire.✤ Every cache hit you get lowers the load on your webservers and databases, which makes them happy and happy servers have longer MTBFs .✤ Varnish is so fast that a varnish cache hit can turn off the light switch and be in bed before the light goes out.
  32. 32. Til Next Time...✤ Come back next month (date TBD) for the thrilling conclusion of our story: Intro toVarnish II: The VCL Strikes Back
  33. 33. Sources & Links✤ Kristian Lyngstol’s blog✤ 90kts’s slideshare deck, Caching with Varnish✤ Varnish Usage Statistics