1. Lecture 8 Processes and events Local and global states Time
Causality and concurrent events
Logical clocks
Message delivery to processes
Synchronous systems
Asynchronous systems
Monitoring models

2. Student presentation next week
Up to 5 minute presentations followed by discussions.
All presentations in Power Point
Format:
Title of project/research
Motivation (why is the problem important)
Background (who did what)
Specific objectives (what do you plan to do)
Literature
Each student will provide feedback about each presentation (grades - A, B, C, F- and comments).

3. Processes and Events Event: change in the state of a process.
Local history
Distributed systems: multiple processes
Local events
Communication events
Space-time diagrams

4. Space-time diagrams

5. Local and global states
The global state of distributed computation with n processes is an n-dimensional vector.
Global state (i,,j,k) of a computation with 3 processes means:
p1 has just experienced event i
p2 has just experienced event j
p3 has just experienced event k

7. Time We need to measure time intervals
We need also the concept of global time.
Timestamps

8. Causality Binary cause-effect relationship between events:
Local events: causality can be derived from local history
Communication events: a receive(m) is causally related to the send(m)
Transitivity.
Concurrent events: events in the global history that are not related by causality.

9. Logical clocks LC(e) – the local variable associated with event e.
Each process timestamps a message sent m: TS(m) = LC(send(m))
Rules to update the local clock:
LC(e) = LC + 1  local event or send(m)
LC(e) = max (LC, TS(m)+1) e=receive(m)
Logical clocks do not allow global ordering of events

11. Message delivery to processes
Delivery rules – how channels deliver messages to processes
FIFO delivery
Causal delivery  extension of causal delivery when a process receives messages from multiple sources. Allows a process to reason about the entire system using only local information.

13. Violation of causal delivery

14. Synchronous systems Delay is bounded. Consensus is possible Examples
Collision-free multiple access systems
Token rings
Multiple access systems based upon collision resolution protocols
FCFS algorithm of Gallagher
Stack algorithm

18. Asynchronous systems
No upper bounds on processing or communication delays.
Any algorithm for an asynchronous system can be used for a synchronous one but the opposite is not true.
RTT – round trip time for TCP.

19. Monitoring models Monitor
Run  total ordering of all events in the global history consistent with the local history of each process.
Cut. The frontier of a cut.
Consistent cut

20. Consistent and inconsistent cuts