Design Document

Design Document

Overview

A design document is the bridge between "what we're building" (Working Backwards output) and "how we're building it." It forces the author to think deeply before writing code, surface trade-offs explicitly, consider operational reality, and invite scrutiny from peers. The document is not documentation—it is a thinking tool. If writing the design document doesn't change your approach at least once, you didn't think deeply enough.

A good design document answers: What problem are we solving? Why this approach over alternatives? What are we trading away? How will we know it's working? How will we operate it at scale?

When to Use

• Before starting implementation of any project estimated at more than 2 engineer-weeks
• When introducing a new service, API, or significant data store
• When making architectural changes that affect other teams
• When changing system boundaries (splitting or merging services)
• When the blast radius of failure includes customer-facing impact
• When multiple valid approaches exist and the choice isn't obvious
• Before any project that will require a Design Bar Raiser review

Amazon Context

At Amazon, writing is thinking. A design document is a 6-page narrative (not PowerPoint) that forces clarity of thought. The design document ties directly to the Working Backwards process: the PR/FAQ and PRFAQ define what and why for customers; the design document defines how for engineers. The Design Bar Raiser (DBR) review process ensures the document meets a high bar before implementation begins.

Design documents are living artifacts during the design phase but become historical records once approved. They capture the reasoning at a point in time—including constraints, assumptions, and rejected alternatives—so future engineers can understand why decisions were made, not just what was decided.

The standard flow: Working Backwards → Design Document → Design Bar Raiser Review → Implementation Plan → Code.

The Process

1. Link to Working Backwards Output

Every design document starts by referencing the approved Working Backwards artifact (PR/FAQ, PRFAQ, or one-pager). This section answers:

• What customer problem are we solving? (1-2 sentences, pulled from WB)
• Who is the customer? (Internal team, external customer, or both)
• What does success look like from the customer's perspective?
• What constraints were established during Working Backwards?

Do NOT re-argue the "what" or "why." That was settled in WB. If you find yourself re-litigating the problem statement, go back and update the WB document first.

2. Requirements

Functional Requirements

List what the system must do. Each requirement should be:

• Testable (you can write a verification for it)
• Prioritized (P0 = launch blocker, P1 = needed within 30 days of launch, P2 = future)
• Traced to a customer need from the WB output

Example format:

• FR-1 (P0): The system shall process payment transactions within 500ms at p99.
• FR-2 (P0): The system shall support idempotent retries for all write operations.

Non-Functional Requirements

Cover these categories explicitly:

• Latency: p50, p95, p99 targets for key operations
• Throughput: Expected TPS at launch, at 10x, and peak
• Availability: Target (e.g., 99.95%) and what "available" means precisely
• Durability: Data loss tolerance (zero? RPO of X minutes?)
• Scalability: How does the system scale? Horizontally? What's the ceiling?
• Security: Data classification, encryption requirements, access control
• Cost: Per-unit economics at steady state and at 10x growth
• Compliance: Regulatory requirements (GDPR, PCI, SOC2, etc.)

3. Proposed Architecture

High-Level Architecture

Describe the system using a clear diagram and narrative. Cover:

• Service boundaries and responsibilities
• Data flows (read path and write path separately)
• External dependencies and how you handle their failure
• Storage choices and why
• Async vs. sync boundaries and why

Detailed Design

For each major component:

• Input/output contract
• State management approach
• Failure modes and recovery
• Scaling characteristics

Data Model

• Schema design with rationale
• Access patterns that drove the design
• Migration strategy from current state (if applicable)
• Growth projections and storage cost implications

4. Alternatives Considered

For each rejected alternative:

• What: Describe the approach clearly enough that a reader could implement it
• Pros: Genuine advantages (not strawmen)
• Cons: Why it was ultimately rejected
• Decision driver: The specific requirement or constraint that made this inferior

You must include at least 2 alternatives. If you can only think of one approach, you haven't explored the design space enough.

Common alternatives to address:

• Build vs. buy
• Single service vs. multiple services
• Synchronous vs. asynchronous
• SQL vs. NoSQL vs. purpose-built
• Push vs. pull architecture
• Architectural patterns from the patterns/ catalog when applicable (see below)

Using patterns/ as a source of alternatives

The patterns/ directory carries architectural patterns whose adoption propagates across the entire system (e.g., cell-based architecture, shuffle-sharding, static stability). Patterns are not a separate mandatory section — they enter the design as alternatives, evaluated against the data and the workload's actual requirements.

When generating alternatives, read patterns/INDEX.md (the lightweight catalog) and identify any pattern whose applies_when criteria plausibly match the workload's characteristics. Only then load the full patterns/<name>/PATTERN.md for matching patterns. A matching pattern becomes a real alternative — it competes with the baseline and other options on its own merits, with concrete pros/cons, cost, and operational impact.

Two important cases:

1. The user already chose a pattern. If the user (or the WB output) is asking to implement cell-based, shuffle-sharding, or any other pattern explicitly, the pattern is the starting point. The design then explores alternatives to the pattern (e.g., "cell-based with Single-AZ cells" vs "cell-based with Multi-AZ cells" vs "the simpler bulkhead approach within a single instance"), not whether to adopt it at all.
2. No pattern fits. If, after scanning the catalog, none of the patterns is a credible match for the workload, do not list patterns just to reject them. Patterns that don't fit don't appear in the design. Quality of thought, not quantity of dismissals.

When a pattern is adopted, copy its Skill Impact Map into the relevant later sections (Architecture, Operational Concerns, Testing) — the map is the contract for how the pattern propagates through the rest of the design and into the specs.

5. Trade-offs

State trade-offs explicitly using this format:

We are choosing X over Y because Z, accepting the consequence that W.

Examples:

• "We are choosing eventual consistency over strong consistency because our read throughput requirement of 100K TPS cannot be met with a single-leader model, accepting that users may see stale data for up to 5 seconds after a write."
• "We are choosing DynamoDB over Aurora because our access patterns are key-value lookups with predictable latency requirements, accepting that ad-hoc queries will require a separate analytics pipeline."

Every non-trivial design has trade-offs. If your document has none, you're hiding them.

6. Cost Estimation

• Infrastructure cost: Compute, storage, network, managed services
• Operational cost: On-call burden, maintenance time, monitoring
• Development cost: Engineer-weeks to build, timeline
• Cost at scale: What happens to unit economics at 10x current projection?
• Cost comparison: How does this compare to alternatives from Section 4?

Use the AWS pricing calculator or equivalent. Show your math. Include both steady-state and peak costs.

7. Operational Concerns

• Deployment strategy: How will this be deployed? (Progressive, blue/green, etc.)
• Rollback plan: How do you undo a bad deployment? What about data migrations?
• Monitoring: What metrics will you emit? What dashboards exist?
• Alarming: What conditions trigger alarms? Who gets paged?
• Runbooks: What manual interventions might be needed? Document them.
• On-call burden: How does this change the on-call experience?
• Dependency management: What happens when each dependency fails?
• Capacity planning: How do you know when to scale? Is it automatic?
• Disaster recovery: RTO and RPO. How do you recover from region failure?

8. Testing Strategy

• Unit tests: What components are unit-testable? Coverage target?
• Integration tests: How do you test service interactions?
• Load tests: How do you verify the system meets throughput/latency requirements?
• Chaos tests: What failure scenarios will you inject?
• Security tests: Penetration testing, SAST/DAST, dependency scanning
• Acceptance tests: How do you verify customer-facing requirements from WB?
• Migration tests: If migrating, how do you verify correctness during transition?

9. Design Bar Raiser Review

Before submitting for Design BR review:

• Ensure all sections above are complete (no TBD or placeholders)
• Have at least one peer pre-review the document
• Prepare a 2-sentence summary of your biggest open question
• Identify which sections you expect the most pushback on
• Schedule the review with adequate lead time (minimum 3 business days for reviewers to read)

See the design-review skill for the full review process.

Mechanisms Over Good Intentions

Common Rationalizations

Red Flags

• Design document has no alternatives considered (author didn't explore the space)
• All alternatives are strawmen with obvious flaws (rigged comparison)
• For a high-criticality workload (≥ 99.99% availability target, RPO < 5s, RTO < 30s, FSI/regulated, ultra-scale, multi-tenant), the alternatives list does not include any pattern from patterns/INDEX.md despite plausibly matching criteria — the author skipped the catalog
• An adopted pattern's Skill Impact Map is not reflected in Architecture, Operational Concerns, or Testing
• No explicit trade-offs stated (author is hiding complexity)
• Non-functional requirements are missing or vague ("high availability," "fast")
• Cost section says "TBD" or is missing entirely
• No operational concerns section (author hasn't thought about running this in production)
• Document was written after implementation started (post-hoc justification, not design)
• No link to Working Backwards output (solving an unvalidated problem)
• Testing strategy only mentions unit tests (no integration, load, or chaos)
• Architecture diagram is a single box labeled "Service" (insufficient detail)
• Document is over 20 pages (scope is too large; break into smaller designs)

Verification

• Document links to an approved Working Backwards artifact
• Functional requirements are testable and prioritized (P0/P1/P2)
• Non-functional requirements include specific numeric targets
• Architecture section includes both high-level and detailed component design
• Data model covers access patterns and growth projections
• At least 2 genuine alternatives are described with real pros/cons
• When patterns/INDEX.md contains a pattern that plausibly fits the workload, it appears as a real alternative (or is the chosen approach) — not silently skipped
• If a pattern is adopted, its Skill Impact Map is reflected in Architecture, Operational Concerns, and Testing
• Trade-offs are explicit ("choosing X over Y because Z, accepting W")
• Cost estimation includes infrastructure, operational, and development costs
• Operational concerns cover deployment, rollback, monitoring, and on-call
• Testing strategy covers unit, integration, load, and chaos testing
• Document has been pre-reviewed by at least one peer before BR review
• No sections contain "TBD" or placeholder text

Tenets

1. Writing is thinking. The document is not overhead; it IS the design work. If you skip the document, you skip the thinking.
2. Explicit trade-offs over implicit assumptions. Every choice has a cost. Name it. If you can't articulate what you're giving up, you don't understand your design.
3. Alternatives are not decorative. You must genuinely consider other approaches. If your alternative section reads like a formality, you've failed to explore the space.
4. Operational reality over architectural elegance. A beautiful design that can't be operated at 3am by an on-call engineer is a bad design.
5. The document serves future engineers. Six months from now, someone will ask "why did we do it this way?" Your document is the answer. Write for them.