> For the complete documentation index, see [llms.txt](https://kinesis-school-of-programming.gitbook.io/nestjs-unleashed/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://kinesis-school-of-programming.gitbook.io/nestjs-unleashed/extra-module-3-openapi-specification.md).

# Extra module 3 - OpenAPI Specification

Documentation on how to interact with the system.

Imagine the following scenario: there is an online service that collects data of products from different online shops. The intention is that a person accessing this service can then compare these products: which ones are available in each shop, their respective prices, customer reviews, etc. In a case like this, it would be interesting to expose a documentation of our backend or API, showing how to communicate with it. This way, the creator of this service would know how to obtain this data from us.

There is a standard that defines rules for this kind of documentation: the **OpenAPI Specification**. It is **language-agnostic** and states that a documentation like this should contain:

* Available operations (routes or endpoints)
* Parameters (what to send & what to be received)
* Authentication methods
* Contact information, license, terms of use, and other information

Fortunately, Nest has integration with a popular tool that implements this specification: **Swagger**. By leveraging it, much of this documenting process will be completely automated, which will save us a lot of work. Let's then install the necessary dependency to start this module.

```sh
yarn add @nestjs/swagger
```