tim-thimmaiah

Initially, Stitch only supported real-time updates and addressed this problem with a MapReduce job named The Restorator that performed the following actions:

• Calculated the expected totals
• Queried Cassandra to get the values it had for each counter
• Calculated the increments needed to apply to fix the counters
• Applied the increments

Meanwhile, to stop the sand shifting under its feet, The Restorator needed to coordinate a locking system between itself and the real-time processors, so that the processors did not try to simultaneously apply increments to the same counter, resulting in a race-condition. It used ZooKeeper for this. Zookeeper

Stitch is a wrapper around a Cassandra database. It has a web application that provides read-access to the counts through an HTTP API. The counts are written to Cassandra in two distinct ways, and it's possible to use either or both of them:

• Real-time: For real-time updates, Stitch has a processor application that handles a stream of events coming from a broker and increments the appropriate counts in Cassandra.

• Batch: The batch part is a MapReduce job running on Hadoop that reads event logs, calculates the overall totals, and bulk loads this into Cassandra. Cassandra