UML Sequence Diagram

Sequence Diagrams show a detailed flow for a specific use case or even just part of a specific use case. They are almost self explanatory; they show the calls between the different objects in their sequence and can show, at a detailed level, different calls to different objects.

Figure uml.05 Sequence Diagram As Figure uml.05 shows, you form a sequence diagram by first placing the objects or roles that participate in the interaction at the top of your diagram, across the horizontal axis. Typically, you place the object or role that initiates the interaction at the left, and increasingly more subordinate objects or roles to the right. Next, you arrange the messages that these objects send and receive along the vertical axis in order of increasing time from top to bottom. This gives the reader a clear visual cue to the flow of control over time. Sequence diagrams have two features that distinguish them from communication diagrams.

First, there is the lifeline. An object lifeline is the vertical dashed line that represents the existence of an object over a period of time. Most objects that appear in an interaction diagram will be in existence for the duration of the interaction, so these objects are all aligned at the top of the diagram, with their lifelines drawn from the top of the diagram to the bottom. Objects may be created during the interaction. Their lifelines start with the receipt of the message create (drawn to box at the head of the lifeline). Objects may be destroyed during the interaction. Their lifelines end with the receipt of the message destroy (and are given the visual cue of a large X, marking the end of their lives).
Second, there is the focus of control. The focus of control is a tall, thin rectangle that shows the period of time during which an object is performing an action, either directly or through a subordinate procedure. The top of the rectangle is aligned with the start of the action; the bottom is aligned with its completion (and can be marked by a return message). You can show the nesting of a focus of control (caused by recursion, a call to a self-operation, or by a callback from another object) by stacking another focus of control slightly to the right of its parent (and can do so to an arbitrary depth). If you want to be especially precise about where the focus of control lies, you can also shade the region of the rectangle during which the object's method is actually computing and control has not passed to another object, but this is rather fussy.

The main content in a sequence diagram is the messages. A message is shown by an arrow from one lifeline to another. The arrowhead points to the receiver. If the message is asynchronous, the line has a stick arrowhead. If the message is synchronous (a call), the line has a filled triangular arrowhead. A reply to a synchronous message (a return from a call) is shown by a dashed arrow with a stick arrowhead. The return message may be omitted, as there is an implicit return after any call, but it is often useful for showing return values. The ordering of time along a single lifeline is significant. Usually the exact distance does not matter; lifelines only show relative sequences, so the lifeline is not a scale diagram of time. Usually the positions of messages on separate pairs of lifelines do not imply any sequencing information; the messages could occur in any order. The entire set of messages on separate lifelines forms a partial ordering. A series of messages establishes a chain of causality, however, so that any point on another lifeline at the end of the chain must always follow the point on the original lifeline at the start of the chain.

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