UMLBehaviour diagrams

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.

July 27, 20265 min readUML 2.5.129 of 35

The short answer

It is an activity diagram whose nodes are whole interactions instead of individual actions - the map above a set of sequence diagrams.
The control-flow notation is identical to an activity diagram. Only what the nodes are changes: interactions rather than actions.
A ref frame points at a sequence diagram you already drew, which is what keeps the overview small.
Worth drawing once one feature has more than about three sequence diagrams and nothing says how they relate.

UML interaction overview diagram. From an initial node the flow enters a ref frame for Authorize payment. A decision then routes to either a ref frame for Capture payment when approved, or a ref frame for Decline payment when declined. Both reach the final node. — Three sequence diagrams and the logic that chooses between them. Each framed box is a reference to an interaction defined elsewhere.

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.

04Common questions#

What is a UML interaction overview diagram?

An activity diagram whose nodes are entire interactions rather than individual actions. It sits above a set of sequence diagrams and shows which one runs when, including the branches and loops between them.

What is an interaction occurrence?

A reference to an interaction defined elsewhere, drawn as a frame labelled ref carrying the name of the sequence diagram it points at. It is what lets an overview diagram stay small, since each node is a whole diagram you have already drawn.

How is it different from an activity diagram?

The control-flow notation is the same: initial and final nodes, decisions, merges, forks and joins. What differs is what the nodes are. An activity diagram's nodes are actions; an interaction overview diagram's nodes are interactions.

When is one worth drawing?

When you have more than about three sequence diagrams describing one feature and nothing says how they relate. It is the map, and a map is only useful once there is enough territory to get lost in.

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