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Lecture 9: Ordered Multicasting

Lecture 9: Ordered Multicasting. TVS: Section 6.2 (CDK: Section 12.4). Contents. What is an ordered multicast ? Why is it useful? How can it be done? Using logical timestamps. Multicast. 1 process sends a message to a group of other processes

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Lecture 9: Ordered Multicasting

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  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 COMP28112 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. COMP28112 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 COMP28112 Lecture 9

  5. Fig 6.11 from TvS COMP28112 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. COMP28112 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. COMP28112 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 …! COMP28112 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 COMP28112 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) COMP28112 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 COMP28112 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 COMP28112 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. COMP28112 Lecture 9

  14. Example (1,0,0) (1,1,0) P0 m* m m P1 (1,1,0) delay m* P2 (0,0,0) (1,0,0) (1,1,0) COMP28112 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. COMP28112 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) COMP28112 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. COMP28112 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 COMP28112 Lecture 9

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