Building SaaS Products: The Architecture Decisions That Compound
Early SaaS architecture decisions are hard to reverse. Here are the ones to get right before your first customer.
SaaS products have specific architectural requirements that don't apply to single-tenant applications. Getting these right early makes scaling to more customers an execution problem rather than a rewrite problem.
Multi-tenancy is the foundational decision. There are three models: shared database with a tenant_id column (simplest to build, hardest to isolate), separate schemas per tenant (good balance of isolation and operational simplicity for most products), and separate databases per tenant (strongest isolation, highest operational overhead). Most early-stage SaaS products should use shared database with tenant_id — it's operationally simple and the isolation trade-off is acceptable until you have enterprise customers with explicit data isolation requirements.
Billing integration shapes your data model. Integrate Stripe (or an equivalent) at the beginning, not as an afterthought. The concepts of subscriptions, plans, usage, and invoices touch the core of your application — retrofitting billing onto an application that wasn't designed for it is painful. Design your feature flagging system to work from subscription status from day one.
Tenant-scoped rate limiting prevents a noisy-neighbor problem — one customer's heavy usage degrading performance for others. Implement rate limits per tenant at the API gateway or middleware level. This is operationally simple but makes a meaningful difference to perceived reliability.
Feature flags and plan gating belong in configuration, not code. Hardcoded if-statements gating features by plan name accumulate into unmaintainable logic. A centralized feature flag system — even a simple database table mapping plan to features — makes adding plans, running trials, and managing grandfathered customers tractable.
Observability per tenant is the last piece most teams add too late. Aggregate metrics don't tell you which tenant is experiencing slow queries or high error rates. Instrument your application to tag logs and metrics with a tenant identifier from the start.