True, and it isn't the full story of system dependencies. Data dependencies aren't really shown here and they are also tricky, although Cassandra is the the primary ("source of truth") for most things. So for example, if a user does something on the site such as Digg a story, you mainly write Cassandra (several times) but in some cases have to write Redis, as well as flush some caches. Meanwhile async jobs can be writing to HDFS and ultimately MySQL.