How to update file content when accessing Gatsby.js?

Question

JavaScript

There is a task: when opening site.com/file.xml, get up-to-date data. This incremental data is taken from an API from another service. How can I ensure that the contents of a file are updated when a link is opened or once in a while?

Application on Gatsby.js, hosted on Netlify.
Now the file is created on build in the public folder via gatsby-node.js.