Legacy Evolution to SOA – Introduction



The Legacy Evolution to SOA (L2SOA) Guide, as an addition to Using TOGAF to Define and Govern Service-Oriented Architectures (TOGAF SOA Guide) [15], contributes to The Open Group vision of Boundaryless Information Flow by leveraging and fostering common understanding of L2SOA. Its main goal is to leverage the collective experiences of L2SOA practitioners to develop legacy evolution best practices and lessons learned to improve the success of L2SOA implementations. The content is therefore based on real projects.

The document describes the following:

  • The background of this Guide, including current state problems and issues
  • Key concepts, principles, and considerations related to L2SOA
  • A high-level (general) approach to enable L2SOA
  • Consolidated best practices, including metrics, architecture styles, technologies, governance, etc.
  • Historical case studies providing insight into how some of the concepts described in the document are currently being applied


Legacy systems are the operations and knowledge backbone of organizations. They are difficult to replace or modernize and crucial to business survival. According to the Gartner Group, it is estimated that more than 70% of corporate data still resides on legacy systems.

These legacy systems are mature systems, created in an era where integration, flexibility, and company mergers were not as prominent as they are today.

Legacy systems have positive and negative qualities:

  • They are reliable (securing daily, supporting primary operations), efficient (developed for a dedicated environment), suitable (implementing company-specific know-how), and valuable (distinguishes companies, company-specific business logic).
  • They are also complex (many changes over time, almost no documentation), hard to modularize and integrate, and expensive to maintain and enhance.

Companies in all industries and of all sizes have expressed a growing need for migrating their existing application assets into new architectures guided by SOA concepts. The rationale is based on:

  • Financial considerations (protecting investments, leveraging existing assets)
  • Employee skill migration and improvement (rather than replacing people)
  • Stability (retaining operational software benefits, maintaining required high uptime)
  • Evolutionary renewal (enabling new and emerging technologies)

Service-Oriented Architecture (SOA) as a style enables a staged transition from a (partly) silo-based system landscape towards an integrated, componentized, and shared service environment. Legacy systems can be integrated and migrated into a modernized landscape using SOA. In this way a big bang modernization scenario is not necessary, but can be done in phases.

This document is based upon the experience of a set of L2SOA projects and organized as follows:

  • Motivation: Common concerns and drivers for modernization.
  • Approach to Enable L2SOA: Approach to enable L2SOA, aligned with the TOGAF Architecture Development Method (ADM).
  • Best Practices: Overview of a set of best practices in a variety of L2SOA evolution aspects.
  • The document ends with a detailed description of two L2SOA cases, including recommendations to improve the specific journey, based upon this Guide.


Legacy Application or System

Gartner defines a legacy application or system as: “an information system that may be based on outdated technologies, but is critical to day-to-day operations. Replacing legacy applications and systems with systems based on new and different technologies is one of the information systems (IS) professional's most significant challenges. As enterprises upgrade or change their technologies, they must ensure compatibility with old systems and data formats that are still in use.” [1]. This definition directly highlights the importance of these systems, but also the challenges for transformation towards new technologies.

The SOA Reference Architecture [3] describes legacy as: “a feature or behavior that is being retained for compatibility with older applications, but which has limitations which make it inappropriate for developing portable applications. New applications should use alternative means of obtaining equivalent functionality.”.

Interestingly, this definition talks about retaining only for compatibility and limitations for portability, while Gartner focuses on the business-critical value. Both perspectives need to be taken into account.


The Open Group defines Service-Oriented Architecture (SOA) is an architectural style that supports service-orientation [2]. Service-orientation is a way of thinking in terms of services and service-based development and the outcomes of services. An architectural style is the combination of distinctive features in which architecture is performed or expressed.

The Open Group SOA Reference Architecture [3] provides layers and building blocks to enable SOA within an organization and guidelines for making architectural, design, and implementation decisions.

Legacy to SOA (L2SOA)

L2SOA is an approach to enable legacy systems to be integrated into a silo-transcending landscape and processes using SOA as the architectural style. This resulting goal of L2SOA is to reach a more adaptive and agile environment, providing an infrastructure for controlled transformation, re-use of important business functionality, etc.

Future Directions

The content of this Guide is input for the further detailing of the TOGAF SOA Guide [15], which will benefit from including L2SOA. Further detailing of aspects in this Guide into specific guidelines is possible depending on the need for it. There is no commitment to any particular additional content and other content not mentioned here may be added.