Service Guide

Service Layer Implementation Guide

Guide for implementing Backend.AI service layer with Actions, Processors, and Service methods.

Standard Operations

Services implement 6 standard operations:

1. create - Create new entity
2. get - Retrieve single entity
3. search - Query with filters
4. update - Update entity
5. delete - Delete entity
6. purge - Permanently remove entity

Multi-target: batch_* (atomic, one statement) vs bulk_* (per-row, partial failures). See /repository-guide.

Method naming (no prefix):

await service.create_user(action: CreateUserAction)
await service.user(action: GetUserAction)
await service.search_users(action: SearchUsersAction)
await service.update_user(action: UpdateUserAction)
await service.delete_user(action: DeleteUserAction)
await service.purge_user(action: PurgeUserAction)

Architecture

API Handler → Processor → Service → Repository

Components:

• Actions: Immutable dataclasses (operations)
• ActionResults: Immutable dataclasses (results)
• Processors: Orchestrate with hooks and metrics
• Service Methods: Business logic
• Repositories: Data access

Directory Structure

services/{domain}/
├── types.py              # Operation enums
├── service.py            # Service protocol + implementation
├── processors.py         # Processor package
└── actions/
    ├── base.py          # Base action classes
    ├── create.py        # Create action
    ├── get.py           # Get action
    ├── search.py        # Search action
    ├── update.py        # Update action
    ├── delete.py        # Delete action
    └── purge.py         # Purge action

Example: src/ai/backend/manager/services/storage_namespace/

Actions and ActionResults

Pattern:

• Frozen dataclasses for immutability
• Base classes define entity_id() and operation_type()
• Concrete actions for each operation (create, get, search, update, delete, purge)
• Corresponding ActionResult types

See complete examples:

• services/storage_namespace/actions/ - Full action implementations
• services/storage_namespace/actions/base.py - Base classes

Processors

Pattern:

• ActionProcessor wraps service methods
• Provides hooks and metrics integration
• Expose processors via properties
• Initialize with service protocol

See complete examples:

• services/storage_namespace/processors.py - Processor package
• services/processors.py - Base processor classes

Service Implementation

Pattern:

• Define Protocol with all operation methods
• Implement service class accepting Actions, returning ActionResults
• Follow: validate → business rules → repository call → return result

Operation patterns:

• create_{entity}: Validate → check existence → create
• {entity}: Fetch → check not found → return
• search_{entities}: Build scope/filters → repository.search
• update_{entity}: Validate → update → return
• delete_{entity}: Soft delete (status change)
• purge_{entity}: Hard delete

Repository Call Pattern

Core principle (preferred):

• Service method SHOULD call a single repository method
• Avoid combining multiple repository calls in service layer
• If complex composition is needed, consider adding specialized repository method

Transaction boundary:

• DB sessions are created and managed at repository method level
• Service methods do NOT create transactions directly
• Repository uses begin_session() (write) or begin_readonly_session() (read)
• Each repository public method defines its own transaction scope

Exception cases:

• Multi-repository coordination is acceptable when business logic requires
• Example: Creating domain requires both domain and user_group repositories
• Document the reason when combining multiple repository calls

See complete examples:

• services/storage_namespace/service.py:38-81 - Single repository call pattern
• services/domain/service.py - Multi-repository coordination (exceptional case)
• repositories/README.md:54-61 - Service-Repository integration principles

Implementation Workflow

Step 1: Define Operations Enum

File: services/{domain}/types.py

Define StrEnum with 6 operations: CREATE, GET, SEARCH, UPDATE, DELETE, PURGE

See: services/storage_namespace/types.py

Step 2: Create Base Actions

File: services/{domain}/actions/base.py

Define base classes with entity_id() and operation_type()

Step 3: Implement Concrete Actions

Files: services/{domain}/actions/{operation}.py

For each operation: create, get, search, update, delete, purge

Step 4: Define Service Protocol

File: services/{domain}/service.py

Protocol with all standard operations

Step 5: Implement Service

Same file

• Initialize with repository
• Implement all operations
• Follow: validate → business rules → repository → return result

Step 6: Create Processor Package

File: services/{domain}/processors.py

• Processor for each operation
• Expose via properties

Step 7: Write Tests

See: /test-guide for complete testing workflow

Critical: Service tests MUST mock repositories (no real DB)

Step 8: Integrate with API

See: /api-guide for REST and GraphQL integration

Common Patterns

Complete examples: src/ai/backend/manager/services/storage_namespace/service.py

Best Practices

Action Design

• Immutable: @dataclass(frozen=True)
• Complete: All operation params
• Self-describing names

Service Method Design

• Single responsibility
• Explicit exceptions (inherit BackendAIError)
• Repository coordination only
• Always return ActionResult
• Single repository call per method (preferred pattern)
• Transaction boundary managed by repository

Processor Design

• Thin orchestration
• Use ActionProcessor for hooks
• Property pattern

Testing

• Mock repositories (never real DB)
• Test business logic
• Verify repository calls

See: /test-guide skill

Related Documentation

• Repository Layer: /repository-guide - Data access
• API Integration: /api-guide - REST/GraphQL
• Testing: /test-guide - Scenario-first testing
• Service README: src/ai/backend/manager/services/README.md

Examples

Complete implementations:

• src/ai/backend/manager/services/storage_namespace/ - Full service
• src/ai/backend/manager/services/domain/ - Multi-repository
• src/ai/backend/manager/services/auth/ - Hook integration

Summary

Standard operations:

• create, get, search, update, delete, purge
• batch_update, batch_delete, batch_purge

Components:

• Actions/ActionResults (immutable)
• Processors (orchestration)
• Service methods (business logic)

Testing: Mock repositories (no real DB)

Next steps:

1. Study example services
2. Define actions and operations
3. Implement service methods
4. Create processors
5. Write tests (/test-guide)
6. Integrate with API (/api-guide)