Modern applications rarely fail because of UI bugs. They fail because of poor system design decisions—especially around authentication, authorization, and tenant isolation.
In this article series, I’ll break down real-world system design patterns used in production systems, focusing on:
Authentication (Auth)
Role-Based Access Control (RBAC)
Multi-tenant architecture
Permissions, policies, and access boundaries
Common mistakes and scaling challenges
This series is not theory-heavy. It’s based on practical patterns, trade-offs, and decisions teams face when building real systems.
Why This Series?
Most tutorials show:
“Add JWT authentication”
“Protect a route”
“Create roles like admin and user”
But real systems need answers to harder questions:
How do you design auth for multiple organizations (tenants)?
How do roles differ per department or project?
How do you avoid permission explosions?
How do you scale RBAC without rewriting everything later?
These are the problems we’ll solve—step by step.
The Core Building Blocks
Before diving deep, let’s clarify the three pillars this series revolves around.
1. Authentication (Who are you?)
Authentication answers one question:
Who is making this request?
Common approaches:
Email/password
OAuth (Google, GitHub, SSO)
Token-based auth (JWT, opaque tokens)
Session-based auth
⚠️ Authentication does not decide what you can do. It only proves identity.
2. Authorization & RBAC (What are you allowed to do?)
Authorization answers:
What can this authenticated user access or modify?
RBAC is the most widely used model:
Users → Roles → Permissions
But in real systems:
Roles vary per tenant
Permissions vary per context
Some actions depend on ownership or hierarchy
We’ll explore:
Simple RBAC
Role-Permission mapping
Context-aware permissions
When RBAC breaks and what to use instead
3. Multi-Tenant Architecture (Where do you belong?)
Multi-tenancy answers:
Which organization’s data and rules apply to this user?
Examples:
SaaS products
Enterprise tools
Internal platforms used by multiple companies
Key challenges:
Data isolation
Tenant-specific roles and permissions
Scaling without duplicating logic
Preventing cross-tenant access bugs
A single missed check here can become a security incident.
A Real-World Example We’ll Build On
Throughout this series, we’ll use a realistic scenario:
A SaaS platform used by multiple companies.
Each company has departments, users, roles, and permissions.
High-level structure:
Organization (Tenant)
└── Company
└── Department
└── Users
└── Roles
└── Permissions
Rules:
A user belongs to one tenant
Roles are defined per tenant
Permissions control access to features and data
Access depends on both role and context
This mirrors how many enterprise systems actually work.
Common Mistakes Teams Make Early
Before we design it properly, let’s call out common mistakes:
❌ Hard-coding roles (if user.role === 'admin')
❌ Assuming one global admin role
❌ Mixing authentication and authorization logic
❌ Ignoring tenant boundaries in queries
❌ Designing RBAC too late (after features grow)
This series will help you avoid these traps.
What This Series Will Cover
Planned articles include:
Authentication in Real Systems
JWT vs sessions
Token lifecycle
Secure login flows
Designing RBAC That Scales
Role vs permission design
Database schema patterns
Avoiding role explosion
Multi-Tenant Authorization
Tenant isolation strategies
Query-level security
Middleware patterns
Advanced Patterns
Policy-based access control
Attribute-based access control (ABAC)
Feature flags & permission gates
Common Security Pitfalls
Privilege escalation bugs
Insecure defaults
Authorization testing strategies
Who This Series Is For
This series is useful if you:
Build backend systems
Work on SaaS products
Design APIs
Want to understand why patterns exist, not just how to implement them
Basic familiarity with backend development is assumed, but the concepts apply across stacks.
What’s Next
In Part 2, we’ll start with:
Authentication design in multi-tenant systems
— how to structure identity, tokens, and tenant context correctly from day one.
If you’re interested in real-world system design patterns, follow along.
About the Author
I’m a software engineer working on real-world web systems, focusing on scalable backend design, authorization models, and clean architecture. I write about lessons learned from building and evolving production systems.
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