Solution architecture

Architecture is the set of decisions that are expensive to reverse. Everything cheap to change later is design, and does not need this level of ceremony.

Start from what constrains, not from what appeals

Architecture is driven by quality attributes, not by features. Establish the ones that actually bind before drawing anything: expected load and its shape, latency the user notices, availability the business will fund, data residency and retention obligations, the rate of change the team needs to sustain, and the size and skills of the team who will operate it.

Vague attributes produce vague architecture. "Scalable" is not a requirement; "20,000 concurrent sessions with p99 under 300ms" is a constraint that eliminates options.

Boundaries are the whole game