Low-Level Design (LLD)

Low-Level Design Document 
 
 Project: {{PROJECT_NAME}}
 Module/Component: {{MODULE_NAME}}
 Version: {{VERSION}}
 Date: {{DATE}}
 Author: {{AUTHOR}}
 Status: Draft | In Review | Approved
 Reviewers: {{REVIEWERS}}
 Related HLD: HLD Document 
 
 Document History 
 
 
 
 Version 
 Date 
 Author 
 Changes 
 
 
 
 
 0.1 
 {{DATE}} 
 {{AUTHOR}} 
 Initial draft 
 
 
 
 
 1. Module Overview 

 Module: {{MODULE_NAME}} 
 Service/Repo: {{REPO_NAME}} 
 Team Owner: {{TEAM_NAME}} 
 Single Responsibility: {{ONE_SENTENCE_WHAT_THIS_MODULE_DOES}} 
 Boundaries: 
 
 Owns: {{WHAT_THIS_MODULE_OWNS}} 
 Does NOT own: {{WHAT_THIS_MODULE_DOES_NOT_OWN}} 
 Delegates to: {{WHAT_IT_DELEGATES}} 
 
 Key Business Rules: 
 
 {{BUSINESS_RULE_1}} 
 {{BUSINESS_RULE_2}} 
 {{BUSINESS_RULE_3}} 
 
 
 2. Class / Module Diagram 

 classDiagram
 class {{ServiceClassName}} {
 -repository: {{RepositoryInterface}}
 -eventBus: EventBus
 -logger: Logger
 +create(dto: Create{{Entity}}Dto): Promise~{{Entity}}~
 +findById(id: string): Promise~{{Entity}} | null~
 +findAll(filter: {{Filter}}Dto): Promise~PaginatedResult~{{Entity}}~~
 +update(id: string, dto: Update{{Entity}}Dto): Promise~{{Entity}}~
 +delete(id: string): Promise~void~
 -validate(dto: unknown): void
 -publishEvent(event: {{DomainEvent}}): Promise~void~
 }

 class {{RepositoryInterface}} {
 <<interface>>
 +findById(id: string): Promise~{{Entity}} | null~
 +findMany(filter: {{Filter}}): Promise~{{Entity}}[]~
 +create(data: Partial~{{Entity}}~): Promise~{{Entity}}~
 +update(id: string, data: Partial~{{Entity}}~): Promise~{{Entity}}~
 +delete(id: string): Promise~void~
 +count(filter: {{Filter}}): Promise~number~
 }

 class {{Entity}} {
 +id: string
 +createdAt: Date
 +updatedAt: Date
 +deletedAt: Date | null
 {{FIELD_1}}: {{TYPE_1}}
 {{FIELD_2}}: {{TYPE_2}}
 +isDeleted(): boolean
 +toJSON(): {{EntityJSON}}
 }

 class Create{{Entity}}Dto {
 {{FIELD_1}}: {{TYPE_1}}
 {{FIELD_2}}: {{TYPE_2}}
 }

 class {{Controller}} {
 -service: {{ServiceClassName}}
 +POST /{{resource}}(body: Create{{Entity}}Dto): Response
 +GET /{{resource}}/:id(): Response
 +GET /{{resource}}(query: {{Filter}}Dto): Response
 +PUT /{{resource}}/:id(body: Update{{Entity}}Dto): Response
 +DELETE /{{resource}}/:id(): Response
 }

 {{ServiceClassName}} --> {{RepositoryInterface}}
 {{ServiceClassName}} --> {{Entity}}
 {{Controller}} --> {{ServiceClassName}}
 {{RepositoryInterface}} ..> {{Entity}}
 
 
 3. Database Schema 

 3.1 Tables 
 {{TABLE_NAME_1}} 

 Purpose: {{TABLE_PURPOSE}} 
 
 
 
 Column 
 Type 
 Nullable 
 Default 
 Constraints 
 Description 
 
 
 
 
 id 
 UUID 
 NO 
 gen_random_uuid() 
 PK 
 Primary key 
 
 
 created_at 
 TIMESTAMPTZ 
 NO 
 NOW() 
 
 Record creation timestamp 
 
 
 updated_at 
 TIMESTAMPTZ 
 NO 
 NOW() 
 
 Last update timestamp 
 
 
 deleted_at 
 TIMESTAMPTZ 
 YES 
 NULL 
 
 Soft delete timestamp 
 
 
 {{COLUMN_1}} 
 {{TYPE}} 
 {{YES/NO}} 
 {{DEFAULT}} 
 {{CONSTRAINTS}} 
 {{DESCRIPTION}} 
 
 
 {{COLUMN_2}} 
 {{TYPE}} 
 {{YES/NO}} 
 {{DEFAULT}} 
 {{CONSTRAINTS}} 
 {{DESCRIPTION}} 
 
 
 {{FK_COLUMN}} 
 UUID 
 NO 
 
 FK → {{OTHER_TABLE}}(id) 
 Reference to {{OTHER_TABLE}} 
 
 
 
 Indexes: 
 
 
 
 Index Name 
 Columns 
 Type 
 Rationale 
 
 
 
 
 {{TABLE_NAME_1}}_pkey 
 id 
 B-tree (PK) 
 Primary key lookup 
 
 
 idx_{{TABLE_NAME_1}}_{{COLUMN}} 
 {{COLUMN}} 
 B-tree 
 {{QUERY_RATIONALE}} 
 
 
 idx_{{TABLE_NAME_1}}_deleted_at 
 deleted_at 
 Partial (WHERE deleted_at IS NULL) 
 Soft-delete filter performance 
 
 
 
 {{TABLE_NAME_2}} 
 Purpose: {{TABLE_PURPOSE}} 
 
 
 
 Column 
 Type 
 Nullable 
 Default 
 Constraints 
 Description 
 
 
 
 
 id 
 UUID 
 NO 
 gen_random_uuid() 
 PK 
 Primary key 
 
 
 {{COLUMN_1}} 
 {{TYPE}} 
 {{YES/NO}} 
 {{DEFAULT}} 
 {{CONSTRAINTS}} 
 {{DESCRIPTION}} 
 
 
 
 3.2 Enums 
 CREATE TYPE {{ENUM_NAME}} AS ENUM (
 '{{VALUE_1}}',
 '{{VALUE_2}}',
 '{{VALUE_3}}'
);
 
 3.3 Migration Notes 

 
 Migration file: {{MIGRATION_FILE_NAME}}.sql 
 Zero-downtime: {{YES_NO}} — {{NOTES}} 
 Backfill required: {{YES_NO}} — {{BACKFILL_STRATEGY}} 
 Estimated migration time: {{ESTIMATE}} 
 
 
 4. API Contract 

 Base Path: /api/v{{VERSION}}/{{RESOURCE}} 
 POST /{{resource}} 
 Summary: Create a new {{entity}} 
 Authentication: Bearer JWT required 
 Request Body: 
 {
 "{{field1}}": "{{type_and_example}}",
 "{{field2}}": "{{type_and_example}}"
}
 
 Success Response — 201 Created : 
 {
 "id": "550e8400-e29b-41d4-a716-446655440000",
 "{{field1}}": "{{value}}",
 "createdAt": "2024-01-01T00:00:00.000Z"
}
 
 Error Responses: 
 
 
 
 Status 
 Code 
 Description 
 
 
 
 
 400 
 VALIDATION_ERROR 
 Request body fails validation 
 
 
 401 
 UNAUTHORIZED 
 Missing or invalid JWT 
 
 
 403 
 FORBIDDEN 
 Insufficient permissions 
 
 
 409 
 CONFLICT 
 {{DUPLICATE_FIELD}} already exists 
 
 
 422 
 BUSINESS_RULE_VIOLATION 
 {{BUSINESS_RULE}} not met 
 
 
 500 
 INTERNAL_ERROR 
 Unexpected server error 
 
 
 
 
 GET /{{resource}}/:id 
 Summary: Retrieve a {{entity}} by ID 
 Path Parameters: 
 
 
 
 Parameter 
 Type 
 Description 
 
 
 
 
 id 
 UUID 
 The {{entity}} identifier 
 
 
 
 Success Response — 200 OK : 
 {
 "id": "550e8400-e29b-41d4-a716-446655440000",
 "{{field1}}": "{{value}}",
 "createdAt": "2024-01-01T00:00:00.000Z"
}
 
 Error Responses: 
 
 
 
 Status 
 Code 
 Description 
 
 
 
 
 401 
 UNAUTHORIZED 
 Missing or invalid JWT 
 
 
 404 
 NOT_FOUND 
 {{entity}} not found 
 
 
 
 
 GET /{{resource}} 
 Summary: List {{entities}} with pagination and filtering 
 Query Parameters: 
 
 
 
 Parameter 
 Type 
 Default 
 Description 
 
 
 
 
 page 
 integer 
 1 
 Page number (1-based) 
 
 
 limit 
 integer 
 20 
 Items per page (max: 100) 
 
 
 sort 
 string 
 createdAt:desc 
 Sort field and direction 
 
 
 {{filter1}} 
 {{type}} 
 — 
 Filter by {{filter1}} 
 
 
 
 Success Response — 200 OK : 
 {
 "data": [],
 "pagination": {
 "page": 1,
 "limit": 20,
 "total": 0,
 "totalPages": 0
 }
}
 
 
 5. Algorithm Specifications 

 5.1 {{ALGORITHM_NAME}} 
 Purpose: {{WHAT_IT_DOES}}
 Complexity: Time O({{TIME_COMPLEXITY}}) | Space O({{SPACE_COMPLEXITY}}) 
 function {{algorithmName}}(input: {{InputType}}): {{OutputType}}
 // Step 1: Validate input
 if input is null or invalid then
 throw ValidationError("{{VALIDATION_MESSAGE}}")
 end if

 // Step 2: {{STEP_2_DESCRIPTION}}
 result = initialize()
 for each item in input.items do
 if item.satisfies({{CONDITION}}) then
 result.add(transform(item))
 end if
 end for

 // Step 3: {{STEP_3_DESCRIPTION}}
 sorted = result.sortBy({{SORT_CRITERIA}})

 // Step 4: Apply business rules
 for each rule in {{BUSINESS_RULES}} do
 sorted = rule.apply(sorted)
 end for

 return sorted
end function
 
 Edge Cases: 
 
 Empty input: Return empty result, do not throw 
 Single item: {{SINGLE_ITEM_BEHAVIOR}} 
 Maximum items: Limit to {{MAX_ITEMS}}, log warning if exceeded 
 
 
 6. Sequence Diagrams 

 6.1 Create {{Entity}} Flow 
 sequenceDiagram
 autonumber
 actor Client
 participant GW as API Gateway
 participant Auth as Auth Middleware
 participant SVC as {{ServiceClassName}}
 participant DB as Database
 participant MQ as Message Queue
 participant Email as Email Service

 Client->>GW: POST /{{resource}} {body}
 GW->>Auth: Validate JWT
 Auth-->>GW: User context {userId, roles}
 GW->>SVC: create(dto, userContext)
 SVC->>SVC: validate(dto)
 alt Validation fails
 SVC-->>Client: 400 ValidationError
 end
 SVC->>DB: BEGIN TRANSACTION
 SVC->>DB: INSERT INTO {{TABLE_NAME_1}}
 DB-->>SVC: {{entity}} record
 SVC->>DB: COMMIT
 SVC->>MQ: publish("{{ENTITY_CREATED_EVENT}}", event)
 MQ->>Email: Trigger confirmation email
 Email-->>Client: Email delivered async
 SVC-->>GW: {{entity}} dto
 GW-->>Client: 201 Created
 
 6.2 {{SECONDARY_FLOW_NAME}} Flow 
 sequenceDiagram
 autonumber
 actor Client
 participant SVC as {{ServiceClassName}}
 participant Cache as Redis
 participant DB as Database

 Client->>SVC: GET /{{resource}}/:id
 SVC->>Cache: GET {{resource}}:{{id}}
 alt Cache hit
 Cache-->>SVC: Cached {{entity}}
 SVC-->>Client: 200 OK (from cache)
 else Cache miss
 Cache-->>SVC: null
 SVC->>DB: SELECT * FROM {{TABLE_NAME_1}} WHERE id = $1
 alt Not found
 DB-->>SVC: null
 SVC-->>Client: 404 Not Found
 end
 DB-->>SVC: {{entity}} record
 SVC->>Cache: SET {{resource}}:{{id}} TTL={{CACHE_TTL}}
 SVC-->>Client: 200 OK
 end
 
 
 7. State Diagrams 

 stateDiagram-v2
 [*] --> {{STATE_DRAFT}} : Created

 {{STATE_DRAFT}} --> {{STATE_PENDING}} : submit()
 {{STATE_DRAFT}} --> [*] : delete()

 {{STATE_PENDING}} --> {{STATE_ACTIVE}} : approve()
 {{STATE_PENDING}} --> {{STATE_REJECTED}} : reject(reason)
 {{STATE_PENDING}} --> {{STATE_DRAFT}} : recall()

 {{STATE_ACTIVE}} --> {{STATE_SUSPENDED}} : suspend(reason)
 {{STATE_ACTIVE}} --> {{STATE_COMPLETED}} : complete()
 {{STATE_ACTIVE}} --> {{STATE_CANCELLED}} : cancel(reason)

 {{STATE_SUSPENDED}} --> {{STATE_ACTIVE}} : reactivate()
 {{STATE_SUSPENDED}} --> {{STATE_CANCELLED}} : cancel(reason)

 {{STATE_REJECTED}} --> {{STATE_DRAFT}} : revise()
 {{STATE_COMPLETED}} --> [*]
 {{STATE_CANCELLED}} --> [*]
 
 State Transition Rules: 
 
 
 
 From 
 To 
 Trigger 
 Guard Condition 
 Side Effect 
 
 
 
 
 DRAFT 
 PENDING 
 submit() 
 All required fields populated 
 Notify reviewers 
 
 
 PENDING 
 ACTIVE 
 approve() 
 Approver has {{PERMISSION}} role 
 Send welcome email 
 
 
 ACTIVE 
 SUSPENDED 
 suspend(reason) 
 Admin only 
 Log audit event 
 
 
 
 
 8. Error Handling Strategy 

 8.1 Error Classification 
 
 
 
 Error Type 
 HTTP Status 
 Retry? 
 Log Level 
 Alert? 
 
 
 
 
 ValidationError 
 400 
 No 
 INFO 
 No 
 
 
 UnauthorizedError 
 401 
 No 
 WARN 
 No 
 
 
 ForbiddenError 
 403 
 No 
 WARN 
 Suspicious patterns only 
 
 
 NotFoundError 
 404 
 No 
 INFO 
 No 
 
 
 ConflictError 
 409 
 No 
 WARN 
 No 
 
 
 ExternalServiceError 
 502 
 Yes (3x) 
 ERROR 
 Yes (if sustained) 
 
 
 DatabaseError 
 500 
 Yes (1x) 
 ERROR 
 Yes 
 
 
 UnexpectedError 
 500 
 No 
 ERROR 
 Yes 
 
 
 
 8.2 Error Response Format (RFC 7807) 
 {
 "type": "https://api.{{DOMAIN}}/errors/{{ERROR_CODE}}",
 "title": "{{Human-readable error title}}",
 "status": 400,
 "detail": "{{Specific error message}}",
 "instance": "/{{resource}}/{{id}}",
 "traceId": "{{TRACE_ID}}"
}
 
 8.3 Retry & Fallback Strategy 
 External API call failure:
 → Retry with exponential backoff: [1s, 2s, 4s]
 → Max retries: 3
 → Circuit breaker: Open after 5 failures in 60s window
 → Fallback: {{FALLBACK_BEHAVIOR}}
 → Alert: PagerDuty if circuit remains open > 5 minutes
 
 
 9. Concurrency & Thread Safety 

 
 
 
 Concern 
 Scenario 
 Mitigation 
 
 
 
 
 Duplicate creation 
 Two requests create same {{entity}} simultaneously 
 Unique constraint on {{UNIQUE_FIELD}} + catch constraint error → 409 
 
 
 Optimistic locking 
 Two updates to same record 
 version column + check-and-increment 
 
 
 Race condition in {{FLOW}} 
 {{RACE_CONDITION_DESCRIPTION}} 
 {{MITIGATION}} (e.g., DB-level lock, Redis SETNX) 
 
 
 
 
 10. Performance Considerations 

 
 
 
 Operation 
 Target (p99) 
 Current Baseline 
 Optimization 
 
 
 
 
 GET /{{resource}}/:id 
 < 50ms 
 {{BASELINE}} 
 Redis cache (TTL={{TTL}}) 
 
 
 GET /{{resource}} (list) 
 < 200ms 
 {{BASELINE}} 
 Indexed query + cursor pagination 
 
 
 POST /{{resource}} 
 < 300ms 
 {{BASELINE}} 
 Async event publishing 
 
 
 Bulk import (1000 items) 
 < 5s 
 {{BASELINE}} 
 Batch insert in chunks of 100 
 
 
 
 Known bottlenecks: 
 
 {{BOTTLENECK_1}}: {{MITIGATION}} 
 {{BOTTLENECK_2}}: {{MITIGATION}} 
 
 
 11. Dependencies 

 Internal Dependencies 
 
 
 
 Dependency 
 Type 
 Purpose 
 Fallback if unavailable 
 
 
 
 
 {{INTERNAL_SERVICE_1}} 
 Synchronous 
 {{PURPOSE}} 
 {{FALLBACK}} 
 
 
 {{SHARED_LIB}} 
 Library 
 {{PURPOSE}} 
 N/A (required) 
 
 
 
 External Dependencies 
 
 
 
 Dependency 
 Version 
 Purpose 
 Fallback if unavailable 
 
 
 
 
 PostgreSQL 
 {{VERSION}} 
 Primary data store 
 None — module unavailable 
 
 
 Redis 
 {{VERSION}} 
 Caching + session 
 Degrade gracefully (skip cache) 
 
 
 {{EXTERNAL_API}} 
 v{{VERSION}} 
 {{PURPOSE}} 
 {{FALLBACK}} 
 
 
 
 
 12. Configuration Parameters 

 
 
 
 Variable 
 Type 
 Default 
 Required 
 Description 
 
 
 
 
 {{MODULE_NAME}}_MAX_PAGE_SIZE 
 integer 
 100 
 No 
 Maximum items per page 
 
 
 {{MODULE_NAME}}_CACHE_TTL_SECONDS 
 integer 
 300 
 No 
 Cache TTL in seconds 
 
 
 {{MODULE_NAME}}_RETRY_ATTEMPTS 
 integer 
 3 
 No 
 External API retry count 
 
 
 {{EXTERNAL_API_KEY_VAR}} 
 string 
 — 
 Yes 
 API key for {{EXTERNAL_SERVICE}} 
 
 
 {{DB_CONNECTION_VAR}} 
 string 
 — 
 Yes 
 Database connection string 
 
 
 
 
 13. Testing Approach 

 
 
 
 Test Type 
 Tool 
 Coverage Target 
 Location 
 
 
 
 
 Unit tests 
 Jest / Vitest 
 > {{UNIT_COVERAGE}}% 
 src/{{module}}/__tests__/unit/ 
 
 
 Integration tests 
 Supertest + TestContainers 
 Key flows 
 src/{{module}}/__tests__/integration/ 
 
 
 Contract tests 
 Pact 
 All public APIs 
 src/{{module}}/__tests__/contract/ 
 
 
 
 Key test scenarios: 
 
 Create {{entity}} — success path 
 Create {{entity}} — validation error (missing required field) 
 Create {{entity}} — conflict (duplicate {{UNIQUE_FIELD}}) 
 Get {{entity}} by ID — found 
 Get {{entity}} by ID — not found (404) 
 List {{entities}} — with filters, pagination, sorting 
 Update {{entity}} — success 
 Update {{entity}} — not found 
 Delete {{entity}} — success (soft delete) 
 State transition — valid transition 
 State transition — invalid transition (guard fails) 
 External service failure — circuit breaker triggers 
 Concurrent creation — duplicate constraint handled 
 
 
 Approval 
 
 
 
 Role 
 Name 
 Date 
 Signature 
 
 
 
 
 Author 
 
 
 
 
 
 Module Owner 
 
 
 
 
 
 Security Review 
 
 
 
 
 
 Tech Lead