Reliable Communication Smita Hiremath CSC 8530

Reliable Client-Server Communication Point-to-Point communication Established by TCP Masks omission failure, crash failure not masked

Cont… RPC Semantic in the presence of Failure 1. The client is unable to locate the server 2. The request message from the client to the server is lost 3. The server crashes after receiving a request 4. The reply message from the server to the client is lost 5. The client crashes after sending the request.

Cont… Client Cannot Locate the Server Server might be down Server evolves and new version of interface is installed Lost Request Messages Easiest one to deal Client stub start the timer when sending the request.

Cont… Server Crashes A server in client-server communication a) Normal case b) Crash after execution c) Crash before execution

Cont… At least once semantics – Guarantees that the RPC has been carried out at least once, but possibly more. At most once semantics – Guarantees that the RPC has been carried out at most once, but possibly none at all Guarantees nothing. RPC may have been carried out anywhere from zero to a large number of times

Cont… Lost Reply Message Some requests can be repeated (idempotent) Assign each request a sequence number Client Crashes Computations are orphans Extermination – the log is kept on the disk or some other medium that survives crash. Reincarnation – divide the time up into sequential numbered epochs.

Cont… Gentle reincarnation – computations are killed if the owner cannot be found. Expiration – RPC is given standard amount of time.

Reliable Group Communication Basic Reliable-Multicasting Message sent to a process group should be delivered to each member of that group. Agreement exists on who is the a member of the group.

Reliable-Multicasting scheme A simple solution to reliable multicasting when all receivers are known and are assumed not to fail a) Message transmission b) Reporting feedback

Scalability in Reliable Multicasting Two different approaches Nonhierarchical Feedback Control Hierarchical Feedback Control

Nonhierarchical Feedback Control Scalable Reliable Multicasting (SRM) Never acknowledges successful delivery of multicast messages. If the receiver missed a message, it multicasts its feedback to the rest of the group, which allows another group member to suppress its own feedback. Only a single feedback message will reach the sender, which in turn subsequently retransmits the message.

Nonhierarchical Feedback Control Several receivers have scheduled a request for retransmission, but the first retransmission request leads to the suppression of others.

Scale reasonably well Problems with this approach Ensuring that only one request for retransmission is returned to the sender requires accurate scheduling of feedback messages at each receiver. Feedback also interrupts those processes to which the message has been successfully delivered. Nonhierarchical Feedback Control

Hierarchical Feedback Control Scalability for very large groups of receivers. The group of receivers are partitioned into number of subgroups, which are organized into trees. Each subgroup appoints a local coordinator, which is responsible for handling retransmission Local coordinator will have its own history buffer.

The essence of hierarchical reliable multicasting. a) Each local coordinator forwards the message to its children. b) A local coordinator handles retransmission requests. Hierarchical Feedback Control

Atomic Multicast Atomic multicast problem – message must be delivered to either all processes or none at all.

Virtual Synchrony The logical organization of a distributed system to distinguish between message receipt and message delivery

Cont… The principle of virtual synchronous multicast.

Message Ordering Four different orderings: 1. Unordered multicasts 2. FIFO-ordered multicasts 3. Causally-ordered multicasts 4. Totally-ordered multicasts

Cont… Process P1Process P2Process P3 sends m1receives m1receives m2 sends m2receives m2receives m1 Three communicating processes in the same group. The ordering of events per process is shown along the vertical axis.

Cont… Process P1Process P2Process P3Process P4 sends m1receives m1receives m3sends m3 sends m2receives m3receives m1sends m4 receives m2 receives m4 Four processes in the same group with two different senders, and a possible delivery order of messages under FIFO- ordered multicasting

Cont… Multicast Basic Message Ordering Total-ordered Delivery? Reliable multicastNoneNo FIFO multicast FIFO-ordered delivery No Causal multicast Causal-ordered delivery No Atomic multicastNoneYes FIFO atomic multicast FIFO-ordered delivery Yes Causal atomic multicast Causal-ordered delivery Yes Six different versions of virtually synchronous reliable multicasting.

Implementing Virtual Synchrony a) Process 4 notices that process 7 has crashed, sends a view change b) Process 6 sends out all its unstable messages, followed by a flush message c) Process 6 installs the new view when it has received a flush message from everyone else