How to Write a Good HLD and LLD Document
Writing HLD and LLD is a learnable skill. Here is how to write documents that developers actually use.
How to Write a Good HLD and LLD Document
A good HLD or LLD document is not long. It is clear. The point is to enable others to build without questions.
1. Start With the Goal
Open with one sentence on what the system does and why. "This is a backend that lets users sign up, swipe, match, and chat in real time." Sets the context for everything that follows.
2. Use Diagrams for HLD
HLD should have an architecture diagram. Boxes for components, arrows for communication, labels for protocols. A good diagram beats three paragraphs.
3. Use Tables for Schemas in LLD
For each collection, use a table: field name, type, required, default, constraints. Tables are scannable. Paragraphs are not.
4. Show API Contracts in LLD
For each endpoint: HTTP method, path, request body shape, response shape, status codes. Use code blocks with JSON examples. No prose where a code block will do.
5. Write the Logic as Steps
Numbered steps for what each handler does. "1. Validate body. 2. Check duplicate. 3. Hash password. 4. Save user. 5. Send email async. 6. Return 201." This is what the developer codes to.
6. List Error Cases Explicitly
400 (validation), 404 (not found), 409 (conflict), 500 (internal). Each error case has a reason and a sample response. Do not leave errors implicit.
7. Include Edge Cases
Empty states, duplicates, race conditions, large inputs, concurrent edits. Each edge case has a behavior the developer should implement.
8. Keep It Short
Skip the fluff. No "we will use MongoDB because it is a popular NoSQL database." Use MongoDB because it fits the data shape; one line is enough.
9. Update the Doc
Designs evolve. Update HLD when the architecture changes. Update LLD when you find edge cases during coding. The doc is alive.
10. Make It Reviewable
Share the doc with a teammate before coding. If they ask questions, the doc is missing something. Fix the doc, not just answer the question.
The Takeaway
A good HLD or LLD starts with a goal, uses diagrams and tables, shows API contracts, writes logic as steps, lists errors and edge cases, is short, is updated, and is reviewable. The test: can a developer code from it without asking questions?
Start with the goal in one sentence. Use an architecture diagram. Label components, communication, and protocols. Keep it short. Skip the fluff about why popular tools are popular.
Use tables for schemas. Show API contracts with method, path, body, response, and status codes. Write logic as numbered steps. List error cases explicitly. Include edge cases. The goal is a developer can code from it without questions.
As short as possible while being clear. HLD often fits on one page with a diagram. LLD is longer because it covers every endpoint, but each section should be just the necessary detail.
Yes. Designs evolve. When you find edge cases during coding, add them to the LLD. When the architecture changes, update the HLD. The doc is alive.
Share it with a teammate before coding. If they ask questions, the doc is missing something. Fix the doc, not just answer the question. The test is: can a developer code from it without questions?
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