How does BizTalk Server work?

At its core, it is an event-processing engine that is based on pub/sub-conventional patterns.

Wikipedia defines pub/sub as:

"Publish/subscribe (or pub/sub) is a messaging paradigm where senders (publishers) of messages are not programmed to send their messages to specific receivers (subscribers). Rather, published messages are characterized into classes, without knowledge of what (if any) subscribers there may be. Subscribers express interest in one or more classes, and only receive messages that are of interest, without knowledge of what (if any) publishers there are. This decoupling of publishers and subscribers can allow for greater scalability and a more dynamic network topology."

Common flow in BizTalk is once a message is received by an adapter, is run through pre-processing in a pipeline, before being data transformed in maps, then being published to the message box.

 * IMPORTANT NOTE: Once a message has entered the messagebox it is immutable (which means CANNOT be changed).

Because of the nature of pub/sub patterns a bit a latency is introduced, which BizTalk makes up for with its star abilities (durability, reliability, and scalability).

The most dynamic use for BizTalk is in SOA or Service bus architecture.  In this, architecture itinerary services were introduced, which basically applied a message with a purpose concept.

What is this message you talk about?

It is the data processed through the BizTalk messaging engine (IE. XML document, Word Document, Flat file etc.).  The message basically contains two critical parts message type and context.  The message type defines it within the bus and is typically a XML namespace and root node.  The message context is a set of name/value pairs.

How does a message get to and from BizTalk?

BizTalk uses receive locations which are defined as endpoints, which use a particular adapter which knows how to receive that specific message.  These receive locations have a particular receive pipeline associated with them, which can be used to modify the message before it becomes immutable.  Now, a receive port is defined as containing more than one receive location, which applies XSLT maps to the message prior to being published to the messagebox.

BizTalk used send ports and orchestrations (Two real types of subscribers within BizTalk), send ports to provide a means of transporting messages from the bus and an orchestration provides an executable business process, which uses the message to complete workflow operations.

Note: BizTalk Orchestration work as both Publisher as well as Subscriber

BizTalk Artifacts - What are they & their role in Architecture?

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See Also

Another important place to find a huge amount of BizTalk related articles is the TechNet Wiki itself. The best entry point is BizTalk Server Resources on the TechNet Wiki.