Advanced Operating System COMMUNICATION Shrada Pradhan Advanced Operating System Csc CSC 8320

Content Flow Basic Reliable-Multicasting Schemes Scalability in Reliable Multicasting Nonhierarchical Feedback Control Hierarchical Feedback Control Atomic Multicast Virtual synchronous multicast

Introduction Services guarantee that messages are delivered to all members in a process group Reliable multicast services are important But reliable multicasting is not that easy.

Basic Reliable-Multicasting Schemes A reliable communication in the presence of faulty process If all the non-faulty group receive message. A reliable communication when processes are assumed to operate correctly Neither processes fail, nor process join or leave the group during transmission. All the group memebers should receive the messages.

Contd.. Weaker multicast Unreliable multicast Condition: The numbers of receivers are limited. Unreliable multicast Messages sent may be lost or not delivered to all the receivers.

Contd.. Solution for Unreliable Multicast Assigning a sequence member to each message Detect missing message Send negative acknowledgement, requesting for retransmission Sender retransmit the message.

Scalability in Reliable Multicasting Problem Cannot handle large number of receivers Senders may be swapped with feedback messages Solution Receiver sending acknowledgement only for missing messages. But overflows history buffer. Removing messages after certain time.

Scalable multicast Non hierarchial feedback control (NHFC) To reduce the number of feedback messages sent. Feedback supression model is used. Receiver sends negative acknowledgement as feedback. Receiver multicasts its feedback to the rest of the group. Schedules feedback messages with random time delay.

Scalable Multicast Hierarchial Feedback Control Large group of receivers Each group is partitioned into smaller subgroups in a tree Each subgroup has a local coordinator responsible for handling retransmission requests. Local coordinaters have own buffer history

Hierarchaial Feedback control

Contd.. Asks its parent subgroup if coordinator misses message. If message and acknowledgement received, clears its history buffer.

Limitation NHFC: HFC: Reasonable accurate scheduling is not easy. Other receivers are forced to receive useless messages. HFC: Construction of tree is not easy.

Automatic Multicast Message is either delivered to all or not essayed at all. Reliable multicast with process failures. Believes in group membership for correctly operating replicas.

Contd. If one process breaks then it leaves the group After repair can join the group And then update with the state of the rest of the members. This maintains non-faulty process maintains consistent view Forces reconciliation when replica recovers and rejoins the group

Atomatic Multicast

Virtual synchronous Reliable multicast property If sender of the message crashes, it is either delivered to remaining process or ignored.

Different Ordering Unordered Multicast FIFO Multicast No guarantee concerning order of delivery of messages FIFO Multicast Should deliver incoming messages from the same process in the same order For eg. M1 is always delivered before M2

Contd. Casually Ordered Multicast Totally Ordered Multicast Preserves potential casuality between messages Even though m1 preceeds m2, communication layer delivers m2 after m1. Totally Ordered Multicast Messages should be delivered in the same ordered to all group members.

Contd. Casually Ordered Multicast Totally Ordered Multicast Preserves potential casuality between messages Even though m1 preceeds m2, communication layer delivers m2 after m1. Totally Ordered Multicast Messages should be delivered in the same ordered to all group members.

Latest Relevant knowledge Optimistic Atomic Multicast[2] Is expensive in terms of message delays a protocol that combines reduced latency increased throughput Messages are delivered optimistically in a single communication step. Does not rely on spontaneous message ordering for fast delivery

Future Work Atomic multicasts if not occurred between database nodes Instead transaction ordering is exposed as a service that is provided by a dedicated set of nodes. Improves transaction throughput and latency [3] So studies can be done in such fields to enhace performance.

References 1. Andrew S. Tanenbaum , and Maarten Van Steen. Distributed Systems Principles and paradigms. Second Edition, 2007 2. Optimistic Atomic Multicast, Distributed Computing Systems (ICDCS), 2013 IEEE 33rd International Conference on 8-11 July 2013. 3. An Atomic-Multicast Service for Scalable In-Memory Transaction Systems, Cloud Computing Technology and Science (CloudCom), 2014 IEEE 6th International Conference on Date 15-18 Dec. 2014