What is a sequence diagram?

Sequence diagrams are a part of UML that helps to explain the behaviour of a system. Here's a simple sequence diagram with two participants:

You read them from top to bottom. As time increases, you depict events happening in the system, and how various parts of the system react to each other.

Be aware, however, of people that abuse sequence diagrams. You can very easily confuse people if you try to describe how long things take in a sequence diagram. Some of the most confusing diagrams that I have ever seen were trying to describe the architecture of the system. Don't do that! Sequence diagrams can only describe behaviour.

Always be aware of the primary purpose of a sequence diagram -- to explain. Here is an example of how some explanatory text along the side can make sequence diagrams more clear.

Sequence diagrams like this can be produced very easily on websequencediagrams.com. Here is all you have to type to produce the diagram above.

title Drop shipping

 With drop shipping, the retailer takes orders
 from a customer, but leaves responsibility for
 for shipping to a wholesaler or producer.  

Customer->Retailer: Order
Retailer->Factory: Order detailsn and shipping address
Factory->Customer: Ships final product

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• Sequence Diagram for a Purchase Order
• How to make different arrows in WebSequenceDiagrams
• Creating sequence diagrams in Eclipse
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