A scalable monolith is where most systems should begin.

Before scale, before traffic, before teams grow large, the real challenge is clarity. When clarity fades, systems slow down even while they continue to function. Changes feel heavier. Understanding takes longer. Confidence erodes.

Many teams respond by adopting microservices early. The motivation feels reasonable. Smaller units appear safer. Deployments seem isolated. Boundaries look cleaner.

Authentication is the first step every application must get right. Users expect sign in to be effortless while developers need the system to be secure, scalable and easy to maintain. OAuth based sign in combined with access and refresh tokens has become the standard, and for good reason. It removes the burden of password handling and provides a clean way to authorize API access.

This guide walks through the entire flow in a clear and practical way.