architecture-primer

Architecture Primer

This skill turns a vague "how should I build this?" into concrete, justified architecture decisions. It is based on the System Design Primer, adapted to favor shipping over over-engineering.

Analogy: do not design a 10-lane highway before knowing if 10 cars or 10 million cars will use it. Measure the traffic first, then build the road.

PHASE 1 — Discovery (mandatory before any decision)

Never propose an architecture before completing discovery. Ask questions 2-3 at a time, in plain language, in these three blocks. Skip any question the user already answered.

Block A — Scale

• How many users do you expect at launch? And in 12 months?
• Are users in one region or several countries?
• Is usage steady, or does it spike (e.g. launches, business hours, campaigns)?

Block B — Nature of the system

• Does it read more than it writes, or the other way around?
• What happens if it goes down for 1 hour — is it critical?
• Does the data need immediate consistency, or is a short delay acceptable?

Block C — Context

• Is there a technical team, or just you?
• Roughly what monthly infrastructure budget is realistic?
• When do you need the MVP live?

PHASE 2 — Decision tree

Apply these rules using the discovery answers. Each rule has a default; deviate only with a concrete reason.

Monolith vs Microservices

• Team under 5 people → monolith, always.
• MVP or under 6 months to ship → monolith, always.
• Over 100k daily active users → evaluate microservices for the parts that actually hurt.
• Rule: start monolith, split when it hurts. Premature microservices add ops cost with no benefit.

SQL vs NoSQL

• Relational data needing integrity → PostgreSQL.
• Flexible documents, horizontal scale → MongoDB.
• Cache / sessions / real-time → Redis.
• Full-text search → Elasticsearch.
• Rule: PostgreSQL by default; switch only with a concrete reason.

Cache

• Same query repeats a lot → Redis.
• Data that rarely changes but is read often → CDN or Redis.
• Data unique per user on every request → no cache (it would not hit).