UMLStructure diagrams

UML composite structure diagrams

The inside of one thing. Its parts, the ports on its boundary, and the connectors wiring them together - the diagram for explaining how a single component is actually built.

July 27, 20266 min readUML 2.5.125 of 35

The short answer

A part is a role an instance plays inside one container, not a type. The same class can appear twice as two differently named parts.
An assembly connector is internal wiring between two parts; a delegation connector crosses the boundary and forwards.
Delegation is how a composite honours an interface without doing the work: what arrives on the port is handled by a part inside.
Draw this when the question is what is inside one thing. A component diagram answers how several things fit together.

UML composite structure diagram showing the inside of the Payment orchestrator. An input port on the left boundary delegates over IPayment to an Authorizer part, which connects to a RiskCheck part and then to a LedgerClient part, which delegates over ILedger to an output port on the right boundary. — One classifier, opened up. The small squares on the boundary are ports; the arrows crossing it are delegation connectors handing an external contract to an internal part.

01What it shows#

A composite structure diagram answers one question: what is this thing made of, on the inside? It takes a single classifier - a class, a component, a subsystem - and draws its internal parts and how they are wired.

The distinction from the diagrams either side of it is worth getting straight. A class diagram shows types and the relationships between them, in general. A component diagram shows several components and the contracts between them. A composite structure diagram zooms into one of those and shows the runtime structure inside it.

02Parts, ports, connectors#

Element	Notation	What it means
Part	rectangle inside the frame	A role played inside the owner, written `roleName: Type`. Can carry a multiplicity in the corner.
Port	small square on the boundary	A named interaction point. Everything crossing the boundary goes through one, which is what makes the boundary meaningful.
Connector	line between parts	A communication path between two parts inside the owner.
Delegation connector	line from a port to a part	Traffic arriving at this external port is handled by that internal part. The outside never learns which one.
Assembly connector	ball in a socket	One part's provided interface wired to another's required one.
Collaboration	dashed ellipse	A named pattern of roles - the structural half of a design pattern, without committing to classes.

03Why ports are the useful bit#

A port is a small square, and it is doing more work than it looks. It says: this is the only way in. Nothing outside reaches a part directly; everything arrives at a port and is delegated inwards.

That produces the property this diagram exists for. The outside of the box depends only on the ports, so the entire inside can be rearranged - parts split, merged, replaced - without anything outside changing, as long as the ports keep their contracts. It is encapsulation drawn as geometry.

Ports also let one component present several distinct faces. A payment orchestrator with a public API port, an admin port, and an event-publishing port is saying something precise about its surface area, and about which of those a firewall or an authorization policy needs to treat differently.

04When to draw one#

Reach for it when

Explaining how one non-obvious component is built internally
Designing something where the same type appears in two different roles
Documenting a component with several distinct interaction points
Embedded, telecoms, or protocol work, where this diagram is genuinely load-bearing

Reach for something else when

The internals are three classes with obvious relationships - a class diagram is enough
You are showing several components, not the inside of one - use a component diagram
The interesting part is what happens over time, not the wiring
Nobody has asked how the inside works

This is one of the less-used UML diagrams and reasonably so - most components do not have interesting internal structure. When one does, though, nothing else expresses it as directly, and the alternative is usually a paragraph of prose that nobody reads twice.

In one line each

01Shows the inside of exactly one classifier: its parts, ports, and wiring.
02Parts are roles played at runtime, not classes - the same class can be two parts.
03Ports are the only crossings of the boundary, which is what makes the boundary real.
04Delegation connectors hand an external contract to an internal part.
05Rearranging the inside is free as long as the ports keep their contracts.

05Common questions#

What is the difference between a part and a class?

A part is a role played by an instance inside one specific containing classifier, not a type in its own right. The same class can appear as two different parts of one composite, each with its own name and multiplicity, because the diagram is about the container's internal structure rather than about types.

What is a delegation connector?

A connector from a port on the outer boundary to a port or part inside it. It says that whatever arrives on the outside is handled by that internal part, which is how a composite honours an interface without doing the work itself.

What is the difference between an assembly and a delegation connector?

An assembly connector joins two parts inside the composite, wiring one's required interface to another's provided one. A delegation connector crosses the boundary, joining an external port to something inside. Assembly is internal wiring; delegation is forwarding.

When would I draw this instead of a component diagram?

When the question is what is inside one thing rather than how several things fit together. A component diagram shows the parts of a system and their contracts; a composite structure diagram opens one of those parts and shows how it is built.

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