Unit II message ordering & snapshots message ordering and group communication: Message ordering paradigms
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- Điều hướng trang này:
- 2.5 Group communication
- CS8603:Distributed Systems Department of CSE
| (4) Message M arrival at Pi from a higher priority process Pj:
At the time a message M is processed by Pi, process Pi executes RECEIVE(M) (which is assumed to be always enabled) and then send(ack(M)) to Pj . (5) Processing when Pi is unblocked: When Pi is unblocked, it dequeues the next (if any) message from the queue and processes it as a message arrival (as per rules 3 or 4). 2.5 Group communication • Processes across a distributed system cooperate to solve a task. Hence there is need for group communication. • A message broadcast is sending a message to all members. • In Multicasting the message is sent to a certain subset, identified as a group. • In unicasting is the point-to-point message communication. • Broadcast and multicast is supported by the network protocol stack using variants of the spanning tree. This is an efficient mechanism for distributing information. CS8603:Distributed Systems Department of CSE 2020 – 2021 2. 10 Jeppiaar Institute of Technology • However, the hardware or network layer protocol assisted multicast cannot efficiently provide the following features: • Application-specific ordering semantics on the order of delivery of messages. • Adapting groups to dynamically changing membership. • Sending multicasts to an arbitrary set of processes at each send event. • Providing various fault-tolerance semantics. • If a multicast algorithm requires the sender to be a part of the destination group, the multicast algorithm is said to be a closed group algorithm. • If the sender of the multicast can be outside the destination group, then the multicast algorithm is said to be an open group algorithm. • Open group algorithms are more general, and therefore more difficult to design and more expensive to implement, than closed group algorithms. • Closed group algorithms cannot be used in in a large system like on-line reservation or Internet banking systems where client processes are short-lived and in large numbers. • For multicast algorithms, the number of groups may be potentially exponential, i.e., O(2 n ). tải về 0.79 Mb. Chia sẻ với bạn bè của bạn:
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