Senior Architect

Overview

Provide architecture-level guidance for system design decisions. This skill produces Architecture Decision Records (ADRs), trade-off analyses, scalability blueprints, and non-functional requirements specifications. Every recommendation includes explicit trade-offs and is grounded in proven patterns.

Announce at start: "I'm using the senior-architect skill for system design and architecture decisions."

Phase 1: Requirements Analysis

Goal: Capture all functional and non-functional requirements before designing.

Actions

1. Identify functional requirements (capabilities)
2. Define non-functional requirements (quality attributes)
3. Identify constraints (budget, team, timeline, compliance)
4. Map integration points with existing systems
5. Establish success criteria and SLOs

STOP — Do NOT proceed to Phase 2 until:

• Functional requirements are listed
• Non-functional requirements are quantified (not vague)
• Constraints are explicit
• Success criteria are measurable

Phase 2: Architecture Design

Goal: Evaluate options and select the approach with the best trade-off profile.

Actions

1. Evaluate architectural styles (monolith, microservices, event-driven)
2. Design component boundaries and interfaces
3. Define data architecture (storage, flow, consistency)
4. Plan infrastructure and deployment topology
5. Address cross-cutting concerns (auth, logging, monitoring)

Architecture Style Decision Table

Default Recommendation

Start with Modular Monolith: clear module boundaries, single deployment. Extract to microservices only when you have proven need for independent scaling, deployment, or team autonomy.

Trade-Off Analysis: Common Pairs

Decision Matrix Template

Weight each quality attribute (1-5), score each option (1-5), multiply and sum.

| Quality Attribute  | Weight | Option A | Option B | Option C |
|--------------------|--------|----------|----------|----------|
| Performance        |   4    |  4 (16)  |  3 (12)  |  5 (20)  |
| Maintainability    |   5    |  5 (25)  |  4 (20)  |  2 (10)  |
| Scalability        |   3    |  3 (9)   |  5 (15)  |  4 (12)  |
| Cost               |   4    |  4 (16)  |  2 (8)   |  3 (12)  |
| Total              |        |    66    |    55    |    54    |

STOP — Do NOT proceed to Phase 3 until:

• At least 2 architectural options have been evaluated
• Trade-offs are explicitly documented
• Decision matrix scores support the recommendation
• Data architecture is defined

Phase 3: Documentation and Validation

Goal: Record decisions and validate against requirements.

Actions

1. Write Architecture Decision Records for key decisions
2. Create system context and container diagrams (C4 model)
3. Validate against non-functional requirements
4. Identify risks and mitigation strategies
5. Define evolutionary architecture guardrails

ADR Format

# ADR-{number}: {Title}

## Status
Proposed | Accepted | Deprecated | Superseded by ADR-{number}

## Context
What is the issue motivating this decision?

## Decision
What change are we proposing?

## Consequences

### Positive
- [Benefit 1]
- [Benefit 2]

### Negative
- [Trade-off 1]
- [Trade-off 2]

### Risks
- [Risk and mitigation]

## Alternatives Considered
| Option | Pros | Cons | Verdict |
|--------|------|------|---------|
| Option A | ... | ... | Chosen |
| Option B | ... | ... | Rejected because... |

C4 Model Levels

STOP — Documentation complete when:

• ADRs written for all key decisions
• System context diagram created
• NFRs validated against design
• Risks documented with mitigations

Scalability Patterns

Horizontal Scaling Decision Table

Sharding Strategy Decision Table

Caching Layers

Client Cache (browser) -> CDN Cache -> API Gateway Cache ->
Application Cache (Redis) -> Database Query Cache -> Database

Non-Functional Requirements Template

## Performance
- Response time: p95 < 200ms, p99 < 500ms for API calls
- Throughput: 1000 RPS sustained, 5000 RPS peak
- Batch processing: 1M records/hour

## Availability
- Target: 99.9% (8.76h downtime/year)
- RTO (Recovery Time Objective): < 15 minutes
- RPO (Recovery Point Objective): < 5 minutes

## Scalability
- Current: 10K DAU
- 12-month target: 100K DAU
- Scale dimension: users, data volume, request rate

## Security
- Authentication: OAuth 2.0 / OIDC
- Authorization: RBAC with resource-level permissions
- Data encryption: at rest (AES-256) and in transit (TLS 1.3)

## Observability
- Logging: structured JSON, 30-day retention
- Metrics: RED method, custom business metrics
- Tracing: distributed tracing across all services
- Alerting: PagerDuty integration, tiered severity

SLO/SLA/SLI Framework

Anti-Patterns / Common Mistakes

Subagent Dispatch Opportunities

Follow the dispatching-parallel-agents skill protocol when dispatching.

Integration Points

Key Principles

• Start with the simplest architecture that could work
• Make decisions reversible when possible
• Design for failure (everything will fail eventually)
• Optimize for team cognitive load, not technical elegance
• Document decisions, not just outcomes
• Prefer boring technology for critical paths
• Every architectural decision has a cost — make it explicit

Skill Type

FLEXIBLE — Adapt architecture recommendations to the specific context. ADRs are strongly recommended for all significant decisions. Trade-off analysis is mandatory. NFRs must be quantified, not described vaguely.