Building AuthKit: Why Self-Hosting Authentication Systems Is Harder Than It Looks

Many developers treat authentication as a black-box service, but building a self-hosted system like AuthKit reveals the sophisticated engineering required to handle multi-tenancy, token rotation, and concurrent refresh requests. Understanding these challenges—from race conditions in token refresh to isolating thousands of tenants on shared infrastructure—explains why services like Clerk command their pricing. For engineers evaluating authentication solutions, this deep dive demonstrates whether DIY or SaaS is the right choice for their use case.