XML Pipeline Semantics

l A node is executed only when all its input ports are "filled".

l A node's input port is "filled" either when it contains a default value that is a reference to a URL or a literal value, or when it is connected to another node's output port and this node provides data that is available. Default values are used only if no pipe is present for that port.

l When data is available on a node's output port, it is provided to the input ports of all the nodes to which it is connected.

l A node's input port can be filled by 0 or 1 value. If more than one value becomes available, this is an error and XML pipeline processing aborts.