# Use of external and open data

**In short:** External data, especially open data, is a useful input for applications. Open data is public domain or openly licensed, free to access, and machine-readable. Architects should plan for it early.

## Explanation

Many applications need data they do not produce themselves. Public bodies publish some of it for anyone to use. Architects should know such sources exist and plan how the application will reach them.

**What makes data open**

The Open Definition sets three basic conditions for an open work:

- **Legal status.** The work is in the public domain or under an open license. An open license lets anyone reuse the work.
- **Access.** The work is available as a whole. It costs no more than a reasonable one-time reproduction fee. Ideally, anyone can download it free over the Internet.
- **Machine-readable.** A computer can process the work readily, without manual conversion.

**Open government data**

Data.gov is the US government's open data site. US law requires government data to be released in open, machine-readable formats. Open government data is data that public bodies publish under conditions like these.

**Finding data across catalogs**

Data catalogs list datasets. DCAT is a W3C vocabulary, built on RDF, that helps catalogs on the Web work together. RDF is a W3C way of describing data. Catalogs that use DCAT can describe their datasets in a shared way.

**What this means for architecture**

An external source is a system outside your control. Show it as an external system in your context view. That view is a diagram of your system and everything it exchanges data with. See the article "Context View" for details. Plan the access path to the source as well.

## Example

A city app shows nearby services on a map. The team finds a government portal that offers a free, openly licensed dataset in a machine-readable form. They add the portal to the context diagram as an external system. They also record in the architecture document how the app will obtain the data.

## Trade-offs

The verified facts cover what open data is and where it is published. They do not cover quality, availability, or legal risks of depending on external sources. Check those for each source before you rely on it.



## Checks

- External data sources the system uses appear as external systems in the context view.
- For each open data source, the document states its license, or that it is in the public domain.
- For each open data source, the document states that the data is machine-readable and how it can be obtained.

## Related

- [Context View](https://arc.fuet.ch/concepts/vision-stakeholders/context-view.md)
- [arc42 Context and Scope](https://arc.fuet.ch/concepts/documentation/arc42-context-and-scope.md)
- [Business and Technical Context](https://arc.fuet.ch/concepts/documentation/business-and-technical-context.md)
- [Data reusability and standardized access](https://arc.fuet.ch/concepts/data-architecture/data-reusability-and-standardized-access.md)
- [Integration](https://arc.fuet.ch/concepts/integration/integration.md)
- [Interoperability](https://arc.fuet.ch/concepts/quality-attributes/interoperability.md)

## Sources

- [Open Definition 2.1 - Open Definition - Defining Open in Open Data, Open Content and Open Knowledge](https://opendefinition.org/od/2.1/en/) — opendefinition.org, accessed 2026-11-09
- [Data Catalog Vocabulary (DCAT) - Version 3](https://www.w3.org/TR/vocab-dcat-3/) — w3.org, accessed 2026-11-09
- [About Us - Data.gov](https://data.gov/about/) — data.gov, accessed 2026-11-09