How does Slack keep a company's messages separate from millions of other companies using the same platform?

When you use Slack, Shopify, or Salesforce, you are sharing infrastructure with thousands, sometimes millions, of other organizations. Yet your data remains completely isolated. You never see another company's messages, and they never see yours.

This is multi-tenancy in action.

Multi-tenancy is the foundation of the SaaS business model. Without it, every customer would need dedicated infrastructure, and software would cost orders of magnitude more than it does today.

But sharing infrastructure while maintaining isolation is not trivial. Get it wrong, and you leak one customer's data to another. Get it right, and you build something that scales to millions of customers without proportionally scaling your costs.

In this chapter, we will cover the core concepts you need to understand multi-tenancy: isolation models and their trade-offs, data separation strategies, the noisy neighbor problem, tenant routing, security considerations, and how companies like Slack and Shopify implement these patterns in practice.

1. What is Multi-Tenancy?