The strategy of extending legacy systems is used to implement new business logic without intrusive modifications to the existing system. It involves building new software that is free from the limitations of the legacy system being extended. Although this approach introduces certain burdens — such as the need to maintain an additional application or environment — the benefits significantly outweigh the costs.
Key Characteristics of Extension Software
Technological Independence
Allows the use of modern technologies best suited to the functionality being implemented and aligned with current standards — including responsive user interfaces, cloud-based solutions, and artificial intelligence mechanisms such as Large Language Models (LLMs).
Reusing Existing Data
Extension software builds on valuable data accumulated in legacy systems, accessed through dedicated interfaces or existing reporting environments (e.g., data warehouses). Often, batch synchronization (e.g., once daily) is sufficient. With the use of a buffer, this also enables data aggregation or exploration enhancements, such as implementing additional indexes to support fast, fuzzy logic–based searches.
Collecting New Data
New business requirements typically involve collecting new types of data (i.e., extending the data model). The extension system has its own, independently structured data model, which can reference the legacy model via appropriately selected unique identifiers.
Updating the Legacy System Data
In many cases, the business process supported by the extension requires sending feedback back to the legacy system. This is often the most complex part of the extension strategy, as it requires implementing or integrating with existing legacy-side interfaces — which may require additional specialists or vendor cooperation, and can be costly. In some cases, manual data entry may be considered by existing business staff or other involved teams (e.g., call center), using a dedicated report or tailored UI element to support this step.
Benefits of the Extension Approach
Fast and Cost-Efficient Implementation
By avoiding time-consuming dependency analyses, development can focus directly on implementing specific business requirements — using proven tools, methodology, and design patterns. This leads to faster delivery and more predictable, controlled project costs.
Low Business Risk
Development and deployment occur independently of existing business processes, eliminating the risk of disruption. Moreover, the high predictability of such projects minimizes the chances of failure or delays.
Data Integration and Unification
An extension system is not limited to one legacy source — it can unify data from various components of the corporate ecosystem. This enables coherent views of key business entities, such as customers, vendors, or service locations.
Reduction of Shadow IT
Providing official extension systems naturally reduces the need for business departments to create local, often insecure or inefficient, workaround solutions — collectively known as Shadow IT. Extension systems can also replace or absorb existing Shadow IT, further decreasing its presence in organizational processes.
Scalability and Ease of Further Development
Once implemented, an extension system serves as a flexible platform for adding future functionality — usually faster than the initial project. The existing integration layer can be reused or gradually expanded without revisiting original challenges such as defining monitoring processes or working with vendors.
Foundation for Gradual Migration
Lifecycle planning for legacy systems typically includes a phase-out, ending in migration. A phased migration — transferring functionality gradually over time — is a safer alternative. Extension software can serve as a natural destination for such migration efforts, especially when no better candidate system exists.