Multi-tenant systems share infrastructure while preserving data and performance boundaries. Isolation may use tenant IDs in shared tables, separate schemas, separate databases, or a hybrid. Choose based on security, customization, cost, operational burden, and recovery requirements.

Tenant identity must come from a trusted authenticated context and be enforced in database queries, cache keys, object paths, search filters, and background jobs. A missing tenant predicate can leak data across customers. Test isolation with direct API calls and adversarial identifiers, not just UI workflows.

Use per-tenant quotas, concurrency limits, queue partitions, and workload priorities to protect fairness. Global rate limits alone do not prevent one tenant from monopolizing expensive work.