This builds on the tail end of lecture 5 (Jim and Sally's ill-fated dating experience).  Continuing to work in our model, this lecture starts by exploring the concept of time in a distributed system: A happens before B, or at the same time as B.  A could be Jim emailing to Sally that they should eat outside, B could be Sally reading an email, etc.

We develop Lamport's ® relation, then show how logical clocks and vector clocks can represent this either in part or exactly.  We'll need them as tools later in the course.

Some discussion of clock synchronization and the concept of an "instant" in time.

Make sure the students realize that our big plan is to use these ideas in practical solutions applicable in data centers and other big systems.  We need to take a step at a time but we'll apply these concepts as soon as we can.