S4: Distributed Stream Computing Platform from Yahoo!

S4 is a general-purpose, distributed, scalable, partially fault-tolerant, pluggable platform that allows programmers to easily develop applications for processing continuous unbounded streams of data. S4 was initially developed to personalize search advertising products at Yahoo!, which operate at a rate of thousands of events per second. MapReduce excels at batch jobs, but is hard to apply to stream computation tasks.