Scenario diagram vs sequence diagram

What's the difference between sequence diagrams and scenari开发者_JAVA百科o diagrams in UML?

I think you mean System Sequence Diagram (SSD) and Sequence Diagram.

The purpose of SSD is to illustrate a particular use-case scenario in a visual format where the system appears as a black box.

Sequence Diagram, on the other hand, shows how system objects interact over time for achieving a particular scenario of a use-case.

"Scenarios" per your link aren't diagrams, they're narrative text. They're also called "Use Case Narratives" among other terms. They are approximately equivalent to other behaviour specifications (sequence diagrams, activity diagrams).

There are generally three differences:

1. Format. Scenarios are textual, sequence/activity diagrams are, well, diagrams
2. Content. Textual narrative is often used to describe the sequence of interactions between the user and the "system" in Use Case descriptions. Hence there are usually only two participants (User & System). In contrast, sequence diagrams are primarily used to show how behaviour is distributed among objects. Consequently there will usually be more than two participants.
3. Formality. Narratives are often used earlier in the cycle to get an idea of what's needed. Consequently they're often less formal than Sequence diagrams - which typically show inter-object messages ("method calls").

For examples of tools that do this, take a look at websequencediagrams.com. Or search for "free UML tools".

There is no diagram type called scenario. Maybe you are thinking of collaboration?

Sequence and collaboration diagrams are actually the same: the Sequence depicts the chronology while the Collaboration depicts the interactions between the collaborators. Together (a tool that was around a decade ago) used to let you literally just change the setting and see the diagram in the other mode.

_{I use OmniGraffle (Mac). It's great for class diagrams. Has support for Sequence diagrams but not Collaboration.}