How to handle the co-exist phase of Strangler pattern gracefully

We are consolidating a number of disparate monoliths, redesigning and implementing a greenfield suite of microservices and micro-frontends. The stack is all new, the design is all new – it couldn’t be more different to the legacy systems.

Things are going fine because we have focussed our efforts on internal efficiency gains through better system integration etc. while we prove the concepts but now we are planning our client facing build out!

Our inclination is to continue to use the strangler pattern in its well documented form but I have major concerns and wonder if any of you could share you experiences.

1. The way our lines of business operate means clients will inevitably need to use a combination of new and old systems. The difference in user experience will be vast, how have people handled this? We are considering the proxy approach and façade patterns. We’ve even discussed iframes and agreed never to speak of that episode again.
2. The legacy authentication and new authentication implementations are vastly different. We have been able to handle this internally a little manually, to our main user base, that is not an option? Options like Okta and OneLogin as an interim are being considered.
3. Authorisation is just breaking us. The legacy system was role based, the new system policy based. The legacy role management is extremely complex and completely incompatible. How have people tackled Authorisation in the co-exist phasing?

There are many more questions of course and we are researching away but I would be interested in a discussion with those that have faced this head on and been scarred by it.