1. Lecture 9: Ordered Multicasting
TVS: Section 6.2
(CDK: Section 12.4)

2. Contents What is an ordered multicast? Why is it useful?
How can it be done?
Using logical timestamps
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COMP Lecture 9

3. Multicast 1 process sends a message to a group of other processes
More efficient than looping in the application sending a message to each
May be able to guarantee that either all recipients get the message or none does.
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COMP Lecture 9

4. Example: duplicate bank records
For speed of response to queries, and improved reliability, suppose a bank keeps its database replicated in geographically distant locations (e.g. London & Edinburgh)
Each update request is sent to both (as a multicast)
But this leads to an ordering constraint
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COMP Lecture 9

5. Fig 6.11 from TvS
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COMP Lecture 9

6. The Updates
Suppose Update 1 credits £100 to an account containing £100
Suppose Update 2 adds 1% interest to the account.
If not done in the same order, can get balances of £201 and £202 in the two databases.
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COMP Lecture 9

7. Totally-Ordered multicast
To get the same answers, we need totally-ordered multicast, i.e. all messages are delivered in the same order to each receiver.
Can be done with Lamport’s logical clocks.
Consider a group of processes multicasting to each other.
Each message is timestamped with the logical time of the sender.
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COMP Lecture 9

8. T-O Multicast (2) Assume multicast also goes to the sender
Assume messages from the same sender are received by any 1 receiver in the order they were sent
Assume that no messages are lost
When message is received, put it in a local queue ordered by its timestamp.
The receiver multicasts an ACK to all processes. (Note ACK is not queued!)
A process can “act on” a queued message when it is at the head of the queue, and has been ACK’d by every other process.
Each process has the same contents in the queue …!
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COMP Lecture 9

9. A will have to process messages in a different order
(message delivery can be anything as long as it is the
same for all processes) A B C
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COMP Lecture 9

10. Implications
Note: the need to receive ACKs from all processes reduces advantage of replication! A crashed process will cause everything to stop ….
Keeping replicas consistent by executing same operations in the same order is a general technique (state machine replication)
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COMP Lecture 9

11. Causally-Ordered Multicasting
Weaker than totally-ordered because unrelated messages can be processed in different orders on different machines.
Useful on, e.g., a bulletin board – may want messages delivered in the correct order where they are related, but not to tie down ordering between messages on unrelated topics
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COMP Lecture 9

12. C-O multicasting
Now we use Vector clocks – but only noticing sends as events ….
Suppose Pj receives message m from Pi with timestamp ts(m)
Delay delivery to application until:
ts(m)[i] = VCj[i] + 1
ts(m)[k] <= VCj[k] for all other k
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COMP Lecture 9

13. C-O multicasting (2)
The 1st condition means that this is the next message Pj was expecting from Pi
The 2nd means that Pj has seen all the messages seen by Pi when it sent message m.
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COMP Lecture 9

14. Example (1,0,0) (1,1,0) P0 m* m m P1 (1,1,0) delay m* P2 (0,0,0)
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COMP Lecture 9

15. Explanation m* from P1 arrives at P2 before m from P0
But m* was sent after m arrived at P1
Therefore delivery of m* in P2 is delayed until after m has been delivered.
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COMP Lecture 9

16. End-to-end argument
Some systems have provided T-O and C-O multicasting (e.g. ISIS)
Some debate about whether this is a Good Thing or not
Two main problems:
Not all causality is real (e.g. 1 sender may send unrelated messages, but the system will believe the order matters)
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COMP Lecture 9

17. E-to-E argument (cont.)
Not all causality may be captured. E.g. users of the bulletin board might discuss issues offline and then post related messages without reading logically preceding ones first.
A particular application will have its own approach, but we may or may not want to build it on top of an infrastructure which assumes the answers.
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COMP Lecture 9

18. Exercise a b d c f g i k m j l p n o q r s
Use (i) Lamport Clocks; (ii) Vector Clocks to order the events
27-Apr-19
COMP Lecture 9