Sequence Diagram
Overview
Sequence diagrams are used primarily to understand the time-elapsed interactions between objects (things, actors, phenomae, etc.) in the sequence that those interactions occur.
In computer science, these diagrams establish a dashed vertical lifeline for every object in the system. There can be as many as needed to describe all the objects participating in the system. When an object is active, a narrow rectangle shows the duration that it is “on.” Perpendicular to the lifelines are arrows denoting some kind of message (a call, a transaction, or other interaction between objects). Sometimes there are conditions that must be met before an action can continue (e.g. you must show identification or pay a fee before acess is granted to a space or an item is received.)
Instructions
Observe a situated activity or environment as it unfolds over time.
Identify all entities or participants: humans, objects, atmospheres, non-human actors, environmental changes - everything that plays a role in shaping interactione. Construct a notation on a roll of paper, by hand, in Astah, or Adobe Illustrator.
Every entity (environment, human, non-human, object) should have its own “lifeline.” When a that entity acts or changes state, a slender rectangle indicates the start and end of that action. Interactions are drawn with various kinds of arrows to indicate that one object/participant has sent a request, object etc. to another. Sometimes a request is literally verbal - “please pass the salt”, but it can take different forms - like a gesture, a non-verbal trigger, a reaction, or an environmental change - “the lights dim”. Sometimes the recipient of a message reacts (passes the salt). Sometimes the recipient replies with a message -“I don’t have the salt.” Sometimes there is no response. And sometimes there are conditions that must be met before a response is generated to a request.
Relevant Tutorials
Required Reading:
IBM Sequence Diagrams, also here.
Optional Reading:
Timothy Barker, Recomposing the Digital Present, 2011
Rebecca Hill, The Interval, 2012
Christophe Girot, Vision in Motion, Representing Landscape in Time.
Lyster, Clare, Landscapes of Exchange.
Other References: