UML interaction overview diagrams
An activity diagram whose steps are entire interactions. The map that sits above a set of sequence diagrams and says which one runs when.
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01What it shows
An interaction overview diagram is an activity diagram in which the action nodes are interactions. The control-flow notation is borrowed wholesale from activity diagrams - initial node, decisions, forks, joins, final node - and the things being sequenced are sequence diagrams.
It exists to solve one specific problem: a sequence diagram that has grown too big. Once a flow has three branches, each with its own dozen messages, the single diagram containing all of it is unreadable. Split it into three named sequence diagrams and draw one interaction overview to say which runs when, and both halves become readable again.
02The notation
| Element | Notation | What it means |
|---|---|---|
| Interaction occurrence | frame tagged ref | A reference to an interaction defined elsewhere, named after it. The common case. |
| Inline interaction | frame tagged sd | A small sequence diagram drawn inside this node rather than referenced. Use only when it is genuinely small. |
| Control flow | arrow | Order between interactions. Same meaning as on an activity diagram. |
| Decision / merge | diamond | Chooses one path by its guard, or brings alternatives back together. |
| Fork / join | solid bar | Interactions that run concurrently, and the point that waits for them. |
| Initial / final | disc, and ring around a disc | Start and end of the whole overview. |
That is the entire notation, and if you have read the activity diagram article you already know most of it. The only genuinely new element is the framed node, and its rule is simple: the frame stands for an interaction, and the tag says whether the interaction is drawn inside it (sd) or somewhere else (ref).
03When to draw one
Reach for it when
- A flow with several alternative interactions, each big enough for its own diagram
- Documenting a protocol with distinct phases - handshake, transfer, teardown
- Giving readers an index into a set of related sequence diagrams
- Showing that two interactions genuinely run in parallel
Reach for something else when
- The whole interaction fits comfortably on one sequence diagram with an alt fragment
- There is only one path - the overview would add a box and no information
- You have not actually drawn the referenced interactions yet
- The steps are work rather than interactions - that is a plain activity diagram
This is the least-used diagram in UML, and its niche is real but narrow. The honest guidance is that most flows are better served by one sequence diagram with an alt fragment. Reach for an interaction overview at the point where that single diagram stops fitting on a screen - which does happen, and when it does, nothing else in the language solves it as cleanly.
In one line each
- 01An activity diagram whose nodes are interactions rather than actions.
- 02Control-flow notation is borrowed unchanged from activity diagrams.
- 03A frame tagged ref points at an interaction defined elsewhere; sd means it is drawn inline.
- 04Its purpose is to break up a sequence diagram that has grown unreadable.
- 05Draw the referenced sequence diagrams first - the overview is an index, not a substitute.
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