I built a login system using Flask OIDC and Keycloak. In my system, there is some endpoints decorated with oidc.require_login() that calls the Keycloak login page.
My goal is, after the user successfully logged in, my system checks if the user name exists in a specific database.
How can I set a function to be called every time someone successfully logged in with Keycloak and do this verification at the database?
According to your needs there are several ways to create the user in the backend.
The easiest way would be to just check the JWT token on every request. OIDC is based on JWT and that token is available on any request (which should already be done to find user roles etc). So your application can check that JWT and extract the username from it (see here for details about the JWT format). With the username you can check your internal database and create the user, if it doesnt exist. But at that time you'll not have access to any user credentials any more. It is just SSO and you need to trust Keycloak and the JWT... Also - you'll never be informed, if the user will be deleted in Keycloak, which could be an issue.
There is a callback API in Keycloak in form of the Admin URL per client. But the documentation is not clear. It says: It’s used by the Keycloak server to send backend requests to the application for various tasks, like logout users or push revocation policies. But I cannot find a complete list of "tasks". I saw only logout events. see Keycloak documentation and the documentation only talks about that. If I add an admin url to a test client, I did not get any requests at login time.
a different but more complicated way would be to create your own UserStorage SPI in Keycloak. It would be Java of course, but only some classes. There is an HTTP example or have a look at the LDAP user storage SPI, which supports registration too. If you choose that for your realm and a user tries to login to Keycloak (Login form), the SPI can call your backend to check the user. It also could be "used" to create the user in the backend by checking the Keycloak local storage and only if there is a local Keycloak user, call the backend. That isn't the reason, why you should implement the UserStorage SPI, but it's possible. If you think, this is a good idea, I would prefer to use your backend storage as the one and only storage or build a different one, that then could call your real backend in case of a new user. I would use this one by not using Keycloak local stored users but, by using your own database.
next (maybe last one). You can write an EventListener SPI to read all events and only filter the login events, see here and here. I think, that would be the easiest one. But be aware. In that case, the HTTP call to your backend coming from the event itself is based on a normal HTTP request (without OIDC at that time).
The last two examples create a JAR (which is explained in the links). That JAR with the SPI must be deployed in keycloaks standalone/deployments folder. The EventListener should be active by default, the UserStorage SPI must be activated per realm.
But - be aware - Keycloak/SSO/JWT - should not be used by creating users in multiple backends. Syncing the users between all backends in a SSO environment is maybe the wrong way. Most information is located in the JWT or can be called by a backend from one central user identity management. Do not store a user more then once. If you need the user reference in your backend - link just to the username or userid (string) instead of a complete entity.