Login flow with an alt fragment
An authentication handshake across an actor, a Jacobson control object, and a database lifeline — synchronous calls open activation bars, a dashed reply returns the row, and an alt combined fragment splits the valid/invalid branches. A note records the session-cookie side effect.
For the backend engineer
The canonical "who calls whom, in what order" diagram — and a good tour of the features that separate a UML sequence diagram from a generic flow.
Lifeline kinds carry meaning. actor, control, and database render their distinct UML/Jacobson heads (stick figure, circled arrow, cylinder), so the diagram tells you what kind of thing each participant is, not just its name.
Activation and replies. ->+ opens an activation bar on the callee; -- draws the dashed reply that returns control (and the row). Bars nest, so you can see Auth is active while it waits on DB.
Branching is a real fragment. The alt block with its [credentials valid] / [invalid] guards is a UML combined fragment, drawn as a labelled box spanning the lifelines — not two separate arrows you have to mentally group.