Activity diagram: Activity diagram represents the flow of use cases. The use of activity diagrams is for the description of the business processes that will describe the functionality of a business system.
An activity diagram consists of the following symbols which are used to draw the flow of activities or events taking place in an activity.
Every activity diagram starts with initial node. It is represented by a small filled circle followed by an arrow represents the initial action state or the start point for any activity diagram. Activity or Action Activity
An activity will be represented by an oval shaped rectangle. These activities will represents the actions taking place while an event occurs.
Action flow also known as control flow will be represented by an arrow pointed towards the next event/activity that will take place and will also show a connection between the two actions.
When an activity requires a decision prior to moving on to the next activity, at that stage a decision is used which is represented in form of a diamond between the two activities. A diamond will show a decision with alternate paths.
Guard conditions are used in an activity diagram to represent the mandatory conditions which needs to be true before we move to the next step in the process.
Sequence Diagrams are interaction diagrams that detail how the operations in an event are carried out. It represents the classes and objects which are involved in event and the sequence of messages exchanged between the objects needed to carry out the functionality of the scenario.
Following are some of the notation that are used to draw a sequence diagram
A role played by an entity which will interact with the external subject. It usually represents roles played by human users, external hardware.
A message will represent a particular communication between the Lifelines of an Interaction.
Call message is a kind of message that will show an initiation of operation of target lifeline.
Return message represents the exchange of information/data back to the caller of a corresponded former message.