System Architecture Document Template

The engineering lead or tech lead for the system, with PM review for the overview and constraints sections. The PM does not need to understand every technical detail but should validate that the constraints and design rationale align with product requirements.

Update it whenever the architecture changes meaningfully: new services added, databases changed, integrations added or removed, deployment topology modified. At minimum, review it quarterly. Many teams make the review part of their quarterly planning cycle.

An architecture document describes the current system as a whole. A technical spec proposes a change to the system. The architecture document is the context. The technical spec is the delta. Technical specs should reference the architecture document to explain where the change fits.

Detailed enough for a new engineer to understand the system. Not so detailed that it is never updated. The component map, data architecture, and deployment topology are the core sections. Security and scalability sections can be briefer if detailed documents exist elsewhere (link to them).

Yes. Performance targets, scalability limits, and reliability expectations are essential context. If the system must handle 10K concurrent users with sub-200ms response times, that shapes every architectural decision. Document these constraints explicitly in the overview section.