Writing Low-Level Designs: Principles, Patterns, and My Experience at Amazon

Why LLDs matter in software engineering?

Low-Level Designs (LLDs) play a crucial role in software engineering as they lay the foundation for actual code implementation and help align expectations across teams. While high-level designs capture the what and why of a system, LLDs focus on the how. They provide clarity on the internal workings of a feature or service, breaking down the logic, data structures, and interactions involved. A strong LLD reduces ambiguity, prevents miscommunication, and saves time during development, debugging, and testing.

---

What is a Low-Level Design (LLD)?

A Low-Level Design document is a detailed technical write-up that explains how a feature, service, or module will be implemented. It dives into class structures, data models, function flows, API contracts, and the interactions between various components. Unlike high-level design which speaks in terms of systems and architecture, an LLD operates at a more granular level, focusing on the actual logic that developers will write and maintain. It serves as a blueprint for implementation and is often reviewed collaboratively to ensure scalability, efficiency, and maintainability.

---

Key Principles of Effective LLD

An effective Low-Level Design doesn’t just describe the implementation, it communicates it clearly to anyone who reads it, whether it's your teammate, a reviewer, or your future self. Here are a few key principles to follow:

1. Clear and Simple Language

Your LLD should be easy to understand. Avoid overly complex jargon or unnecessary abstraction. Write it in a way that even someone unfamiliar with the exact feature can grasp the design. Stick to concise explanations and focus on why you made certain design choices.

2. Logical Flow and Structure

Organize your document with a logical progression, start with a summary or problem statement, followed by component-wise breakdowns, data structures, class diagrams, and flowcharts. This helps readers navigate the document efficiently.

3. Visuals: Diagrams and Flowcharts

A picture is worth a thousand lines of code. Use class diagrams, sequence diagrams, and flowcharts wherever needed. Visuals make it easier to understand interactions, flow of control, and responsibilities of different components.

4. Highlight the Grey Areas

No design is perfect or complete on day one. Call out assumptions, trade-offs, and edge cases you’re unsure about. This not only invites early feedback but also demonstrates awareness of potential pitfalls - something that's valued a lot in companies like Amazon.

5. Focus on Modularity and Scalability

Break your solution into reusable components. Mention how your design scales horizontally, vertically, or organizationally. Think from a long-term perspective.

---

Common Design Patterns in LLD

There are many ways to structure a Low-Level Design document, and over time, most companies and engineers tend to converge on patterns that prioritize clarity, completeness, and collaboration. While the content may vary slightly based on the team or project, the structure I’m sharing below is one of the most effective and comprehensive ones I’ve used, especially during my time at Amazon.

This pattern ensures that every stakeholder - engineers, reviewers, product managers, and even QA, has a clear understanding of what’s being built, why it's being built, and how it will be tested.

Title [ eg. [LLD] Implementing the new payment method for the EU region ]

Author : @Aayush
Contributors : ...

1. Glossary

2. Introduction

3. Background

4. Requirements
    4.1. In scope
    4.2. Out of scope

5. Current Architecture

6. Desired Architecture

7. Approaches
    7.1. Approach 1 [ Recommended ]
        - Pros
        - Cons

    7.2. Approach 2
        - Pros
        - Cons

    7.3. Approach 3
        - Pros
        - Cons

8. Testing Details

9. Important links and References

10. Open Questions [ Optional ]

11. Sign off table

12. Appendix

13. Discussions / MOMs

This format works well because it:

• Keeps discussions focused with clear scoping

• Encourages visibility of alternatives (which reviewers love)

• Drives collaboration via contributor fields and open questions

• Leaves a paper trail for future audits or revisits

During my time at Amazon, this template helped in multiple scenarios — from introducing small but critical changes in payment workflows to planning larger backend refactors. It allowed teams to catch edge cases early, align faster in reviews, and reduce ambiguity when implementation began.

---

My Amazon Experience with LLDs

My experience at Amazon has been incredibly valuable when it comes to writing Low-Level Designs and documentation. One key lesson I learned is just how important it is to surface ambiguities, pain points, and potential blockers before jumping into implementation. A well-prepared LLD acts as a thinking tool, it forces you to confront design trade-offs, edge cases, and integration challenges early on.

At Amazon, implementation is almost the final step. The real work begins with thorough discussions, deep dives, and collaborative reviews. We focus heavily on raising all the critical flags early in the design phase, which drastically reduces rework and surprises during development. Over time, I’ve realized that a solid LLD isn’t just documentation, it’s a shared understanding that sets the entire team up for success.

---

Tips to Write Better LLDs

Writing great LLDs is a skill that improves with practice, clarity of thought, and an understanding of real-world systems. Here are a few practical tips I’ve learned through experience:

1. Raise Concerns Early — Even if They Block Implementation

Don’t hesitate to document anything that seems uncertain, risky, or potentially blocking. One of the key purposes of an LLD is to surface these issues early so they can be discussed and resolved before actual development begins.

2. Use Highlights and Notes Effectively

Important notes, assumptions, or edge cases should be clearly marked using callouts, bold text, or even separate “Note” sections. This ensures readers and reviewers don’t miss critical information.

3. Treat It as a Living, Shareable Document

A good LLD isn’t just for the present, it becomes a reference for future developers who might work on the same module or feature. It saves time by providing context, decisions made, and reasons behind those decisions.

4. Deep-Dive into the Implementation

Understanding the nitty-gritty details of the system you’re designing for will reflect in your LLD. The more you dive into the actual implementation layers, the better your design and the easier it becomes to justify technical choices.

5. Review, Revisit, and Refactor

Just like code, LLDs can be improved. Post-discussion updates, review comments, and design pivots should all be captured to keep the document relevant and useful.

---

Conclusion

Low-Level Designs are a critical part of software engineering and often serve as the blueprint for successful implementation. If you want to grow as a developer, mastering the art of writing clear, thoughtful, and thorough LLDs can set you apart. It’s not just about documentation, it’s about thinking deeply, communicating effectively, and reducing ambiguity for everyone involved.

By investing in better LLDs, you not only improve your own understanding but also set a benchmark for your team, making it easier for others to follow, collaborate, and build on your work.