Antony_nganga explores how great engineers make architectural decisions by balancing trade-offs and documenting outcomes—featuring ADRs, the Azure Well-Architected Framework, an ATAM-lite checklist, and practical team guidelines.

How Great Engineers Make Architectural Decisions — ADRs, Trade-offs, and an ATAM-Lite Checklist

Every engineering choice involves balancing trade-offs—such as reliability versus cost, performance versus maintainability, or speed versus safety. Instead of seeking perfect outcomes, effective engineers make informed, transparent decisions and document their reasoning for future teams.

Why Decision-Making Matters

Architecture Decision Records (ADRs) capture the reasoning, context, and trade-offs behind technical decisions. This shared record:

  • Lives alongside code in the repository
  • Explains reasoning in plain language
  • Survives personnel and version changes, becoming the engineering team’s memory

The Five Pillars of Trade-offs: Azure Well-Architected Framework

At Microsoft, all major design discussions leverage the Azure Well-Architected Framework, evaluating against five key pillars:

  1. Reliability – System resilience and recovery from failures
  2. Performance Efficiency – Latency and throughput targets
  3. Cost Optimization – Efficient resource use
  4. Security – Limiting blast radius and exposure
  5. Operational Excellence – Deployment, monitoring, and fast recovery

No single decision perfectly optimizes all five, so documenting conscious trade-offs is critical.

A Practical Decision Flow

  1. Frame It: Clarify the problem, constraints, and quality goals (e.g., SLOs, cost caps)
  2. List Options: Identify 2-4 viable approaches
  3. Score Trade-offs: Use a Decision Matrix to rate each option against the five pillars
  4. ATAM-Lite Review: List scenarios, sensitivity points, and risks
  5. Record It as an ADR: Document everything as a markdown file in the code repository

Example Table

Step What to Do Output
Frame It Clarify problem, constraints, quality goals Problem statement
List Options Identify 2-4 realistic approaches Options list
Score Trade-offs Decision Matrix (rate 1–5 against pillars) Table of scores
ATAM-Lite Review List scenarios, sensitivity points, risks Risk notes
Record It as an ADR Document in markdown next to code ADR file

Example: Adding a Read-Through Cache

  • Decision: Add a Redis cache in front of Cosmos DB to reduce read latency
  • Context: Average P95 latency from DB is 80 ms (goal: less than 15 ms)
  • Options:
    • A) Query DB directly
    • B) Add read-through cache using Redis

Trade-offs:

  • Performance: Massive improvement in read speed
  • Cost: Fewer RU/s consumed on Cosmos DB
  • Reliability: Risk of stale data if cache invalidation fails
  • Operational: More complexity (need for monitoring, TTLs)

Templates You Can Re-use

ADR Template

# ADR-001: Add Read-through Cache in Front of Cosmos DB

Status: Accepted
Date: 2025-10-21
Context: High read latency; P95 = 80ms, target <15ms
Options:
A) Direct DB reads
B) Redis cache for hot keys ✅
Decision: Adopt Redis cache for performance and cost optimization.
Consequences:
- Improved read latency and reduced RU/s cost
- Risk of data staleness during cache invalidation
- Added operational complexity
Links: PR#3421, Design Doc #204, Azure Monitor dashboard

Decision Matrix Example

Pillar Weight Option A Option B Notes
Reliability 5 3 4 Redis clustering handles failover
Performance 4 2 5 In-memory reads
Cost 3 4 5 Reduced RU/s
Security 4 4 4 Same authentication posture
Operational Excellence 3 4 3 More moving parts

Weighted total = Σ(weight × score); highest score indicates best option

Team Guidelines for ADRs

  • Store ADRs in a /docs/adr folder within each code repository
  • One ADR per major change; supersede outdated ones instead of editing history
  • Link relevant ADRs in design reviews and pull requests
  • Revisit ADRs when constraints change (incidents, new SLOs, cost shifts)
  • Publish lessons learned as follow-up knowledge sharing posts

Why It Works

Recording decisions with ADRs, rated against Azure Well-Architected pillars and reviewed through a lightweight ATAM process, aligns team culture with reliability and transparency. This practice supports onboarding, accelerates design reviews, and provides traceable technical history.

Join the Conversation

Have you used ADRs or ATAM-like processes in your architecture work? Share thoughts or link public templates so the community can evolve better architectural reasoning.

Published Oct 21, 2025

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