Messaging is an important means of communication between two systems.

There are 2 types of messaging.

- Synchronous messaging.

- Asynchronous messaging.

Some observations: Client/Server computing is generally based on a model of synchronous communication:

Client and server have to be active at the time of communication

Client issues request and blocks until it receives reply

Server essentially waits only for incoming requests, and subsequently processes them

Client cannot do any other work while waiting for reply

Failures have to be dealt with immediately (the client is waiting)

Connection overhead; Higher probability of failures.

There are two ways to handle failures: 1. Transactional RPC

Enable more complex interactions with some execution guarantees .

2. Service replication and load balancing.

Prevents system from shutting down is service is not available.

MOM stands for message oriented messaging.

Asynchronous forms of interaction Communication by exchanging messages More dynamic Use of queues acting as buffers.

MOM Core

client application server application

inbound queue

queuedmessages

Both synchronous and asynchronous messaging.

Guaranteed message delivery. Heterogeneous platforms. Message priority possible. Broadcast messages. Filtering of messages.

Feature MOM RPCMetaphor Post-office like Telephone likeClient/Server time relationship

Asynchronous. Clients and Servers may operate at different times and speeds

Synchronous. Clients and Servers must run concurrently. Servers must keep up with clients.

Client/Server Sequencing

No fixed sequence Servers must first come up before clients can talk to them.

Style Queued Call-ReturnPartners need to be available No YesTransactional Support Yes (Some Products)

Message Queue can participate in the commit synchronization

No. Requires a Transactional RPC.

Message Filtering Yes No