Shared Kernel

The Shared Kernel is a strategic pattern from Domain-Driven Design (DDD) that contains code shared across multiple bounded contexts (modules) in your application.

The Shared Kernel should be small, well-defined, and changed only with careful coordination, as modifications impact all modules that depend on it.

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What is the Shared Kernel?

In a modular hexagonal architecture, each module (bounded context) should be as independent as possible. However, some concepts are truly generic and used across multiple modules. The Shared Kernel is where you place these common building blocks.

Purpose

The Shared Kernel serves to:

1. Prevent Code Duplication - Avoid reimplementing the same value objects or utilities in every module
2. Ensure Consistency - Guarantee that common concepts (like Email, Money, UUID) behave identically everywhere
3. Maintain Independence - Allow modules to share code without creating tight coupling between them
4. Express Ubiquitous Language - Centralize domain concepts that transcend individual bounded contexts

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Directory Structure

src/
├── Module/                    # Your bounded contexts
│   ├── User/
│   │   └── Account/
│   ├── Blog/
│   │   └── Post/
│   └── Order/
│       └── Checkout/
└── Shared/                    # Shared Kernel
    ├── Domain/
    │   ├── ValueObject/       # Generic value objects
    │   │   ├── Email.php
    │   │   ├── Money.php
    │   │   ├── Uuid.php
    │   │   └── PhoneNumber.php
    │   ├── Exception/         # Base exceptions
    │   │   ├── DomainException.php
    │   │   └── ValidationException.php
    │   └── Event/             # Base event classes
    │       └── DomainEvent.php
    ├── Application/
    │   └── Service/           # Generic application services
    │       └── Clock.php      # Time abstraction
    └── Infrastructure/
        ├── Persistence/       # Generic persistence utilities
        │   └── Doctrine/
        │       └── Type/      # Custom Doctrine types
        └── Messaging/         # Shared messaging infrastructure

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What Belongs in Shared Kernel?

Good Candidates for Shared Kernel

Generic Value Objects

• Email - Used by User, Newsletter, Support modules
• Money - Used by Order, Invoice, Payment modules
• Uuid - Used across all modules for entity IDs
• PhoneNumber - Used by User, Shipping, Contact modules
• Address - Used by User, Order, Shipping modules (if truly generic)

Base Domain Concepts

• Abstract base exceptions (DomainException)
• Domain event interfaces
• Common enums (Country, Currency, Language)
• Measurement units (Weight, Distance)

Infrastructure Utilities

• Clock interface for testable time
• Common Doctrine custom types
• Shared event bus configuration

What Should NOT be in Shared

Context-Specific Logic

• UserEmail (includes user-specific validation like “no admin emails”) → Keep in User module
• OrderTotal (includes tax calculation logic) → Keep in Order module
• ProductPrice (includes pricing rules) → Keep in Product module

Business Rules

• Any validation that differs by context
• Behavior specific to one domain

Premature Abstractions

• Code used by only 1-2 modules (wait until 3+)
• “Might be shared someday” code

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Examples

Example 1: Shared Email Value Object

<?php
// src/Shared/Domain/ValueObject/Email.php

namespace App\Shared\Domain\ValueObject;

final readonly class Email
{
    public function __construct(private string $value)
    {
        if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
            throw new \InvalidArgumentException(
                "'{$value}' is not a valid email address"
            );
        }
    }

    public function getValue(): string
    {
        return $this->value;
    }

    public function equals(self $other): bool
    {
        return $this->value === $other->value;
    }

    public function __toString(): string
    {
        return $this->value;
    }
}

Used across multiple modules:

// User module - User entity
namespace App\User\Account\Domain\Model;

use App\Shared\Domain\ValueObject\Email;

final class User
{
    public function __construct(
        private UserId $id,
        private Email $email,  // ← Shared Email
        private string $name
    ) {}
}

// Newsletter module - Subscriber entity
namespace App\Newsletter\Domain\Model;

use App\Shared\Domain\ValueObject\Email;

final class Subscriber
{
    public function __construct(
        private SubscriberId $id,
        private Email $email,  // ← Same shared Email
        private bool $active
    ) {}
}

// Support module - Ticket entity
namespace App\Support\Ticket\Domain\Model;

use App\Shared\Domain\ValueObject\Email;

final class Ticket
{
    public function __construct(
        private TicketId $id,
        private Email $customerEmail,  // ← Same shared Email
        private string $subject
    ) {}
}

Example 2: Shared Money Value Object

<?php
// src/Shared/Domain/ValueObject/Money.php

namespace App\Shared\Domain\ValueObject;

final readonly class Money
{
    public function __construct(
        private int $amount,      // Store as cents/minor units
        private Currency $currency
    ) {
        if ($amount < 0) {
            throw new \InvalidArgumentException('Amount cannot be negative');
        }
    }

    public function getAmount(): int
    {
        return $this->amount;
    }

    public function getCurrency(): Currency
    {
        return $this->currency;
    }

    public function add(self $other): self
    {
        $this->assertSameCurrency($other);
        return new self($this->amount + $other->amount, $this->currency);
    }

    public function subtract(self $other): self
    {
        $this->assertSameCurrency($other);
        return new self($this->amount - $other->amount, $this->currency);
    }

    public function multiply(int $multiplier): self
    {
        return new self($this->amount * $multiplier, $this->currency);
    }

    private function assertSameCurrency(self $other): void
    {
        if (!$this->currency->equals($other->currency)) {
            throw new \InvalidArgumentException(
                'Cannot operate on different currencies'
            );
        }
    }

    public function equals(self $other): bool
    {
        return $this->amount === $other->amount
            && $this->currency->equals($other->currency);
    }
}

// src/Shared/Domain/ValueObject/Currency.php
enum Currency: string
{
    case USD = 'USD';
    case EUR = 'EUR';
    case GBP = 'GBP';
}

Used in Order and Invoice modules:

// Order module
use App\Shared\Domain\ValueObject\Money;
use App\Shared\Domain\ValueObject\Currency;

final class Order
{
    private Money $total;

    public function calculateTotal(): void
    {
        $this->total = new Money(0, Currency::USD);
        foreach ($this->items as $item) {
            $this->total = $this->total->add($item->getPrice());
        }
    }
}

// Invoice module
use App\Shared\Domain\ValueObject\Money;

final class Invoice
{
    public function __construct(
        private InvoiceId $id,
        private Money $amount,     // ← Shared Money
        private Money $taxAmount   // ← Shared Money
    ) {}
}

Example 3: Shared UUID Value Object

<?php
// src/Shared/Domain/ValueObject/Uuid.php

namespace App\Shared\Domain\ValueObject;

use Symfony\Component\Uid\Uuid as SymfonyUuid;

abstract readonly class Uuid
{
    protected function __construct(private string $value)
    {
        if (!SymfonyUuid::isValid($value)) {
            throw new \InvalidArgumentException("Invalid UUID: {$value}");
        }
    }

    public static function generate(): static
    {
        return new static(SymfonyUuid::v4()->toRfc4122());
    }

    public static function fromString(string $value): static
    {
        return new static($value);
    }

    public function getValue(): string
    {
        return $this->value;
    }

    public function equals(self $other): bool
    {
        return $this->value === $other->value;
    }

    public function __toString(): string
    {
        return $this->value;
    }
}

Each module extends it with their own typed ID:

// User module
namespace App\User\Account\Domain\ValueObject;

use App\Shared\Domain\ValueObject\Uuid;

final readonly class UserId extends Uuid {}

// Order module
namespace App\Order\Checkout\Domain\ValueObject;

use App\Shared\Domain\ValueObject\Uuid;

final readonly class OrderId extends Uuid {}

// Blog module
namespace App\Blog\Post\Domain\ValueObject;

use App\Shared\Domain\ValueObject\Uuid;

final readonly class PostId extends Uuid {}

Why extend instead of direct use?

• Type Safety - UserId ≠ OrderId at compile time
• Clarity - Intent is explicit in method signatures
• Flexibility - Each module can add specific behavior later

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When to Move Code to Shared

The “Rule of Three”

Wait until 3+ modules need it:

Module A needs Email → Keep in Module A
Module B also needs Email → Duplicate or extract to Shared? → Wait
Module C also needs Email → Now extract to Shared!

Decision Checklist

Ask these questions before moving code to Shared:

1. Is it truly generic?
   • Yes: Email - Same validation everywhere
   • No: UserEmail - Might have user-specific rules
2. Does it have zero business logic?
   • Yes: PhoneNumber - Just format and validation
   • No: CustomerDiscount - Contains pricing rules
3. Will all modules use it the same way?
   • Yes: Uuid - Identity concept is universal
   • No: Status - Each module has different status workflows
4. Is it stable?
   • Yes: Money - Well-established pattern
   • No: Notification - Still evolving per module needs

If you answer “Yes” to all → Move to Shared If you answer “No” to any → Keep in module

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Anti-Patterns to Avoid

Shared Becoming a “Junk Drawer”

Bad:

Shared/
├── Utils/
│   ├── StringHelper.php
│   ├── ArrayHelper.php
│   └── MiscFunctions.php  ← Avoid!

Good:

Shared/
├── Domain/
│   └── ValueObject/
│       ├── Email.php       ← Clear purpose
│       └── Money.php       ← Clear purpose

Premature Extraction

Bad:

// After first use in User module
// "This might be shared someday..."
mv User/ValueObject/Email.php Shared/ValueObject/Email.php  ← Too early!

Good:

// After 3rd module needs it
// "Now it's proven to be generic"
mv User/ValueObject/Email.php Shared/ValueObject/Email.php  ← Right time!

Business Logic in Shared

Bad:

// Shared/Domain/ValueObject/Price.php
final class Price
{
    public function applyDiscount(): self
    {
        // Discount logic belongs in Order or Product module!
        if ($this->customer->isPremium()) {
            return $this->multiply(0.9);
        }
    }
}

Good:

// Shared/Domain/ValueObject/Money.php
final readonly class Money
{
    // Pure value object - no business rules
    public function multiply(float $factor): self
    {
        return new self((int)($this->amount * $factor), $this->currency);
    }
}

// Order/Domain/Service/PricingService.php
final class PricingService
{
    // Business logic stays in module
    public function applyDiscount(Money $price, Customer $customer): Money
    {
        if ($customer->isPremium()) {
            return $price->multiply(0.9);
        }
        return $price;
    }
}

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Generating Shared Code

Generate Shared Value Objects

# Generate in Shared namespace
bin/console make:hexagonal:value-object shared Email

This creates:

src/Shared/Domain/ValueObject/Email.php

Generate Shared Exceptions

bin/console make:hexagonal:exception shared ValidationException

This creates:

src/Shared/Domain/Exception/ValidationException.php

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Managing Shared Kernel Changes

Coordination is Key

Changes to Shared affect all modules. Follow these rules:

1. Backward Compatibility - Don’t break existing modules
2. Team Agreement - Discuss changes with all module owners
3. Version Carefully - Consider Shared as an internal “library”
4. Test Thoroughly - Changes impact multiple contexts

Safe Change Example

Before:

final readonly class Email
{
    public function getValue(): string
    {
        return $this->value;
    }
}

After (backward compatible):

final readonly class Email
{
    public function getValue(): string
    {
        return $this->value;
    }

    // New method - doesn't break existing code
    public function getDomain(): string
    {
        return explode('@', $this->value)[1];
    }
}

Breaking Change (Avoid!)

Bad:

final readonly class Email
{
    // Renamed - breaks all modules!
    public function value(): string  // was getValue()
    {
        return $this->value;
    }
}

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Best Practices

1. Keep it Small - Shared Kernel should be minimal
2. Wait for Patterns - Don’t prematurely extract
3. No Business Logic - Only pure, generic concepts
4. Document Well - Clear docs on what belongs in Shared
5. Version Changes - Treat Shared as a contract
6. Test Coverage - High test coverage for Shared code

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Summary

The Shared Kernel is for:

• Generic value objects (Email, Money, UUID)
• Common base exceptions
• Infrastructure utilities
• Concepts used by 3+ modules

The Shared Kernel is NOT for:

• Business logic specific to one context
• Code used by only 1-2 modules
• Unstable or evolving concepts
• Context-specific variations

Key principle: When in doubt, keep it in the module. Extract to Shared only when the need is proven and clear.