Prd V05 Technical Stack Selection

Technical Stack Selection

Position in workflow: v0.5 Risk Discovery Interview → v0.5 Technical Stack Selection → v0.6 Architecture Design

Technical stack selection is about making build/buy/integrate/research decisions for every technical capability your product needs.

Decision Categories

Rule: Default to Buy for everything that isn't a core differentiator. Build only when you must.

Technology Layers to Address

For each layer, decide Build/Buy/Integrate/Research:

Decision Process

1. Inventory needs from FEA- (features) and SCR- (screens)

   • What technical capabilities are required?

2. Check RISK- constraints

   • Do any risks constrain technology choices?
   • Performance? Security? Compliance?

3. Categorize each need: Build | Buy | Integrate | Research

   • Use decision framework below

4. Research specific options for Buy/Integrate

   • Evaluate against criteria

5. Define research scope for Research items

   • What must be learned? By when?

6. Create TECH- entries with full rationale

Build vs. Buy Decision Framework

Strong Build signals:

• "This is why users choose us over competitors"
• "No existing solution fits our specific need"
• "We need to change this weekly based on learning"

Strong Buy signals:

• "Every SaaS product needs this"
• "Multiple proven solutions exist"
• "We'd just be recreating what vendors already built"

TECH- Output Template

TECH-XXX: [Technology/Capability Name]
Category: [Build | Buy | Integrate | Research]
Layer: [Frontend | Backend | Database | Auth | Payments | Infrastructure | Integrations | AI/ML | Analytics | DevOps]
Purpose: [What problem this solves]

Features Served: [FEA-XXX, FEA-YYY]
Screens Affected: [SCR-XXX, SCR-YYY]
Risk Constraints: [RISK-XXX that influenced this]

Decision: [Specific choice made]
Rationale: [Why this choice over alternatives]
Alternatives Considered:
  - [Option A]: [Why rejected]
  - [Option B]: [Why rejected]

Trade-offs:
  - Pro: [Advantage]
  - Con: [Disadvantage]

Cost: [Pricing model, estimated monthly cost]
Integration Complexity: [Low | Medium | High]
Lock-in Risk: [Low | Medium | High]

Research Needed: [If Category = Research]
Evaluation Criteria: [How to decide]
Decision Deadline: [When we must decide]

Example TECH- entry (Buy):

TECH-001: Authentication Service
Category: Buy
Layer: Auth
Purpose: User authentication, session management, social login

Features Served: FEA-010 (login), FEA-011 (signup), FEA-012 (social auth)
Screens Affected: SCR-000 (login), SCR-001 (signup), SCR-010 (settings)
Risk Constraints: RISK-008 (security compliance requirement)

Decision: Clerk
Rationale: Best DX for Next.js, includes social login, compliant
Alternatives Considered:
  - Auth0: Higher price at scale, more complex setup
  - Firebase Auth: Google ecosystem lock-in, less polished
  - Build custom: Would take 2+ weeks, security risk

Trade-offs:
  - Pro: <1 day integration, battle-tested security
  - Con: Vendor dependency, per-MAU pricing at scale

Cost: Free tier sufficient for MVP, ~$25/mo at 1K MAU
Integration Complexity: Low
Lock-in Risk: Medium (user data exportable, but migration work)

Example TECH- entry (Research):

TECH-005: Vector Database for Semantic Search
Category: Research
Layer: Database
Purpose: Store and query embeddings for AI-powered search

Features Served: FEA-025 (semantic search)
Screens Affected: SCR-008 (search results)
Risk Constraints: RISK-012 (query latency <200ms requirement)

Decision: TBD after POC
Rationale: Multiple viable options with different trade-offs
Alternatives to Evaluate:
  - Pinecone: Managed, easy setup, higher cost
  - Weaviate: Self-hosted option, more control
  - pgvector: Uses existing Postgres, simpler stack

Research Needed:
  - Latency benchmark with 1M vectors
  - Cost projection at 10M vectors
  - Query accuracy comparison

Evaluation Criteria:
  - P99 latency < 200ms
  - Cost < $500/mo at scale
  - Supports metadata filtering

Decision Deadline: End of EPIC-02

Evaluation Criteria for Buy/Integrate

Anti-Patterns to Avoid

Quality Gates

Before proceeding to Architecture Design:

• All technology layers addressed
• Build decisions justified as differentiators
• Buy decisions have cost estimates
• Research items have clear criteria and deadlines
• RISK- constraints reflected in choices
• No obvious vendor lock-in without acknowledgment

Downstream Connections

TECH- entries feed into:

Detailed References

• Tech stack examples by product type: See references/examples.md
• TECH- entry template: See assets/tech.md
• Evaluation scorecard: See assets/evaluation-scorecard.md