SAMP — Simple Application Messag- ing Protocol Version 1.11 - IVOA

not discussed further in this document, but the intention is to provide anarchitecture which is sufficiently open to allow for such th<strong>ing</strong>s in the futurewith little change to the basics.1.3 Types of <strong>Messag</strong><strong>ing</strong><strong>SAMP</strong> is currently limited to inter-application desktop messag<strong>ing</strong> with theidea that the basic framework presented here is extensible to meet futureneeds, and so it is beyond the scope of this document to outline the manytypes of messag<strong>ing</strong> systems in use today (these are covered in detail in manyother documents). While based on established messag<strong>ing</strong> models, <strong>SAMP</strong> isin many ways a hybrid of several basic messag<strong>ing</strong> concepts; the protocol ishowever flexible enough that later versions should be able to interact fairlyeasily with other messag<strong>ing</strong> systems because of the shared messag<strong>ing</strong> models.The messag<strong>ing</strong> concepts used within <strong>SAMP</strong> include:Publish/Subscribe <strong>Messag</strong><strong>ing</strong>: A publish/subscribe (pub/sub) messag<strong>ing</strong>system supports an event driven model where information producersand consumers participate in message pass<strong>ing</strong>. <strong>SAMP</strong> applications“publish” a message, while consumer applications “subscribe” to messagesof interest and consume events. The underly<strong>ing</strong> messag<strong>ing</strong> systemroutes messages from producers to consumers based on the messagetypes in which an application has registered an interest.Point-to-Point <strong>Messag</strong><strong>ing</strong>: In point to point messag<strong>ing</strong> systems, messagesare routed to an individual consumer which maintains a queueof “incom<strong>ing</strong>” messages. In a traditional message queue, applicationssend messages to a specified queue and clients retrieve them. In <strong>SAMP</strong>,the message system manages the delivery and rout<strong>ing</strong> of messages, butalso permits the concept of a directed message meant for delivery to aspecific application. <strong>SAMP</strong> does not, however, guarantee the order ofmessage delivery as with a traditional message queue.Event-based <strong>Messag</strong><strong>ing</strong>: Event-based systems are systems in which producersgenerate events, and in which messag<strong>ing</strong> middleware deliversevents to consumers based upon a previously specified interest. Onetypical usage pattern of these systems is the publish/subscribe paradigm,however these systems are also widely used for integrat<strong>ing</strong> loosely coupledapplication components. <strong>SAMP</strong> allows for the concept that an“event” occurred in the system and that these message types may haverequirements different from messages where the sender is try<strong>ing</strong> to invokesome action in the network of applications.5