Low-Level Design (LLD) Template

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1. Problem Statement

• Clearly describe the problem in 2–3 lines
• Mention what needs to be achieved and why

Example:

Admin dashboard should display the count of pending transfers so that operations team can monitor incomplete transactions and take timely action.

2. Scope

• What is included
• What is excluded (if any)

3. Solution Approach (Technical Design)

3.1 Overview

• High-level explanation of the solution
• Mention whether:
  • Existing services will be reused
  • New services/APIs will be created
  • Existing services will be enhanced

3.2 Components / Modules Involved

• List all impacted modules:
  • Backend services
  • Frontend/UI
  • Database
  • External integrations (if any)

3.3 API Design

• For each API:
  • API Name:
  • Method: (GET/POST/PUT/DELETE)
  • Endpoint:
  • Request:
  • Response:
  • Auth Required: Yes/No
  • Service Handling It:

3.4 Data Flow

• Step-by-step flow of how data moves across systems
• Mention:
  • Source of data
  • Processing logic
  • Final output

3.5 Database Impact

• New tables (if any)
• Changes in existing tables
• New fields/columns
• Indexing (if needed)
• Query changes

3.6 Business Logic

• Core logic/conditions
• Rules to calculate/derive values

Example:

Pending transfer = transactions where status IN (initiated, processing, on-hold)

3.7 Service Design

• Number of services impacted/added
• Description of each:
  • New service
  • Modified service
  • Reusable service

3.8 Error Handling

• Failure scenarios
• Retry mechanisms
• Logging strategy
• User-facing error messages

3.9 Edge Cases

• List all possible edge cases

Example:

• Transfer stuck due to insufficient liquidity
• Partial transaction completion
• Duplicate requests

3.10 Performance Considerations

• Query optimization
• Caching (if required)
• Pagination / batching
• Expected load

3.11 Security Considerations

• Authentication / Authorization
• Data masking (if applicable)
• Sensitive data handling

4. Sequence Diagram

• Visual flow of request → processing → response
• Must include:
  • UI
  • Backend service(s)
  • DB
  • External systems (if any)

5. Unit Test Criteria

• Define test scenarios:

Example:

• Valid pending transfers count
• No data scenario
• Error response handling
• Edge case validation

6. Integration / QA Test Scenarios

• End-to-end scenarios
• APIUI validation

7. Dependencies

• External services
• Other modules
• Data availability

8. Assumptions

• Any assumptions made during design

9. Risks & Mitigation

• Potential risks
• How to handle them

10. Deployment / Rollout Plan

• Any migration required
• Backward compatibility
• Feature flag (if needed)

11. Open Questions

• Any pending clarifications