Good Presentation on how to scale ROR web apps
How to scale your web app
1. How to scale  (with ruby on rails) George Palmer [email_address] 3dogsbark.com 2. Overview One server  Two servers Scaling the database Scaling the web server User clusters Final architecture Caching Cached architecture Links Questions 3. How you start  out Shared Hosting One web server and DB on same machine Application designed for one machine Volume of traffic will depend on host DB Web Server Shared Hosting 4. Two servers Possibly  still shared hosting Web server and DB on different machine Minimal changes to code Volume of traffic will depend on whether made it to dedicated machines DB Web Server 5. Scaling the database  (1) DB setup more suited to read intensive applications (MySQL replication) Should be on dedicated hosts Minimal changes to code Master DB Web Server Slave Slave Slave 6. Scaling the database  (2) DB setup more suited to equal read/write applications (MySQL cluster) Should be on dedicated hosts Minimal changes to code Master DB Web Server Master DB MySQL Cluster 7. Scaling the web  server Web Server comprises of “Worker threads” that process work as it comes in DB Farm Worker thread Worker thread Worker thread Worker thread Web Server 8. Load balancing App  Server depends: Rails (Mongrel, FastCGI) PHP J2EE Some changes to code will be required DB Farm App Server App Server App Server Load balancer 9. The story so  far… App servers continue to scale but the database side is somewhat limited… App Server App Server App Server Load balancer Master DB Slave Slave Slave 10. User Clusters For  each user registered on the service add a entry to a master database detailing where their user data is stored UserID DB Cluster Basic authorisation details such as username, password, any NLS settings 11. User Clusters (2)  App Server Master DB User  Cluster 1 User Cluster 2 User clusters are themselves one of the two database setups outlined earlier SELECT * FROM users WHERE  username=‘Bob’ AND … user_id=91732db_cluster=2 12. User Clusters (3)  ID management becomes an issue Best to use master DB id as user_id in user cluster If let cluster allocate then make sure use offset and increment (not auto_increment) Other DBs such as session must reference a user by id and DB cluster Serious code changes may be required Will want to have ability to move use users between clusters 13. The final architecture  As number of app servers grow it’s a good idea to add a database connection manager (eg SQLRelay) Extract out session, search, translation databases onto own machines Use MySQL cluster (or equivalent) for any critical database In replication setup can make a slave a backup master Add a NFS/SAN for static files 14. The final architecture  (2) Load balancer Master DB App Server 1 App Server 2 App Server 50 … DB Connection Manager Master DB Session DB Search DB NLS DB Master Slave Slave Slave Master Slave Slave Slave User Cluster 2 User Cluster 1 NFS/SAN 15. Issues Load balancer  and database connection manager are single point of failure Easy solved 2PC needed for some operations.  For example a user wants to be removed from search database 2PC not supported in rails Rails doesn’t support database switching for a given model Can do explicitly on each request but expensive due to connection establishment overhead Can get round if using connection manager but a proper solution is required (I may write a gem to do this) 16. Making the most  of your assets In a lot of web applications a huge % of the hits are read only.  Hence the need for caching: Squid A reverse-proxy (or webserver accelerator) Memcached Distributed memory caching solution 17. Squid Lookup of  pages is in memory, storing of files is on disk Can act also act as a load balancer Pages can be expired by sending DELETE request to proxy Squid App Server 1 App Server 2 NFS/SAN In cache Not in cache … 18. Memcached Location of  data is irrespective of physical machine A really nice simple API SET GET DELETE In rails only a fews LOC will make a model cached Also useful for tracking cross machine information – eg dodge user behaviour App Server DB Farm Memcached Physical  Machine App Server Memcached Physical  Machine (Not in memcached) 19. Cached Architecture Introduce  Squid Acts as load balancer (note there are higher performing load balancers) Introduce memcached Can go on every machine that has spare memory Best suited to application servers which have high CPU usage but low memory requirements 20. Cached architecture Squid  Master DB App Server 1 App Server 2 App Server 50 … DB Connection Manager Master DB Session DB Search DB NLS DB Master Slave Slave Slave Master Slave Slave Slave User Cluster 2 User Cluster 1 NFS/SAN M C M C M C MC=memcached 21. Cached architecture Wikipedia  quote a cache hit rate of 78% for squid and 7% for memcached So only 15% of hits actually get to the DB!! Performance is a whole new ball game but we recently gained 15-20% by optimising our rails configuration But don’t get carried away - at some point the time you spend exceeds the money saved 22. Cached architecture –  1 machine Squid Master DB App Server 1 App Server 2 App Server 5 … DB Connection Manager Master DB Session DB Search DB NLS DB Master Slave Slave Slave User Cluster 1 NFS/SAN Memcached Physical Machine 23. How far can  it go? For a truly global application, with millions of users - In order of ease: Have a cache on each continent Make user clusters based on user location Distribute the clusters physically around the world Introduce app servers on each continent If you must replicate your site globally then use transaction replication software, eg GoldenGate 24. Useful Links http://www.squid-cache.org/  http://www.danga.com/memcached/ http://sqlrelay.sourceforge.net/ http://railsexpress.de/blog/ 25. Questions?  