1. Distributed Computing Systems
CSCI 6900/4900

2. Message-Oriented Communication
RPCs and RMIs are not always appropriate
Both assume that receiver is executing when a request is issued
Inherently synchronous
Buffer-based network model
Hosts are connected to communication servers
Message buffers at end hosts and communication servers
Example: system

3. Buffer-based Network Model
2-20
General organization of a communication system in which hosts are connected through a network

4. Types of Communication
Persistence
Persistent communication – Stores message until communicated to user
Transient communication – Stored only when sending and receiving processes are alive
Transport level protocols provide transient communication
Synchronicity
Asynchronous – Sender continues after sending message
Synchronous – Sender blocks until message is stored at receiver's local buffer, delivered to receiver or processed by receiver

5. Example of Persistent Asynchronous Comm.
Persistent communication of letters back in the days of the Pony Express.

6. Persistence and Synchronicity in Communication (3)
2-22.1
Persistent asynchronous communication
Persistent synchronous communication

7. Persistence and Synchronicity in Communication (4)
2-22.2
Transient asynchronous communication
Receipt-based transient synchronous communication

8. Persistence and Synchronicity in Communication (5)
Delivery-based transient synchronous communication at message delivery
Response-based transient synchronous communication

9. Which Communication Is Needed?
Appropriateness depends upon the semantics of the application
Underlying system should support all types

10. Message Oriented Transient Communication -Berkeley Sockets
Interface for transport layer
A communications end point
Primitive
Meaning
Socket
Create a new communication endpoint
Bind
Attach a local address to a socket
Listen
Announce willingness to accept connections
Accept
Block caller until a connection request arrives
Connect
Actively attempt to establish a connection
Send
Send some data over the connection
Receive
Receive some data over the connection
Close
Release the connection
Socket primitives for TCP/IP.

11. Connection-oriented communication pattern using sockets.
Berkeley Sockets (2)
Connection-oriented communication pattern using sockets.

12. Message Oriented Persistent Comm.
Message Queuing Systems or Message-Oriented Middleware (MoM)
Incorporates support for persistent asynchronous communication
Offers intermediate-term storage capacity
Neither sender or receiver need to be active for the entire communication duration

13. Message Queuing Model
Applications communicate through messages stored in queues
Each application has its own (local) queue
Guarantees that the message will be inserted in recipient’s queue
No guarantee about time or whether receiver reads the message
Supports loosely coupled communication

14. Message-Queuing Model (1)
2-26
Four combinations for loosely-coupled communications using queues.

15. Message-Queuing Model (2)
Primitive
Meaning
Put
Append a message to a specified queue
Get
Block until the specified queue is nonempty, and remove the first message
Poll
Check a specified queue for messages, and remove the first. Never block.
Notify
Install a handler to be called when a message is put into the specified queue.
Basic interface to a queue in a message-queuing system.

16. General Architecture of a Message-Queuing System (1)
Source Queue/Destination Queue
Each queue has a unique name
Queuing system maps queue names to network address
Queue managers and relays

17. General Architecture of a Message-Queuing System (2)
2-29
The general organization of a message-queuing system with routers.

18. Message Brokers
Message brokers convert messages to a format required by destination application
2-30