MSMQ applications communicate using messages. A message is simply a unit of information or data being sent between computers. The message can contain text or binary data. Transactional messages can be used to pair the sending or receiving of any message with an action in another operation. Using transactional messages ensures that the unit of work is carried out as an atomic operation¾that is, the operation succeeds or fails as a whole. Transactional messages can also be used to ensure that a message is delivered only once and to ensure that all messages sent from one computer to another are delivered in order. Positive and negative acknowledgments can be used to confirm that messages reached or were retrieved from the destination queue.
MSMQ supports two delivery methods: express and recoverable. Choosing between express and recoverable delivery is a matter of trading performance and resource use for reliability and failure recovery. In general, express messages use fewer resources and are faster than recoverable messages. However, express messages cannot be recovered if the computer storing the memory-mapped message files fails. Recoverable messages use more resources and are slower than express messages, but can be recovered regardless of which computer fails.
MSMQ uses public and private queues to store and forward messages. All MSMQ queues, regardless of their function, can be manipulated with the same MSMQ functions. This includes the special journal, dead letter, transactional dead letter, administration, system, and report queues. Each of the queues is simply a standard MSMQ queue used for a specific purpose. For more information on the MSMQ API, see the Microsoft Message Queue Server Software Development Kit (MSMQ SDK).
Queue quotas and computer quotas specify the cumulative limit for messages in a queue or in all queues on a computer. The queue and computer quotas are based on size and can be set independently. When a queue quota is reached, messages can no longer be sent to the queue until one or more messages are removed from the queue. When a computer quota is reached, messages can no longer be sent to any queues on the computer until one or more messages are removed from one of the queues.
MSMQ routes and delivers messages based on a combination of queue priority and message priority. Messages are routed and delivered first by queue priority, and then by message priority.
MSMQ supports dependent clients, independent clients, and servers. Both independent clients and servers run the MSMQ service and can communicate asynchronously. MSMQ dependent clients require synchronous access to an MSMQ server.
Some MSMQ servers hold copies of the MSMQ information store (MQIS) database. The MQIS is a distributed database that holds enterprise topology, enterprise settings, computer information, and queue information. MSMQ-based applications can query the MQIS to find queues and get queue properties.
All computers operate within one MSMQ enterprise. The enterprise is divided into sites, where communication between any two computers is fast and inexpensive. Sites are connected through site links. Site-link costs define the cost of sending messages between sites. Computers running in MSMQ communicate over connected networks (CNs). A CN is a collection of computers in which any two computers can communicate directly. MSMQ servers designated as in-routing servers (InRSs), out-routing servers (OutRSs), and site gates can be used to control the flow of messages and provide session concentration. MSMQ servers take all these factors into account when routing messages within your MSMQ enterprise.