Context Mapping: How Contexts Relate

Context Mapping is the practice of identifying relationships between bounded contexts and choosing appropriate integration patterns.

TL;DR

Pattern	Relationship	When to Use
Partnership	Teams collaborate closely	Co-evolving contexts
Shared Kernel	Shared code/model	Tightly coupled teams
Customer-Supplier	Upstream serves downstream	Clear dependency
Conformist	Downstream adopts upstream model	No negotiating power
ACL	Translation layer	Protect from external models
OHS	Published API/protocol	Many consumers
Published Language	Standard interchange format	Industry standards
Separate Ways	No integration	Independent contexts

Key Patterns

Anti-Corruption Layer (ACL)

Protect your domain from messy external models:

// External legacy system (messy)
public class LegacyCustomerRecord
{
    public string CUST_ID { get; set; }
    public string CUST_NM { get; set; }
    public int CUST_TYP_CD { get; set; }
}

// ACL Translator
public class CustomerTranslator
{
    public Customer Translate(LegacyCustomerRecord legacy)
    {
        return new Customer(
            id: CustomerId.From(legacy.CUST_ID),
            name: new CustomerName(legacy.CUST_NM),
            type: TranslateType(legacy.CUST_TYP_CD)
        );
    }
}

Customer-Supplier

Upstream serves downstream with negotiating power:

// Downstream consumes Upstream API
public class InventoryClient
{
    public async Task<StockAvailability> CheckStock(ProductId productId)
    {
        var response = await _httpClient.GetAsync($"/api/stock/{productId}");
        return await response.Content.ReadFromJsonAsync<StockAvailability>();
    }
}

Open Host Service (OHS)

Published API for many consumers:

[ApiController]
[Route("api/v1/users")]
public class UsersController : ControllerBase
{
    [HttpGet("{userId}")]
    public async Task<ActionResult<UserDto>> GetUser(Guid userId)
    {
        // Published API for all consumers
        var user = await _userService.GetById(userId);
        return Ok(new UserDto { Id = user.Id, Email = user.Email });
    }
}

Choosing the Right Pattern

Do you control the upstream?
├── NO → Conformist or ACL
│         └── Is upstream model acceptable?
│             ├── YES → Conformist
│             └── NO → ACL
│
└── YES → Partnership, Customer-Supplier, or OHS
          └── How many consumers?
              ├── One → Customer-Supplier
              ├── Many → OHS
              └── Mutual dependency → Partnership

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