General purpose of this lecture is to presentation on UML Sequence Diagrams. UML sequence diagram is definitely an interaction diagram that models just one scenario executing in the system. Sequence diagrams can be somewhat close to the code level. Here briefly explain why not just code up that algorithm instead of drawing it as the sequence diagram. Here also discuss on Key parts of a sequence diagrams, Indicating method calls, linking sequence diagrams and Decentralized system control with examples.