# Federation Contract
For a federation to work, all providers need to come up with a federation contract to agree on. See the overview about the existing federation concepts and issues. So to make the user experience good, a lot of alignment between the providers is required. The following pages document the "contract" that the federated back-ends have agreed upon.
To join the federation, it is required to (mostly) fulfill these requirements and document differences for users in the "Federation Aspects and Known Issues". Nevertheless, these requirements are negotiable if there are good arguments for a change as the current state of the "contract" is just the compromise that the existing providers have agreed upon and if a new back-end joins the federation new compromises may need to be made.