HLD

High-Level Design

High-level design: the shape, the constraints, the risks.

Format.md
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# HLD: <System / Feature name>

- **Owner:** <name>
- **Status:** Draft | Reviewed | Approved
- **Last updated:** YYYY-MM-DD

## Overview

One paragraph: what this system does and the value it delivers.

## Scope

### In scope
- …
- …

### Out of scope
- …
- …

## Architecture

High-level diagram (link to a `.puml` in this repo) and a paragraph walking the
reader through the major components and their responsibilities.

```
[Client] -> [API Gateway] -> [Service A] -> [Datastore]
                          \-> [Service B] -> [Queue] -> [Worker]
```

### Components
- **<Component A>** — responsibility, tech, owner.
- **<Component B>** — responsibility, tech, owner.

## Key Flows

### Flow 1 — <name>
Step-by-step, or link to a sequence diagram.

### Flow 2 — <name>
…

## Non-Functional Requirements

| Concern        | Target                                |
|----------------|---------------------------------------|
| Performance    | p95 < 200ms                           |
| Scale          | 10k RPS sustained, 25k peak           |
| Availability   | 99.9% monthly                         |
| Security       | <auth model, data classification>     |
| Compliance     | <SOC2 / GDPR / HIPAA / …>             |

## Risks

| Risk                        | Likelihood | Impact | Mitigation                   |
|-----------------------------|------------|--------|------------------------------|
| <e.g. vendor lock-in>       | Medium     | High   | Abstract behind interface X  |

## Dependencies

- **Upstream:** systems we rely on.
- **Downstream:** systems that rely on us.
- **External:** vendors, third-party APIs, contractual obligations.

## Open Questions

- [ ] …

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Others in Architecture document templates

ADR — Architecture Decision RecordContext, decision, consequences. The alternatives section is the valuable one.
RFC — Request for CommentsFor proposing a change and inviting disagreement before building.
Tech Spec / LLDLow-level design. What an implementer needs that the HLD does not say.
NFR CatalogueThe requirements that get discovered late and expensively.
RunbookWhat to do at 3am. Written for someone tired and unfamiliar.
Discovery / Spike ReportFindings from a time-boxed investigation, including the dead ends.
Post-Mortem / Incident ReviewBlameless incident review. Timeline first, conclusions second.
Solution One-PagerA solution summary for people who will not read the HLD.