1. Outline Theoretical Foundations - continued Vector clocks - review
Casual ordering of messages
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2. Announcements This is no TA for this class
I will do the grading and everything else by myself
Homework #1 is due this Thursday, Feb. 6, 2003
Turn your homework in hardcopy
For the last problem, attach a hardcopy of your program
Lab 1 will be assigned today
You can work as a team with most two members on each team
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3. Lamport’s Logical Clocks – review
Implementation rules
[IR1] Clock Ci is incremented between any two successive events in process Pi
Ci := Ci + d ( d > 0)
[IR2] If event a is the sending of message m by process Pi, then message m is assigned a timestamp tm = Ci(a). On receiving the same message m by process Pj, Cj is set to
Cj := max(Cj, tm + d)
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4. An Example
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5. Total Ordering Using Lamport’s Clocks
If a is any event at process Pi and b is any event at process Pj, then a => b if and only if either
Where is any arbitrary relation that totally orders the processes to break ties
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6. A Limitation of Lamport’s Clocks - review
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7. Vector Clocks Implementation rules
[IR1] Clock Ci is incremented between any two successive events in process Pi
Ci[i] := Ci[i] + d ( d > 0)
[IR2] If event a is the sending of message m by process Pi, then message m is assigned a timestamp tm = Ci(a). On receiving the same message m by process Pj, Cj is set to
Cj[k] := max(Cj[k], tm[k])
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8. Vector Clocks – cont.
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9. Vector Clocks – cont.
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10. Vector Clocks – cont. Assertion
At any instant,
Events a and b are casually related if ta < tb or tb < ta. Otherwise, these events are concurrent
In a system of vector clocks,
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11. Causal Ordering of Messages
The causal ordering of messages tries to maintain the same causal relationship that holds among “message send” events with the corresponding “message receive” events
In other words, if Send(M1) -> Send(M2), then Receive(M1) -> Receive(M2)
This is different from causal ordering of events
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12. Causal Ordering of Messages – cont.
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13. Causal Ordering of Messages – cont.
The basic idea
It is very simple
Deliver a message only when no causality constraints are violated
Otherwise, the message is not delivered immediately but is buffered until all the preceding messages are delivered
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14. Birman-Schiper-Stephenson Protocol
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15. Schiper-Eggli-Sando Protocol
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16. Schiper-Eggli-Sando Protocol – cont.
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17. Schiper-Eggli-Sando Protocol – cont.
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18. Regarding Lab 1
For this project, you can work as a team with at most two members per team
Each team only needs to turn in one report
However, all the members are required to understand all the parts as you may be tested on the midterm or the final exam
Three processes
Bank Account Server
Bank Office Clients
Bank Office Mutual Exclusion Processes
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