Adapters provide connectivity for BizTalk to and from the outside world. The rest of the BizTalk engine is thus isolated from whatever requirements or communication semantics a remote system may impose.
In most cases, adapters work with a transport and have no underlying knowledge of the data that will be sent or received. They receive a byte stream from the wire and feed the BizTalk engine. Adapters are for the most part data agnostic — that is, that they are not concerned with whether they are receiving XML, flat file, or binary data.
In the receive case, the adapter is responsible for listening or polling for data and wrapping that data in a BizTalk message. In the case of listening, it may listen on a endpoint (or URL) for incoming data and will communicate with the remote system in whichever way is required (authentication, transactions, handshaking, etc.). In the polling scenario, the behavior will typically involve periodically checking an endpoint for new data that needs to be received into BizTalk.
In the send case, the adapter is responsible for receiving messages from the BizTalk MessageBox and transmitting them to the remote system, using their associated transport or protocol.
The SQL adapter retrieves an XML message from the BizTalk MessageBox and ultimately creates rows in a remote SQL Server database, all based on the contents of the XML message. The adapter is responsible for transferring the message from BizTalk to the remote system without the ...