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 applicationserver application inbound queue queued messages

 Both synchronous and asynchronous messaging.  Guaranteed message delivery.  Heterogeneous platforms.  Message priority possible.  Broadcast messages.  Filtering of messages.

FeatureMOMRPC MetaphorPost-office likeTelephone like Client/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 SequencingNo fixed sequenceServers must first come up before clients can talk to them. StyleQueuedCall-Return Partners need to be available NoYes Transactional Support Yes (Some Products) Message Queue can participate in the commit synchronization No. Requires a Transactional RPC. Message Filtering YesNo