Group Communication. Group oriented activities are steadily increasing. There are many types of groups: Open and Closed groups Peer-to-peer and hierarchical groups. We focus on how the members of a group communicate. Major issues. Different types of multicast Atomic multicast
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Group oriented activities are steadily increasing.
There are many types of groups:
Open and Closed groups
Peer-to-peer and hierarchical groups
We focus on how the members of a group communicate
A multicast is called atomic, when the message is delivered to every correct member, or to no member at all.
How can we implement atomic multicast?
Basic multicast does not consider crash failures.
Reliable multicast does.
Three criteria for basic multicast:
Liveness. Each process must receive every message
Integrity. No spurious message received
No duplicate. Accepts exactly one copy of a message
Sender’s programReceiver’s program
i:=0; ifm is new
do i ≠ n accept it;
send message to i; multicast m;
i:= i+1 m is duplicate discard m
Tolerates process crashes.
Sometimes, certain features available in the infrastructure of a distributed system simplify the implementation of multicast. Examples are
Although multicasts can be implemented using point-to-point communications, there are some practical forms of multicasts that make use of the inherent multicasting ability of the underlying medium. IP multicast is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to multiple clients.
Applications that take advantage of multicast include distance learning, videoconferencing, and distribution of software, stock quotes, and news. The source sends only one copy, which is replicated by the routers.
Routers maintain and update distribution trees when members join / leave a group.
Concern: Too much load on routers. Application layer multicast overcomes this.Distribution trees
(a.k.a. Single source FIFO)
Total order multicast is useful in the consistent update of replicated servers
Causal order multicast is relevant in implementing bulletin boards
Local order multicast is useful in updating cache memories in multi-computersOrdered multicasts
Distributed implementation without a sequencer
Uses the idea of 2PC
Why does it work?
Tolerates process crashes. The additional requirements are as follows:
Only correct processes are required to receive the messages from all correct processes in the group. Multicasts by faulty processes will either be received by every correct process, or by none at all.
In an asynchronous distributed system, total order reliable multicastscannot be implemented when even a single process undergoes a crash failure.
Why? Since it will violate the FLP impossibility result.
IP multicast or application layer multicast has to detect the loss of messages and use retransmission for achieving reliability. For large groups (like distance learning applications) scalability is a major problem.