Last night, Tathagata Das resolved SPARK-11290, "Implement trackStateByKey for improved state management", which will bring a 7x performance improvement to Spark Streaming when Spark 1.6 is released in December, 2015.
trackStateByKey() offers three benefits over updateStateByKey(), which has served as the workhorse of Spark Streaming since its inception in 2012:
Internally, the performance improvement is achieved by looking at only the key/state pairs for which there is new data. The chart above, which comes from Tathagata's design document, illustrates a typical use case, where 4 million keys are being tracked (for example, 4 million concurrent users on a website or app, or streaming audio or video) but only 10,000 had some activity during the past micro-batch (of, say, two seconds duration). With updateStateByKey(), all 4 million key/state pairs would have had to have been touched due to updateStateByKey()'s internal use of cogroup.
Capability to time out states is built in as a first class option. You no longer have to cobble together your own timeout mechanism. The downside is that if you use this option, you lose the performance improvement mentioned above because the timeout mechanism requires examining every key/state pair.
Ability to return/emit values other than just state as a result of having examined the state. Returning to the example of tracking a web or app user, trackStateByKey() could be maintaining a running logged-in time and emit that together with some metadata for the purposes of populating a dashboard. Not only does one avoid dual-purposing the state per key for two different purposes, but the performance benefit of touching only the modified keys is also realized.