Multicast operations are the operations that are sent from one process to a set of processes and the membership of a group is usually transparent for a sender . However simple multicast protocol does not guarantee any ordering or message delivery. Therefor, stronger assumptions should be made in a frame of the nowadays distributed systems, such as, reliability. Some systems  relies on the reliable multicast, in which any transmitted message is either received by all or none processes. In other words, there could not be a situation where a client accesses a server just before it crashes and observe an update that no other server will process. This property is called uniform agreement. Moreover, to maintain a consistent and fault-tolerant system a total order assumption should be made additionally to reliable uniform multicast.
The simplest specification of the uniform reliable total order multicast can be defined in terms of two primitives , which are TO-multicast(m) and TO-deliver(m), where m is some message. When a process issued a uniquely identified message m as TO-multicast(m), this assumes following properties :
• Validity. If a correct process TO-multicast a message m, then it eventually TO-delivers m.
• Uniform Agreement. If a process TO-delivers a message m, then all correct processes eventually TO-deliver m.
• Uniform Integrity. For any message m, every process TO-delivers m at most once, and only if m was previously TO-broadcast by the sender.
• Uniform Total Order. If two processes,p and q, both TO-deliver message m and m’, then p TO-deliver m before m’, if and only if q TO-delivers m before m’.
If all these properties satisfied then reliable total order multicast takes place. Uniformity in the system is presented as not allowance to deliver a message out of order by any process at any time.
Internally in NOMX, multicast communication is used for most of the subsystems as it is the only fast and reliable way to guarantee consistency and agreement within all nodes with minimal cost.
Although there are three main ways to maintain total order, e.g., symmetric messaging, collective agreement [Birman and Joseph 1987], sequencer based [Kaashoek 1989]. The system that I am developing within my master project uses the single sequencer ordering mechanism as the more efficient in comparison to the consensus one. The simpliest presentation of the total order ordering is illustrated on the picture down. This figure shows that no matter when the messages were issued they will be delivered in the same order to all the processes. For the sequenced mechanisms the main problem is a possible bottleneck and critical point of failure in sequencer part. Moreover, sequencer may limit the scalability of the system. It can be overcomes using the replicated standby sequencer that is delivers all messages issued by the primary one and takes over in case of failure.
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