This is an example of accessing Butler data using a web service. This was presented during the "Butler client-server" talk at the SQRE services bootcamp.
Most of the meat is in the get_coadd_url() FastAPI handler in
src/butlerexampleservice/handlers/external.py -- the rest of this is just a
Safir template project.
An example for a Phalanx deployment of this service can be found here.