Strategic DDD

Domain discovery and boundary definition at the organizational level. Strategic DDD answers: "Where are the boundaries?" and "How do contexts relate?"

Subdomain Classification

Subdomains exist in the problem space. Classify before modeling.

Type	Competitive advantage	Build or buy	DDD investment	Staffing
Core	Yes, unique differentiator	Build in-house	Full strategic + tactical	Senior talent
Supporting	No, enables core	Build (simplified) or outsource	Pragmatic tactical	Mid-level
Generic	No, commodity	Buy/integrate (Stripe, Auth0)	Minimal/none	Junior/integration

Decision heuristic: If two competitors solve this the same way, it's Generic. If your approach gives you an edge, it's Core. Everything else is Supporting.

Bounded Context Discovery

A bounded context is a software boundary within which a model is consistent. Different from subdomains (problem-space). They should align but are independent concepts.

Discovery Techniques

Language divergence (primary signal): When the same word means different things to different groups, you've found a boundary. "Account" in billing vs. "Account" in identity. "Product" in catalog vs. "Product" in inventory.

Organizational structure: Teams that communicate frequently often share a context. Teams with formal handoffs often have separate contexts. Conway's Law applies.

Consistency requirements: Where true invariants must be maintained transactionally defines aggregate boundaries, which cluster into contexts.

EventStorming Big Picture: Domain events placed on timeline. When events cluster into distinct swimlanes with different actors and vocabulary, those are context candidates.

Domain Storytelling: Actors perform activities on work objects. When actors, activities, or work objects change meaning, you've crossed a boundary.

Boundary Validation Checklist

• Each context has its own ubiquitous language (no term means two things)
• One team owns one context (no shared ownership)
• Context is independently deployable (no mandatory co-deployment)
• Internal model changes don't ripple to other contexts
• Data that crosses boundaries is explicitly mapped (no implicit sharing)

Common Mistakes

1. Context = microservice: A context may contain multiple services; a service may handle a subdomain within a larger context
2. Too large: Conflicting models coexist, vocabulary is ambiguous -- split
3. Too small: Excessive cross-context communication, simple operations need sagas -- merge
4. Database-first: Sharing a database between contexts couples their models implicitly

Context Mapping Patterns

Nine patterns describing how bounded contexts relate. Draw the context map before writing code.

Mutually Dependent

Partnership: Two contexts evolve together. Both teams coordinate releases. Use when: genuinely reciprocal dependency, tight collaboration feasible. Risk: high coordination cost.

Shared Kernel: Small shared model subset both contexts depend on. Must be small, explicitly versioned, jointly owned. Changes require agreement from both teams.

Upstream/Downstream

Customer-Supplier: Upstream serves downstream's needs. Downstream negotiates what upstream provides. Structured relationship with explicit API contracts.

Conformist: Downstream adopts upstream's model as-is. No negotiation. Use when: integration simplicity outweighs design freedom, or upstream won't accommodate.

Anti-Corruption Layer (ACL): Downstream creates a translation layer to isolate its model from upstream's. Essential for: legacy integration, poor upstream models, protecting domain purity.

Open Host Service (OHS): Upstream provides a well-defined API for multiple downstream consumers. Standardized protocol (REST, gRPC, GraphQL).

Published Language: Shared schema or format for inter-context communication. Examples: iCalendar, vCard, JSON Schema, Protocol Buffers.

Independent

Separate Ways: No integration. Contexts have no meaningful dependency. Duplication is acceptable when coupling cost exceeds duplication cost.

Big Ball of Mud: Recognition pattern only. Demarcate and quarantine legacy systems. Draw a boundary around them, communicate through ACL.

Selection Heuristic

Do the teams need to coordinate releases?
  YES -> Do both depend on each other equally?
    YES -> Partnership (if feasible) or Shared Kernel (if small overlap)
    NO  -> Customer-Supplier (negotiate API)
  NO  -> Is upstream model acceptable?
    YES -> Conformist (adopt as-is)
    NO  -> Anti-Corruption Layer (translate)
  Independent? -> Separate Ways
  Legacy mess? -> Big Ball of Mud + ACL

Ubiquitous Language

A rigorous vocabulary shared between developers and domain experts, scoped per bounded context.

Rules

1. Domain terms in code: class names, method names, variable names reflect domain language
2. Same term, different context: "Order" in Sales vs. "Order" in Fulfillment are different concepts -- this is expected and healthy
3. When the model is hard to express in code, the language needs refinement
4. Resolve conflicts immediately: if developers and domain experts use different words for the same concept, pick one and enforce it

Building the Language

• Maintain a glossary per bounded context (not per system)
• Review glossary during modeling sessions
• When new terms emerge, add them; when terms change, update code
• Code review should catch language violations ("this variable should be called X, not Y")

Context Map Visualization

Use Mermaid flowchart for context maps:

flowchart LR
    subgraph Core
        OrderCtx[Order Context]
        CatalogCtx[Catalog Context]
    end
    subgraph Supporting
        NotifCtx[Notification Context]
    end
    subgraph Generic
        PaymentCtx[Payment Context]
        AuthCtx[Identity Context]
    end

    OrderCtx -->|Customer-Supplier| CatalogCtx
    OrderCtx -->|ACL| PaymentCtx
    OrderCtx -->|OHS| NotifCtx
    CatalogCtx -.->|Conformist| AuthCtx

Label every arrow with the context mapping pattern. Core contexts in the center.