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2 Overview

This specification defines the Message Service within the Application Interface Specification

(AIS).

2.1 Message Service

The Message Service specifies a buffered message-passing system based on the

concept of a message queue for processes on the same or on different nodes1. Messages

are written to message queues and read from them. A single message queue

permits a multipoint-to-point communication. Message queues are persistent or nonpersistent.

The Message Service must preserve messages that have not yet been

consumed when the message queue is closed.

Processes sending messages to a message queue are unaware that the process

which was originally processing these messages, has been replaced by another process

acting as a standby in case the original process fails or switches over.

Message queues can be grouped together to form message queue groups. Message

queue groups permit multipoint-to-multipoint communication. They are identified by

logical names, so that a process is unaware of the number of message queues and of

the physical location of the message queues to which it is communicating. The

sender addresses message queue groups by using the same mechanisms that it

uses to address single message queues. The message queue groups can be used to

distribute messages among message queues pertaining to the message queue

group. Regardless of the number of message queues to which messages are distributed,

the message queue group remains accessible under the same name.

Message queue groups can be used to maintain transparency of the sender process

to faults in the receiver processes, represented by the message queues in the message

queue groups. The sender process communicates with the message queue

group. If a receiver process fails, the sender process continues to communicate with

the message queue group and is unaware of the fault, because it continues to obtain

service from the other receiver processes.

With message queues, the Message Service uses the model of n senders to 1

receiver whereas with message