java-design-patterns/anti-corruption-layer/README.md

---
title: Anti-corruption layer
category: Integration
language: en
tag:
    - Architecture
    - Decoupling
    - Isolation
---

## Also known as

* ACL
* Interface layer
* Translation layer

## Intent

Implement a façade or adapter layer between different subsystems that don't share the same semantics. It translates
between different data formats and systems, ensuring that the integration between systems does not lead to corruption of
business logic or data integrity.

## Explanation

### Context and problem

Most applications rely on other systems for some data or functionality. For example, when a legacy application is
migrated to a modern system, it may still need existing legacy resources. New features must be able to call the legacy
system. This is especially true of gradual migrations, where different features of a larger application are moved to a
modern system over time.

Often these legacy systems suffer from quality issues such as convoluted data schemas or obsolete APIs. The features and
technologies used in legacy systems can vary widely from more modern systems. To interoperate with the legacy system,
the new application may need to support outdated infrastructure, protocols, data models, APIs, or other features that
you wouldn't otherwise put into a modern application.

Maintaining access between new and legacy systems can force the new system to adhere to at least some of the legacy
system's APIs or other semantics. When these legacy features have quality issues, supporting them "corrupts" what might
otherwise be a cleanly designed modern application. Similar issues can arise with any external system that your
development team doesn't control, not just legacy systems.

### Solution

Isolate the different subsystems by placing an anti-corruption layer between them. This layer translates communications
between the two systems, allowing one system to remain unchanged while the other can avoid compromising its design and
technological approach.

### Programmatic example

#### Introduction

The example shows why the anti-corruption layer is needed.

Here are 2 shop-ordering systems: `Legacy` and `Modern`.

The aforementioned systems have different domain models and have to operate simultaneously. Since they work
independently the orders can come either from the `Legacy` or `Modern` system. Therefore, the system that receives the
legacyOrder needs to check if the legacyOrder is valid and not present in the other system. Then it can place the
legacyOrder in its own system.

But for that, the system needs to know the domain model of the other system and to avoid that, the anti-corruption
layer(ACL) is introduced. The ACL is a layer that translates the domain model of the `Legacy` system to the domain model
of the `Modern` system and vice versa. Also, it hides all other operations with the other system, uncoupling the
systems.

#### Domain model of the `Legacy` system

```java
public class LegacyOrder {
    private String id;
    private String customer;
    private String item;
    private String qty;
    private String price;
}
```

#### Domain model of the `Modern` system

```java
public class ModernOrder {
    private String id;
    private Customer customer;

    private Shipment shipment;

    private String extra;
}

public class Customer {
    private String address;
}

public class Shipment {
    private String item;
    private String qty;
    private String price;
}
```

#### Anti-corruption layer

```java
public class AntiCorruptionLayer {

    @Autowired
    private ModernShop modernShop;

    @Autowired
    private LegacyShop legacyShop;

    public Optional<LegacyOrder> findOrderInModernSystem(String id) {
        return modernShop.findOrder(id).map(o -> /* map to legacyOrder*/);
    }

    public Optional<ModernOrder> findOrderInLegacySystem(String id) {
        return legacyShop.findOrder(id).map(o -> /* map to modernOrder*/);
    }

}
```

#### The connection

Wherever the `Legacy` or `Modern` system needs to communicate with the counterpart the ACL needs to be used to avoid
corrupting the current domain model. The example below shows how the `Legacy` system places an order with a validation
from the `Modern` system.

```java
public class LegacyShop {
    @Autowired
    private AntiCorruptionLayer acl;

    public void placeOrder(LegacyOrder legacyOrder) throws ShopException {

        String id = legacyOrder.getId();

        Optional<LegacyOrder> orderInModernSystem = acl.findOrderInModernSystem(id);

        if (orderInModernSystem.isPresent()) {
            // order is already in the modern system
        } else {
            // place order in the current system
        }
    }
}
```

## Applicability

Use this pattern when:

* A migration is planned to happen over multiple stages, but integration between new and legacy systems needs to be
  maintained
* Two or more subsystems have different semantics, but still need to communicate
* When integrating with legacy systems or external systems where direct integration might pollute the domain model of
  the new system
* In scenarios where different subsystems within a larger system use different data formats or structures
* When there is a need to ensure loose coupling between different subsystems or external services to facilitate easier
  maintenance and scalability

## Tutorials

* [Microsoft - Anti-Corruption Layer](https://learn.microsoft.com/en-us/azure/architecture/patterns/anti-corruption-layer)
* [Amazon - Anti-Corruption Layer](https://docs.aws.amazon.com/prescriptive-guidance/latest/cloud-design-patterns/acl.html)

## Known Uses

* Microservices architectures where individual services must communicate without being tightly coupled to each other’s
  data schemas
* Enterprise systems integration, especially when integrating modern systems with legacy systems
* In bounded contexts within Domain-Driven Design (DDD) to maintain the integrity of a domain model when interacting
  with external systems or subsystems

## Consequences

Benefits:

* Protects the integrity of the domain model by providing a clear boundary
* Promotes loose coupling between systems, making the system more resilient to changes in external systems
* Facilitates cleaner and more maintainable code by isolating integration code from business logic

Trade-offs:

* Introduces additional complexity and potential performance overhead due to the translation process
* Requires extra effort in design and implementation to ensure the layer is effective without becoming a bottleneck
* Can lead to duplication of models if not carefully managed

## Related Patterns

* [Facade](https://java-design-patterns.com/patterns/facade/): The Anti-Corruption Layer can be seen as a specialized
  form of the Facade pattern that is used to isolate different subsystems
* [Adapter](https://java-design-patterns.com/patterns/adapter/): The Anti-Corruption Layer can be implemented using the
  Adapter pattern to translate between different data formats or structures
* [Gateway](https://java-design-patterns.com/patterns/gateway/): The Anti-Corruption Layer can be used as a Gateway to
  external systems to provide a unified interface

## Credits

* [Domain-Driven Design: Tackling Complexity in the Heart of Software](https://amzn.to/3vptcJz)
* [Implementing Domain-Driven Design](https://amzn.to/3ISOSRA)