# Component diagram

Component diagrams show structure at the level of deployable or replaceable parts, with explicit provided and required interfaces. That interface detail is what separates them from a block diagram.

Useful when the contract between parts is the subject.

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## When to draw one

* Documenting a plugin or module architecture.
* Explaining what a component needs in order to run.
* Design reviews where interface boundaries are the debate.

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## What is on the diagram

| **Element**        | **Meaning**                                      |
| ------------------ | ------------------------------------------------ |
| Component          | A replaceable part with defined interfaces       |
| Provided interface | A lollipop: what it offers                       |
| Required interface | A socket: what it needs                          |
| Port               | A named connection point on a component          |
| Assembly connector | A provided interface plugged into a required one |

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## A worked example

Components with provided and required interfaces:

![Component diagram](https://help.gocapable.com/images/att1417641992.svg)

The source, which you can paste into a new diagram and edit:

```
@startuml
package "Order platform" {
  [Order API] as API
  [Pricing engine] as Pricing
  [Inventory service] as Inv
  [Notification service] as Notif
}

database "Orders DB" as DB
queue "Event bus" as Bus

API -(0- Pricing : price quote
API -(0- Inv : stock check
API --> DB
API --> Bus
Bus --> Notif

interface "REST /orders" as REST
REST - API
@enduml
```

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## A few things that catch people out

* Draw the interfaces, not just the boxes. Without them it is a block diagram.
* A component that requires six interfaces is doing too much, and the diagram says so.
* Keep deployment out of it; that is a deployment diagram's job.

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## Related

[Deployment diagramWhat runs where.](https://help.gocapable.com/diagrams/deployment-diagram.html)

[Object diagramInstances at a moment in time.](https://help.gocapable.com/diagrams/object-diagram.html)

[UML diagramsBack to the UML overview.](https://help.gocapable.com/diagrams/uml-diagrams.html)

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_Four types cover most of it._