System Architecture

Step 2 - Identify domains. Group by business capability, not technical layer. Look for things that change for different reasons and at different rates.

Step 3 - Map data flow. Trace: where does data enter → how does it transform → where does it exit? Make the flow obvious.

Step 4 - Draw boundaries. Every boundary needs a reason: different team, different change rate, different compliance requirement, or different scaling need.

Step 5 - Run complexity checklist. Before adding any non-trivial pattern:

[ ] Have I tried the simple solution?
[ ] Do I have evidence it's insufficient?
[ ] Can my team operate this?
[ ] Will this still make sense in 6 months?
[ ] Can I explain why this complexity is necessary?

If any answer is "no", keep it simple.

Step 6 - Present the architecture using the output template below.

Output Template

### System: [Name]

**Constraints**:
- Scale: [current and expected load]
- Team: [size and structure]
- Lifespan: [prototype / MVP / long-term]

**Architecture**:
[Component diagram or description of components and their relationships]

**Data Flow**:
[How data enters → transforms → exits]

**Key Boundaries**:
| Boundary | Reason | Change Rate |
|----------|--------|-------------|
| ... | ... | ... |

**Trade-offs**:
- Chose X over Y because [reason]
- Accepted [limitation] to gain [benefit]

**Complexity Justification**:
- [Each non-trivial pattern] → [why it's needed, with evidence]

Core Principles

1. Boundaries at real differences. Separate concerns that change for different reasons and at different rates.
2. Dependencies flow inward. Core logic depends on nothing. Infrastructure depends on core.
3. Follow the data. Architecture should make data flow obvious.
4. Design for failure. Network fails. Databases timeout. Build compensation into the structure.
5. Design for operations. You will debug this at 3am. Every request needs a trace. Every error needs context for replay.

For concrete good/bad examples of each principle, see examples.md.

Anti-Patterns

Success Criteria

Your architecture is right-sized when:

1. You can draw it - dependency graph fits on a whiteboard
2. You can explain it - new team member understands data flow in 30 minutes
3. You can change it - adding a feature touches 1-3 modules, not 10
4. You can delete it - removing a component needs no archaeology
5. You can debug it - tracing a request takes minutes, not hours
6. It matches your team - deployable units ≤ number of teams

When the Simple Solution Isn't Enough

If the complexity checklist says "yes, scale is real", see scaling-checklist.md for concrete techniques covering caching, async processing, partitioning, horizontal scaling, and multi-region.

Iterative Architecture

Architecture is discovered, not designed upfront:

1. Start obvious - group by domain, not by technical layer
2. Let hotspots emerge - monitor which modules change together
3. Extract when painful - split only when the current form causes measurable problems
4. Document decisions - record why boundaries exist so future you knows what's load-bearing

Every senior engineer has a graveyard of over-engineered systems they regret. Learn from their pain. Build boring systems that work.