What is a state invariant in a sequence diagram?

What is a state invariant in a sequence diagram?

• A. An invariant is a condition applied to a specific lifeline at a point preceding a particular event occurrence and must hold true at that moment for a valid execution
• B. A condition applied to the entire interaction
• C. A condition that must always be true across the diagram
• D. A condition used to denote loop iterations

Answer: (A)

State invariants in a sequence diagram are constraints placed on the state of a single lifeline at a specific moment in time. They express something that must be true just before (or after) a particular message occurs, and that truth is required for the interaction to be valid. That’s why describing a condition tied to a specific lifeline at the moment before an event happens is the best fit. For example, you might require that a user session is active and a user’s account is verified just before a login attempt proceeds. This focuses on the state of one object at a precise point, rather than across the whole interaction or the diagram. A constraint applied to the entire interaction would be a global condition, not a moment-specific one on a single lifeline. A condition that must always be true across the diagram is even more general than a local, momentary invariant. And a constraint used to denote loop iterations serves a different purpose—guiding repeated execution—rather than describing the state just before a specific event.

State invariants in a sequence diagram are constraints placed on the state of a single lifeline at a specific moment in time. They express something that must be true just before (or after) a particular message occurs, and that truth is required for the interaction to be valid. That’s why describing a condition tied to a specific lifeline at the moment before an event happens is the best fit. For example, you might require that a user session is active and a user’s account is verified just before a login attempt proceeds. This focuses on the state of one object at a precise point, rather than across the whole interaction or the diagram.

A constraint applied to the entire interaction would be a global condition, not a moment-specific one on a single lifeline. A condition that must always be true across the diagram is even more general than a local, momentary invariant. And a constraint used to denote loop iterations serves a different purpose—guiding repeated execution—rather than describing the state just before a specific event.