Distributed (Operating) Systems -Message-oriented Communication- -Stream-oriented Communication-

1. Distributed (Operating) Systems -Message-oriented Communication- -Stream-oriented Communication- Fall 2011 Kocaeli University Computer Engineering Department

2. Communication Issues • Message-oriented communication • Transient vs Persistent • Synchronous vs asynchronous • Stream-oriented communication

3. Message-oriented Transient Communication • Many distributed systems built on top of simple message-oriented model • Example: Berkeley sockets

4. Berkeley Socket Primitives

5. Message-Passing Interface (MPI) • Sockets designed for network communication (e.g., TCP/IP) • Support simple send/receive primitives • Abstraction not suitable for other protocols in clusters of workstations or massively parallel systems • Need an interface with more advanced primitives • Large number of incompatible proprietary libraries and protocols • Need for a standard interface • Message-passing interface (MPI) • Hardware independent • Designed for parallel applications (uses transient communication)

6. MPI • Key idea: communication between groups of processes • Each endpoint is a (groupID, processID) pair • MPI are intent to be used in clusters • It is basically used for scientific applications and parallel systems • It overcomes some of the TCP communications • MPI basically run on UDP • Socket based communication is also transient but it is based on synch communication • but here MPI also support asynchronous communication

7. MPI Primitives

8. Message-oriented Persistent Communication • Message queuing systems • Support asynchronous persistent communication • Intermediate storage for message while sender/receiver are inactive • Example application: email • Communicate by inserting messages in queues • Sender is only guaranteed that message will be eventually inserted in recipient’s queue • No guarantees on when or if the message will be read • “Loosely coupled communication”

9. Message-Queuing Model (1)

10. Message-Queuing Model

11. General Architecture of a Message-Queuing System (2) • Queue manager and relays • Relays use an overlay network • Relays know about the network topology and how to route

12. Message Brokers • Message broker: application level gateway in MQS • Convert incoming messages so that they can be understood by destination (format conversion) • Also used for pub-sub systems

13. IBMʼsWebSphere MQ • Queue managers manage queues • Connected through message channels • Message channel agent (MCA) • Checks queue, wraps into TCP packet, send to receiving MCA

14. Stream Oriented Communication • Message-oriented communication: request-response • When communication occurs and speed do not affect correctness • Timing is crucial in certain forms of communication • Examples: audio and video (“continuous media”) • 30 frames/s video => receive and display a frame every 30ms • Characteristics • Isochronous communication • Data transfers have a maximum bound on end-end delay and jitter • Push mode: no explicit requests for individual data units beyond the first “play” request

15. To Push or Pull ? • Client-pull architecture • Clients pull data from servers (by sending requests) • Example: HTTP • Pro: stateless servers, failures are easy to handle • Con: limited scalability • Server-push architecture • Servers push data to client • Example: video streaming, stock tickers • Pro: more scalable, Con: stateful servers, less resilient to failure • When/how-often to push or pull?

16. Examples

17. Streams and Quality of Service • Properties for Quality of Service: • The required bit rate at which data should be transported. • The maximum delay until a session has been set up • The maximum end-to-end delay . • The maximum delay variance, or jitter. • The maximum round-trip delay.

18. Quality of Service (QoS) • Time-dependent and other requirements are specified as quality of service (QoS) • Requirements/desired guarantees from the underlying systems • Application specifies workload and requests a certain service quality • Contract between the application and the system

19. Specifying QoS: Token bucket • The principle of a token bucket algorithm • Parameters (rate r, burst b) • Rate is the average rate, burst is the maximum number of packets that can arrive simultaneously

20. Enforcing QoS (1) • Observation: There are various network-level tools, such as differentiated services by which certain packets can be prioritized. • Also: use buffers to reduce jitter

21. Enforcing QoS (2) • Problem: How to reduce the effects of packet loss (when multiple samples are in a single packet)? • Solution: simply spread the samples:

22. Stream synchronization • Multiple streams: • Audio and video; layered video • Need to sync prior to playback • Timestamp each stream and sync up data units prior to playback • Sender or receiver? • App does low-level sync • 30 fps: image every 33ms, lip-sync with audio • Use middleware and specify playback rates

23. Synchronization Mechanism • Problem: Given a complex stream, how do you keep the different substreams in synch? • Example: Think of playing out two channels, that together form stereo sound. Difference should be less than 20–30 μsec! • Alternative: multiplex all substreams into a single stream, and demultiplex at the receiver. Synchronization is handled at multiplexing/demultiplexing point (MPEG).

24. Multicasting • Multicast: sending data to multiple receivers • Network- and transport-layer protocols for multicast bogged down at the issue of setting up the communication paths to all receivers. • Group communication • IP multicast versus application-level multicast • Construct an overlay multicast tree rooted at the sender • Send packet down each link in the tree • Issues: tree construction, dynamic joins and leaves

25. Application-Level Multicasting • IP multicasting works in IP layer. • Why not just use that? • Often difficult to configure, requires too much administrative support, agreement, etc. • Application-level seeks to address those issues. • Peer-to-peer communication using structured overlays can use application-layer protocols to support multicast • The overlay network is used to disseminate information to members

26. Application-Level Multicasting • Essence: Organize nodes of a distributed system into an overlay networkand use that network to disseminate data. • Two possible structures: • Tree: unique path between every pair of nodes • Mesh: multiple neighbors ensure multiple paths (more robust)