[keycloak-user] Check to see if user exists with Admin REST API

[Benjamin Hansmann [alphaApps]]

I hope this is my last question to this list :-)

I want to check if a user exists with given username/or email address
from within an Android App while the user is typing.

Hence I want to provide a REST endpoint to let the app check. I am using
the Admin REST API for backchannel requests to keycloak from within my
servlet.

/admin/realms/{my-realm}/users/{username} (or the get(username)
equivalent of admin-client) does not work for checking if the email
address is already in use.

/admin/realms/{my-realm}/users?search={username} works for email, but
it's greedy. I think "%" is added at beginning and end of the query
parameters before the database request, so that search hits are quasi
guaranteed. E.g. if someone has a registered Email Address of
john.smith at example.com and someone tries to register with
smith at example.com or smith at example.com.br it is shown as already in use.

Any ideas? How is this solved in the web frontend? Maybe the Admin REST
API endpoint should support username and email as path params when those
two can be used to login?

Best Regards
Benjamin