Scaling authentication across systems primarily relies on decoupling the identity provider from service providers. This is achieved through delegated authentication models, where users authenticate once with a central identity provider and then receive a secure token. These tokens, often JSON Web Tokens (JWTs), are cryptographically signed and contain necessary claims, allowing resource servers to verify authenticity locally without repeatedly contacting the identity provider. Single Sign-On (SSO) solutions, built upon protocols like OAuth 2.0 and OpenID Connect, are crucial for this, enabling users to access multiple applications with a single set of credentials. For high availability and performance, the identity provider itself must be horizontally scalable, employing load balancing, caching mechanisms, and stateless session management for efficient token issuance and validation. This token-based approach minimizes direct database lookups for each request, significantly enhancing overall system throughput and reducing latency.