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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