The UML Sequence diagram is used to model implementation details of a scenario of the system. The Sequence diagram represents an interaction, which is a set of messages exchanged among objects within a collaboration to effect a desired operation or result.
A Sequence diagram shows objects involved in the scenario by vertical dashed lines, called object lifelines. Horizontal vectors between the object lifelines represent the messages passed between the objects. The messages are drawn chronologically from the top of the diagram to the bottom; the horizontal placing or spacing of objects is arbitrary.
A message from one object to another can be defined by the method called, or invoked, by the sending object on the receiving object. The method called must belong to the definition of the class instantiated by the receiving object.
Drawing messages on sequence diagram
Messages on a Sequence Diagram are drawn chronologically from the top of the diagram to the bottom, although the chronological order can be reversed if desired. The distance between the message lines does not usually hold any particular meaning, but can describe actual time metrics if the modeler desires. The horizontal placing or spacing of objects is arbitrary.
