The Legacy Evolution to SOA (L2SOA) Guide, as an addition to Using TOGAF to Define and Govern Service-Oriented Architectures (TOGAF SOA Guide) , 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.
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:
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:
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:
Gartner defines a legacy application or system as: “an information system that may be based on outdated technologies, but is critical to day-to-day operations.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.” . This definition directly highlights the importance of these systems, but also the challenges for transformation towards new technologies.
The SOA Reference Architecture  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 . 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  provides layers and building blocks to enable SOA within an organization and guidelines for making architectural, design, and implementation decisions.
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.
The content of this Guide is input for the further detailing of the TOGAF SOA Guide , 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.