Legacy systems

Legacy systems are older-generations systems that are incompatibility with current generation standards and systems.Such systems may still contain valuable data,and may support critical applications.The legacy of today are typically those built with technologies such as database that use the network or hierarchical data models,or use Cobol and file systems without a database.Porting legacy applications to move a modren environment is often costly in terms of both time and money,sice they are often very large consisting of millions of lines of code developed by teams of programmers,over several decades.Thus,it is important to support these older generation or legacy systems and to facilitate their inter operation with newer systems.One approach used to inter operation between relational databases and legacy database is to build a layer,called a wrapper,on top of the legacy systems that can make the legacy systems appear to be a relational database.The wrapper may provide support for ODBC or other interconnection standards such as OLE-DB,which can be used to query and update the legacy systems.The wrapper is responsible for converting relational queries and updates into queries and updates on the legacy systems.When an organisation decides to replace a legacy systems by a new system,it must follow a process called reverse engineering,which consists of going over the code of the legacy systems to come up with schema designs in the required data model.

Reverse engineering also examines the code to find out what procedures and processes were implemented ,in order to get a high level model of the system.Reverse engineering is needed because legacy system usually do not have high level documentation of their schema and overall system design.
When coming up with th design of a new system,the design is reviews,so that of can be improved rather just reimplemented as is.Extensive coding is required to support all the functionality that were provided by the legacy system.The overall process is called re-engineering.When a new system has been built and tested,the system must be populated with data from the legacy systems,and all further activities must be carried out on the new system.However,abruptly transitioning,to a new system which is called the big-bang approach,carries several risks.First,users may not be familiar with the interface of the new system.Second there may be bugs or performance problem in the new system that were not discovered when it was tested.Such problem may lead to great losses for companies since their ability to carry out critical transactions such as sales and purchase may be severely affected.In some,extreme cases the new system have been abandoned and the legacy systems reused,after an attempted switchover failed.An alternative approach called the chicken little approach,incrementally replaces the functionality of the legacy system.for example,the new user interfaces may be used with the old system in the back end or vice versa.Another option is to use the new system only for some functionality that can be decoupled from the legacy system.In either case,the legacy and new systems coexist for some time.There is required functionality to inter operate with the new system.This approach,therefore has a higher development cost associated with it.