RESEARCH ARTICLE *Email Address: ranaehsan@gmail.com 1710 Adv. Sci. Lett. Vol. 24, No. 3, 2018 doi: 10.1166/asl.2018.11142 Copyright © 2018 American Scientific Publishers Advanced Science Letters All rights reserved Vol. 24, 1708-1711, 2018 Printed in the United States of America A Review of Cloud Migration Techniques and Models for Legacy Applications: Key Considerations and Potential Concerns Muhammad Ehsan Rana*, Wan Nurhayati Wan Ab Rahman Faculty of Computer Science and IT, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Legacy systems refer to the applications designed for a particular client that have been in use for a long period of time and developed using obsolete technologies. They are often business-critical systems, therefore any changes here inevitably will affect the other parts of the system. Although these systems are considered to be outdated but are too costly and risky for an organization to replace it. Cloud computing provides a new platform for organization that promises flexible scalability, business agility, high availability and reduction in costs. Considering these benefits, migration of legacy systems to cloud is a lucrative option for many organizations. However the architecture of these legacy applications require a tested, fool-proof and risk free approach for migration. Limited migration models or frameworks have been proposed which caters issues of legacy systems migration to cloud platform. Many of these cloud adoption techniques and models emphasize on the generic phases and procedures on migrating the applications and data to cloud. However some of these models are more flexible, and provide better approaches compared to others. This research explores the issues associated with the legacy applications with regards to their migration on cloud and reviews the existing techniques and models that have been proposed in this context. Keywords: Legacy Systems, Cloud Computing, Cloud Migration Techniques, Cloud Migration Models. 1. INTRODUCTION A. Legacy Systems Legacy system refers to a computer-based system that may still in use but no longer receiving vendor support (cannot be upgraded). Not only hardware and software are included in the system, legacy processes and procedures are also part of the system since the ways of doing things are hard to be changed as they rely on the legacy software. As a result, legacy system is usually incompatible with newly purchased systems and may be problematic due to compatibility issues, obsoleting and lack of security support. These are often business-critical systems and changes on the system inevitably will affect the other parts of the system. This requires the system to be maintained although no updates are available from the vendors, but is too risky to be replaced from the business perspective. For example, bank customer accounting system mostly are one of the earliest system which organizational policies and procedures will rely on. Bank might be having high business risk, if the system replaced did not work as it should be, when the bank replaces the existing system which may run on expensive mainframe hardware. Addition, the changes on existing procedures may cause difficulties with the bank’s auditors 1-3 . COBOL based systems are good examples of legacy systems. COBOL was introduced in late 1950s but there are estimated 200 billion lines of COBOL codes still running in business and enterprise systems 4 . These