IS 651: Distributed Systems HW3 Sisi Duan Assistant Professor Information Systems sduan@umbc.edu

HW3 Q1. Why E1->E10 and E8||E9? Definition (->): We define e->e’ if… Logical ordering: e->e’ if e->e’ for any process I Messages: send (m) -> receive (m) for any message m Transitivity: e->e’‘ if e->e’ and e’->e’’

HW3 Three rules: At process i, increment Li before each event To send a message m at process i, apply rule 1 and then include the current local time in the message: i.e., send(m,Li) To receive a message (m,t) at process j, set Lj = max(Lj,t) and then apply rule 1 before time-stamping the receive event Alice sends sync request to Bob Bob receives sync request from Alice Alice sends sync request to Carol Bob sends sync acknowledgment to Alice Bob sends sync request to Carol Carol receives sync request from Bob Alice receives sync acknowledgment from Bob Carol sends sync acknowledgment to Bob Carol receives sync request from Alice Bob receives sync acknowledgment from Carol Carol sends sync acknowledgment to Alice Alice receives sync acknowledgment from Carol Alice combines the results and sends the final report to Bob Alice sends final report to Carol Carol receives final report from Alice Bob receives final report from Alice

HW3 Q3.B. Sequential consistency does not admit a scalable implementation because it requires a total ordering of all operations in the system.  Q3.E. Sequential consistency is sufficient to guarantee that an airline never overbooks its seats when multiple clients reserve seats on the same airplane.

HW3 Q3.C. Obeying all causal orderings is sufficient to guarantee convergence of state maintained by multiple replicas.  Q3.D. Causal consistency is sufficient to guarantee that an airline never overbooks its seats when multiple clients reserve seats on the same airplane.  Q3.A. Sequential consistency guarantees causal ordering.  In shared memory scenario, I think it’s true. But this is not always true.

Causal order vs Total order in the broadcast scenario [Lamport, Comm. ACM 1978] [Lamport, Distrib. Comput. 1986] Causal order If the broadcast of message m1 "happens before" or "causally precedes" the broadcast of message m2, then no correct process delivers m2 before it delivers m1. m1 “Chase charges 10%!” m2 “That is outrageous!”

Causal order vs Total order in the broadcast scenario ✓ Causal order Chase: “Charge 10%” Outrageous! $100 $90 Chase: “Charge 10%” Outrageous! $100 $90 $100

Causal order vs Total order in the broadcast scenario ✘ Causal order Chase: “Charge 10%” Outrageous! $100 $100 $90 Chase: “Charge 10%” Outrageous! $100 $90 $90 $100

Causal order vs Total order in the broadcast scenario ✓ ✘ Causal order Total order Chase: “Charge 10%” Client 1: “Deposit $100” Outrageous! $100 $90 $190 Client 1: “Deposit $100” Chase: “Charge 10%” Outrageous! $100 $200 $180 $100

Causal order vs Total order in the broadcast scenario ✘ ✓ Causal order Total order Chase: “Charge 10%” Outrageous! $100 $100 $90 Chase: “Charge 10%” Outrageous! $100 $100 $90 $100

Causal order vs Total order in the broadcast scenario ✓ ✓ Causal order Total order Chase: “Charge 10%” Client 1: “Deposit $100” Outrageous! $100 $90 $190 Chase: “Charge 10%” Client 1: “Deposit $100” Outrageous! $100 $90 $190 $100