Unit II message ordering & snapshots message ordering and group communication: Message ordering paradigms

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(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. 
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., 

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