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Architecture of legacy systems

From the book: "Migrating Legacy Systems: Gateways, Interfaces & the Incremental Approach" (Morgan Kaufmann Series in Data Management Systems; ASIN: 1558603301 - April 1995) by Michael L. Brodie, Michael Stonebraker (Contributor), Michael Stonebraker

 Decomposable legacy IS Architecture
 Semi-Decomposable legacy IS Architecture
 Non-Decomposable legacy IS Architecture
 Hybrid legacy IS Architecture

  

   Decomposable legacy IS

  A collection of application modules that each interacting with a database service and each, potentially, with its own user interface and system level interface through which it interacts with one or more Information Systems.

 The prime requirements for an architecture to be decomposable are that the application modules are independent of each other (e.g. no hierarchical structure) and interact only with the database service.

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   Semi-Decomposable legacy IS

 Only user interface and system interfaces are separate modules. The application and database service are not separable since their structure is more complex, not adequately engineered in accordance with current standards, or is poorly understood.

  The lack of desirable structure makes analysis and migration more complex and error prone.

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   Non-Decomposable legacy IS

 Such information systems are, from a system point of view, black boxes since no functional components are separable.

 End users and information systems interact directly with one, apparently unstructured, legacy information systems.

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   Hybrid legacy IS

 In general, the architecture of most legacy ISs may be neither decomposable, semi-decomposable, nor non-decomposable. During its decade long evolution, a legacy IS may have had parts added that fall into each architectural category resulting in a hybrid architecture.

 Some interface and application modules are inseparable from the legacy database service while others are modular and independent.

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