What do the shop modules share in the example?

A confirmed order reference with a coordinated contract.

What belongs in a Shared Kernel?

Only a deliberately shared and maintained model fragment with identical business meaning.

When is separating the models worthwhile?

When shared changes cost more coordination than the shared semantics provide value.

Standalone conceptConceptChapter I · Domain · Building block 17 of 18

Shared Kernel

The shared part stays small, visibly versioned and protected by tests and direct team coordination.

By Michael Meierhoff · Meierhoff Systems

KernelSmall shared model fragment.
ResponsibilityTeams coordinate changes.
TestsProtect the shared contract.

Context

Why this is important

Keep a jointly owned model fragment small and coordinate its change explicitly.

Strategic DDD and collaboration around business models.

Shop example · Mapping

Shared Kernel and separate models

The shop remains a modular monolith with seven business modules and a shared deployment.

This is how you read the picture

Kernel Scope
Responsibility Collaboration
Tests Compatibility

Shared Kernel and separate models
Criterion	Shared Kernel	Separate models
Meaning	Exactly shared	Translated per context
Change	Coordinate jointly	Evolve independently
Coupling	Higher	Lower

The choice weighs shared semantics against coordination effort.

Benefits & limitations

Helps if:

A small business concept must be shared exactly for a clear reason.
The teams involved can plan, test and release changes together.

Boundaries

A shared codebase couples model and release decisions.
As the shared part grows, separate models with explicit translation are often clearer.

In practice

Name the shared domain fragment and check whether both contexts truly need the same meaning. Record that shared meaning and its maintenance ownership in an agreement for both teams.
Limit the kernel to necessary types and rules, version changes transparently, and establish shared tests. Agree which teams approve changes and verify compatible releases.
For each extension, assess whether shared semantics still provide more value than the coordination effort. Remove elements shared only for convenience or given a different meaning in one context.

Read the detailed explanation

A Shared Kernel is a deliberate exception to otherwise separate models. Teams define which types or rules belong in it and how changes are reviewed together. Sharing reduces translation effort but couples release and model decisions. A large shared library can become a hidden integration contract when ownership and change cadence diverge.

Shared Kernel is a Context Map relationship. A Context Map frames collaboration; each Bounded Context defines its model boundary.

Check your understanding

Test yourself in three stages

ReadingCheck in the picture
What do the shop modules share in the example?
View the solution with justification
A confirmed order reference with a coordinated contract.
ApplyingTransferred to a new situation
What belongs in a Shared Kernel?
View the solution with justification
Only a deliberately shared and maintained model fragment with identical business meaning.
EvaluateWeighing an alternative
When is separating the models worthwhile?
View the solution with justification
When shared changes cost more coordination than the shared semantics provide value.

Explore further

These building blocks continue the thoughts of this page.

References

Continue on the learning path

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18 Building blocks

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