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which was originally processing these messages, has been replaced by another process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