Another example of distributed snapshot: Communicating State Machines

1. Another example of distributed snapshot: Communicating State Machines

2. Something unusual Let machine i start Chandy-lamport snapshot before it has sent M along ch1. Also, let machine j receive the marker after it sends out M’ along ch2. Observe that the snapshot state is down  up M’ Doesn’t this appear strange? This state was never reached during the computation!

3. Understanding snapshot

4. Understanding snapshot The observed state is a feasible state that is reachable from the initial configuration. It may not actually be visited during a specific execution. The final state of the original computation is always reachable from the observed state.

5. Discussions What good is a snapshot if that state has never been visited by the system? - It is relevant for the detection of stable predicates. - Useful for checkpointing.

6. Discussions What if the channels are not FIFO? Study how Lai-Yang algorithm works. It does not use any marker LY1. The initiator records its own state. When it needs to send a message m to another process, it sends a message (m, red). LY2. When a process receives a message (m, red), it records its state if it has not already done so, and then accepts the message m. Question 1. Why will it work? Question 1 Are there any limitations of this approach?

7. Questions Distributed snapshot = distributed read. Distributed reset = distributed write How difficult is distributed reset?

8. Global state collection Some applications - computing network topology - termination detection - deadlock detection Chandy Lamport algorithm does a partial job. Each process collects a fragment of the global state, but these pieces have to be stitched together to form a global state.

9. Once the pieces of a consistent global state become available, consider collecting the global state via all-to-all broadcast At the end, each process will compute a set V, where V= {s(i): 0 ≤ i ≤ N-1 } A simple exercise s(i) s(j) i j s(k) s(l) k l

10. Program broadcast (for process i} define V.i, W.i : set of values; initially V.i={s(i)}, W.i =  andevery channel is empty do V.i ≠ W.i send (V.i \ W.i) to every outgoing channel; W.i := V.i  ¬ empty (k, i) receive X from channel(k, i); V.i := V.i  X od All-to-all broadcast Assume that the topology is strongly connected graph V.i W.i V.k W.k (i,k) Acts like a “pump”

11. Lemma. empty (i. k) W.i V.k. (Upon termination) i: V.i = W.i, and all channels are empty. So, V.i  V.k. On a cyclic path, V.i = V.k must be true. Since s(i)V.i, s(i)V.k Proof V.i W.i V.k W.k (i,k)