Reliable messaging allows programs to exchange messages over a network with certainty. Simply speaking, it ensures that an attempt to deliver a message will have one of two possible outcomes:
Basically, reliable messaging becomes very important for mission-critical applications, such as when money is at stake. You wouldn't want your non-refundable order for a $1M widget to be accidentally delivered twice. You wouldn't want your request for 10,000 custom gadgets to be lost when you absolutely must have them by next week or else your business will fail.
Reliable messaging has been around for years. Although several proprietary solutions exist, there does not appear to be any free software  solutions. Likely this is because the free software community has had little interest in systems incorporating financial transactions.
Things may soon change, however, as we realize we have the power to create our own economies. We can create systems for micropayments, donations, etc., instead of relying on proprietary services. At the same time, businesses already well-aware of the applications will be pleased (hopefully in the contributing-money-and-resources kind of way) to have open source alternatives available.
It's about time for the ryokai library.
The ryokai library is an agent for reliable messaging. Applications hand a message to the agent, and from there the agent is responsible for coordinating reliable delivery. It keeps a safe copy of the message until sucessful receipt has been confirmed. It can be queried for the status of pending messages at any time. Communication between the application and the agent is synchronous, while the delivery of messages is asynchronous.
The protocol that the library uses for reliable messaging is not fixed. Various protocols may be plugged in, employing technologies such as WS-ReliableMessaging, HTTPR, etc.
Ryokai is a Japanese word often used in a communication context to mean "roger", "ten-four", or "yes, sir!". Listen carefully to the dialog of your robot battle anime and you'll hear it. (Strictly speaking, the proper Roman spelling is ryoukai. As for pronunciation, RO-KAI will get you close enough.)
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