Communication Part II Message-Oriented Communication Chapter 4

n Most of the lecture notes are based on slides by Prof. Jalal Y. Kawash at Univ. of Calgary n 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 A message is stored: until delivered at A’s host at the first comm server e.g.:

Persistent Synchronous Communication A message is stored: until delivered at B’s host at B’s comm server

Transient Asynchronous Communication A message is stored: at A’s host at the first comm server If B is not running, message is dropped e.g.: asynch. RPC, UDP

Transient Synchronous Communication (1) Weakest: Receipt-based

Transient Synchronous Communication (2) Moderate: Delivery-based Strongest: Response-based

Appendix

General Architecture of a Message-Queuing System (1) Source Queue Destination Queue 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 AB

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 NameNetwork 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 ASystem B A-specific message format B-specific message format Message Broker

Message Brokers The general organization of a message broker in a message-queuing system. 2-30