High Level Design Doc

High-Level Design (HLD) Document Generator

Produce a single .docx document capturing the design reasoning behind a system — principles followed, decisions made and why, how key user journeys play out across components, and the non-functional bar the design targets. The structural "what components exist" view belongs in the companion architecture-document skill; this document assumes that context and builds on it.

Sections (in this order)

Cover page, Version History, Document Approval, Revision Log, and TOC are automatic — your sections array starts at "1. Design Principles".

Diagram Policy

Applies to the .docx path only — Markdown embeds Mermaid source directly instead of rendering anything (see "Output Format" below).

Prefer a real Mermaid diagram — rendered via mmdc and embedded with B.diagramImage() — over diagramPlaceholder(), but only once you have concrete structure to draw (real names, not "TBD"). Read references/diagram-generation.md when you're actually about to render one — it has the full build order, the bundled config (assets/mermaid-config.json), the exact mmdc flags, and token-saving tips. Skip it entirely if this document ends up needing no diagrams.

Missing Information Policy

Never invent design decisions, rationale, or NFR targets you have no basis for — a plausible-sounding fabricated rationale is worse than an honest gap, since a new team might treat it as the real reason a choice was made. Use toBeCompleted("...") for sections you lack real input for, explaining what's needed, and still generate everything else.

Output Format

Ask the user which output format they want, unless they've already said so in this request (e.g., "as a docx", "in markdown," "just give me an .md file") — a quick single-choice question is enough, don't block on it otherwise:

• Word document (.docx) — the default assumption if the person hasn't specified and their context suggests a formal deliverable. Follow "Using the builder" below.

• Markdown (.md) — no script needed, write the file directly. Use these conventions so it stays structurally equivalent to the docx version:

  • Front matter: instead of a cover page, open with the title as an # heading, the project name as an italic subtitle line, then a metadata table instead of separate Version History / Approval / Revision Log tables:

    # High-Level Design (HLD)
    *Acme Order Platform*
    
    | Field | Value |
    |---|---|
    | Version | 0.1 |
    | Author | Jane Doe |
    | Date | 2026-07-19 |
    | Status | Draft |
    | Approved By | Jane Doe (Tech Lead) |
    

  • Headings: #/##/###/#### matching the same section levels used in the table above — don't flatten everything to one level, that's what keeps the document skimmable and consistent with the docx version.

  • Tables: standard Markdown tables.

  • Diagrams — unlike the .docx path, don't render or embed an image here. Write the actual Mermaid source directly in a fenced code block; GitHub, GitLab, Obsidian, and most modern Markdown viewers render mermaid code blocks natively, so this is a real diagram, not a placeholder:

    ```mermaid
    flowchart TD
        A[Client] --> B[API Gateway]
        B --> C[Order Service]
        C --> D[(Database)]
    ```
    

    Only fall back to a text placeholder if you don't yet have concrete enough detail to draw something real (mirrors toBeCompleted above):

    > 📊 **DIAGRAM PLACEHOLDER — TO BE COMPLETED**
    > Not enough detail yet to draw the System Architecture Diagram — need
    > the actual component names and how they connect.
    

  • "To be completed" callout — same blockquote treatment:

    > ⚠️ **TO BE COMPLETED**
    > Explanation of what input is needed to fill this in.
    

  • Save as High_Level_Design.md instead of High_Level_Design.docx.

Workflow

Ask the output format first (see "Output Format" above). For Markdown, skip straight to writing the file using those conventions — the numbered steps below describe the .docx path.

1. Gather what's available — architecture decision records, past design discussions, or ask directly about key trade-offs made and NFR targets.
2. Draft each section as data using the builder functions below.
3. Build with scripts/docx_builder.js.
4. Skip PDF conversion by default — docx_builder.js is already tested and hardened (table widths and text alignment are enforced at the library level), so routine generations don't need a re-render just to confirm it worked. Only convert to PDF and view it if the user explicitly asks for visual verification, or if something about this generation is unusual (e.g., a new kind of content the library hasn't handled before, or a reported rendering problem). When you do need it: soffice --headless --convert-to pdf <file>.docx (or libreoffice --headless ...), then pdftoppm -jpeg -r 100 <file>.pdf page and view the images.
5. Save to /mnt/user-data/outputs/High_Level_Design.docx (or .md if that's the chosen format) and present it.

Using the builder (for the .docx path)

This library requires the docx npm package. Before running any script, check it's available with node -e "require('docx')"; if that fails, install it with npm install docx in the working directory first — don't assume it's pre-installed, since that varies by environment.

const B = require("./scripts/docx_builder.js");

const sections = [
  B.h1("1. Design Principles"),
  ...B.bullets(["Stateless services for horizontal scaling", "Event-driven communication between modules"]),

  B.h1("2. Key Design Decisions"),
  B.table(["Decision", "Rationale", "Alternatives Considered"],
    [["Chose PostgreSQL over DynamoDB", "Need for relational integrity across orders/inventory", "DynamoDB, MongoDB"]],
    [3200, 3200, 2600]),

  B.h1("3. Module Design"),
  B.h2("3.1 Order Service"),
  B.para("Owns order lifecycle state and validation."),

  // Prefer B.diagramImage(path, {caption}) with a real mmdc render when
  // possible (see "Diagram Policy" above) — diagramPlaceholder() is the fallback:
  B.diagramPlaceholder({
    name: "Order Placement Sequence Diagram",
    purpose: "Show the interaction between client, Order Service, Inventory Service, and the database during order placement.",
    recommendedContent: ["Client", "API Gateway", "Order Service", "Inventory Service", "Database"],
  }),

  B.h1("6. Non-Functional Requirements"),
  B.table(["Category", "Requirement"], [["Performance", "p95 latency < 200ms"]], [2400, 6800]),

  B.h1("8. Risks"),
  B.toBeCompleted("Risk register wasn't provided — ask for known risks and their mitigations, or check the project's risk log if one exists."),
];

await B.buildDocument("/mnt/user-data/outputs/High_Level_Design.docx", {
  docLabel: "High-Level Design",
  title: "High-Level Design (HLD)",
  subtitle: "Acme Order Platform",
  versionHistory: [["0.1", "2026-07-19", "Jane Doe", "Initial draft"]],
  approvers: [["Jane Doe", "Tech Lead", "", ""]],
  revisionLog: [],
  sections,
});

Available functions: h1/h2/h3/h4, para, bullets, table(headers, rows, widths), pageBreak, toBeCompleted(explanation), diagramPlaceholder({name, purpose, recommendedContent, notes}), and buildDocument(path, options). Always use heading functions for titles so Word's Table of Contents and Navigation Pane work correctly.

Success Criteria

A senior engineer new to the project should finish this document understanding not just what was built, but why it was built that way — able to make consistent decisions when extending the system later.