The Strangler Fig: A method for Legacy System Migration

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The Strangler Fig: A method for Legacy System Migration

Read More


The Strangler Fig: A method for Legacy System Migration

Read More

In the world of software development, the weight of a mature, complex legacy system can feel like a millstone around a developer's neck. Adding new features becomes an exercise in contortionism, and maintenance feels like trying to mend a cracked foundation. While a complete rip-and-replace might seem tempting, the reality is that such a drastic approach is often impractical and disruptive. This is where the Strangler Fig pattern emerges as a compelling solution, offering a more gradual and controlled path towards modernization.

The Burden of Legacy Systems

Legacy systems, once innovative solutions, can become a hindrance over time. Their codebases become tangled, documentation fades into obscurity, and adding new features feels akin to threading a needle in a hurricane. This escalating complexity not only stifles innovation but also increases the risk of bugs and security vulnerabilities.

Phased Migration

One common approach to address legacy systems is a phased migration to a new platform. This involves meticulously carving off sections of the legacy system and transplanting them into modern architecture. While this allows for a controlled transition, it creates a period of "duality" where both the old and new systems co-exist. This presents its own set of challenges:

  • Maintaining Two Codebases: Development teams now face the burden of maintaining and updating two separate codebases, which can be a significant drain on resources.

  • User Confusion: Users might be left grappling with feature location, as some functionalities reside in the old system and others have transitioned to the new.

  • Disjointed User Experience: The user experience may feel fragmented as users interact with both systems, potentially leading to frustration.

The Strangler Fig Pattern

The Strangler Fig pattern offers an elegant solution to these challenges. Inspired by the strangler fig, a plant that slowly envelops and ultimately replaces its host tree, this pattern advocates for gradual substitution of functionalities within the legacy system.

Here's how it works:

  • Façade: The Gatekeeper

A crucial component of the Strangler Fig pattern is a "façade." This layer acts as a gatekeeper, intercepting requests from the user interface. The façade then intelligently routes the request based on the functionality being accessed:

* **Legacy Functionality:** If the request pertains to a feature still residing in the legacy system (e.g., AssetCapture or EventInterception), the façade directs it to the appropriate component within the old system.* **Modernized Functionality:** If the request relates to a feature that has already been migrated to the new system, the façade routes it to the corresponding service in the modern architecture.

Benefits of the Strangler Fig Pattern

  • Better User Experience: Users remain blissfully unaware of the ongoing migration behind the scenes. They continue to interact with a single user interface, regardless of whether the functionality resides in the legacy or modern system.

  • Reduced Risk: By migrating functionalities incrementally, the risk of introducing bugs or disruptions is minimized. Issues can be addressed in a controlled manner within the isolated new functionality.

  • Phased Retirement: As functionality is progressively migrated to the new system, the legacy system becomes increasingly irrelevant. This allows for its eventual retirement without causing a service outage or impacting users.

Considerations and Mitigating Risks with the Strangler Fig

While the Strangler Fig pattern offers a compelling approach, it's not without its considerations:

  • The complexity of the Façade: The logic within the façade needs to be meticulously designed to ensure proper routing and seamless integration with both the legacy and new systems.

  • Testing Challenges: Testing becomes more complex as functionalities are split between two systems. Developers need to devise comprehensive testing strategies that encompass both the legacy and new components.

  • Long-Term Commitment: The Strangler Fig pattern may require a longer timeframe to complete the migration compared to a complete rip-and-replace approach.

Despite these considerations, several strategies can help mitigate risks:

  • Start Small: Begin by migrating low-risk, well-defined functionalities to gain experience and confidence with the approach.

  • Invest in Automation: Utilize automated testing tools to streamline testing efforts and ensure consistent functionality across both systems.

  • Clear Communication: Maintain open communication with stakeholders throughout the migration process, keeping them informed of progress and potential challenges.


The Strangler Fig pattern offers a powerful tool for developers struggling with the burden of legacy systems. By using this approach we can facilitate a smoother transition to a more modern robust system.

Janith Dissanayake


IT Powered Business Solutions

We build professional software and AI solutions for complex business problems.

IT Powered Business Solutions

We build professional software and AI solutions for complex business problems.