Communication

Part II

Message-Oriented Communication

Chapter 4

Most of the lecture notes are based on slides by Prof. Jalal Y. Kawash at Univ. of Calgary

I have modified them and added new slides

Giving credit where credit is due:

CSCE455/855 Distributed Operating Systems

RPC and Message Passing

• RPC enhances access transparency by hiding communication

• RPC is inherently synchronous and is not always appropriate

• What if the receiving side is not executing when the request is sent?

• Sometimes, we need to resort to message-passing

Persistence and Synchronicity in Communication (1)

General organization of a communication system in which hosts are connected through a network

2-20

Persistence and Synchronicity in Communication (2)

Persistent communication of letters back in the days of the Pony Express.

Persistence in Communication

• Persistence: A (sent) message is stored by the communication system until it is delivered.

• Transient: A (sent) message is stored by the communication system as long as both the sender and receiver applications are executing

Synchrony in Communication

• Asynchronous: The sender does not block until the message is delivered, blocks until stored in a buffer at the sending host, or the first communication server

• Synchronous: The sender blocks until the message is stored in a buffer at the receiving host, or delivered to the receiver

Persistent Asynchronous Communication

2-22.1

A message is stored: • until delivered

• at A’s host

• at the first comm server

• e.g.: email

Persistent Synchronous Communication

2-22.1

A message is stored: • until delivered

• at B’s host

• at B’s comm server

Transient Asynchronous Communication

2-22.2

A message is stored: • at A’s host

• at the first comm server

• If B is not running, messageis dropped

• e.g.: asynch. RPC, UDP

Transient Synchronous Communication (1)

2-22.2

Weakest: Receipt-based

Transient Synchronous Communication (2)

Moderate: Delivery-based Strongest: Response-based

Appendix

General Architecture of a Message-Queuing System (1)

SourceQueue

DestinationQueue

• Nearby the sender

• Messages can be put to this queue only

• Nearby the receiver

• Message contains information about this queue

• How the message travels between A and B is the responsibility of the queuing system

A B

General Architecture of a Message-Queuing System (2)

• The collection of queues is distributed across multiple machines

• The system must maintain a (possibly distributed) database of queues (queuing tables)

Queue Name Network Address

Queuing table entry

General Architecture of a Message-Queuing System (3)

The relationship between queue-level addressing and network-level addressing.

General Architecture of a Message-Queuing System (4)

• Queues are managed by queue managers

• A queue manager interacts directly with the sending/receiving application

• Some (special) queue managers act like routers or relays

General Architecture of a Message-Queuing System (5)

• Routing is static• Medium-level scalability (manually configure the routing tables)

2-29

Integrating Differing Message-Based Systems

System A System B

A-specific message format

B-specific message format

MessageBroker

Message Brokers

The general organization of a message broker in a message-queuing system.

2-30