Communication - Distributed Operating Systems - Lecture Slides, Slides of Operating Systems

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Communication

Message-Oriented Communication

Stream-Oriented Communication

Review

• In a distributed system, processes
  • run on different machines
  • exchange information through message passing
• Successful distributed systems depend on communication models that hide or simplify message passing

Types of Communication

• Persistent versus transient
• Synchronous versus asynchronous
• Discrete versus streaming

Persistent versus Transient Communication

• Persistent : messages are held by the middleware

comm. service until they can be delivered (e.g.,

email)

• Sender can terminate after executing send
• Receiver will get message next time it runs
• Transient : messages exist only while the sender

and receiver are running

• Communication errors or inactive receiver cause the

message to be discarded

• Transport-level communication is transient
• RPC?

Evaluation

• Fully synchronous primitives may slow processes down, but program behavior is easier to understand
• In multithreaded processes, blocking is not as big a problem because a special thread can be created to wait for messages

Discrete versus Streaming Communication

• Discrete : communicating parties exchange discrete messages
• Streaming : one-way communication; a “session” consists of multiple messages from the sender that are related either by send order (TCP streams), temporal proximity (multimedia streams), etc.

Message Passing Interface (MPI)

• Designed for parallel applications using transient communication
• MPI is
  • a standardized and portable message-passing system designed by a group of researchers from academia and industry
  • used in many environments, e.g., clusters
  • platform independent

Message Primitives

• Asynchronous: e.g. MPI_bsend
• Synchronous: e.g. MPI_send, MPI_ssend, MPI_sendrecv:

Message-Oriented Middleware (MOM) - Persistent

• Processes communicate through message queues
  • Queues are maintained by the message-queuing system
  • Sender appends to queue, receiver removes from queue
  • Neither the sender nor receiver needs to be on-line

when the message is transmitted

4.4 Stream-Oriented Communication

• RPC and message-oriented communication are based on the exchange of discrete messages - Timing might affect performance, but not correctness
• In stream-oriented communication the message content (multimedia streams) must be delivered at a certain rate, as well as correctly - e.g., music or video

Data Streams

• Data stream = sequence of data items
• Can apply to discrete, as well as continuous media
  • e.g. UNIX pipes or TCP/IP connections which are both byte oriented (discrete) streams
  • Messages are related by send order
• Audio and video require continuous time-based data streams

Data Streams

• Asynchronous transmission mode: the order is

important, and data is transmitted one after the

other, no restriction to when data is to be delivered

• Synchronous transmission mode defines a

maximum end-to-end delay for individual data

packets

• Isochronous transmission mode has a maximum

and minimum end-to-end delay requirement (jitter

is bounded)

• Not too slow, but not too fast either