4sq Outrage caused by RAM (64G on EC2) max out (not CPU bounded)
badge calculation based on entire check-in history
Solid state hard drive, significant improvement on matching with RAM
Hadoop for analytics, separate data collection branch on logs
customized MongoDB build with C++ code for location search given prefix
sharding from day 1 with MongoDB, (not using replica set yet)
validating objects in app, correcting if possible
no map reduce on MongoDB yet
original 4sq code is PHP with ... SQL
"10% on open source everything"
"XML is wrong"
"if using Java, not with Scala ... , is wrong"