he best way to leverage past IT investment is to reuse the legacy
application or pieces of legacy application in order to build future applications.
As such, we will use a phased approach in order to minimize risks related to code
migration. Strategies for migration
Strategies for migrationCold Turkey
Involves rewriting a legacy IS from scratch to produce the target IS using modern software techniques and hardware of the target environment.Chicken Little
Involves migrating the legacy IS in place, by small incremental steps until the desired long term objective is reached.
Cold Turkey vs. Chicken Little
Cold Turkey risks and disadvantages:A better system must be promised (if the only payoff is to lower future maintenance costs)
Business condition never stand still
Specifications rarely exist
Undocumented dependencies frequentely exist
Legacy ISs can be too big to cut-over
Management of large projects is hard (Brooks effect)
Lateness is seldom tolerated
Large project tend to bloat
Chicken Little advantages:Alternative, low-risk and novel strategy
Each step requires a relatively small resource allocation, a short time, and produces a specific, small result towards the desired goal
If a Chicken Little step fails, only the failed step must be repeated, rather than the entire project
Each problem cited previousely can be addressed in an incremental fashion
Chicken Little is in sharp contrast to the vast resource requirements of a complete rewrite (e.g. hundreds of persons years), a multi-tier development, and one massive result
Since steps are designed to be relatively inexpensive, such incremental steps do not need to promise dramatic new function to be funded
Chicken Little is much safer and more feasible than Cold Turkey
Critical Success Factors of Chicken Little
The selection of independent increments (components) to migrate i.e. portions of legacy interfaces, applications and databases that can be migrated independently of each other.
The sequencing of the increments to achieve the desired goal.
Dealing with unavoidable problems i.e. dependencies between migration steps.
The methodThe method presented here is tailorable and includes the following requirements:
Migrate in place.
Ensure continuous, safe, reliable, robust access to mission critical functions and information at performance levels adequate to support business's workload.
Make as many fixes, improvements, and enhancements as reasonable, to address current and anticipated requirements.
Make as few change as possible to minimize risk.
Alter the legacy code as little as possible to facilitate future evolution.
Establish as much flexibility as possible to facilitate future evolution.
Minimize the potential negative impacts of change, including those on users, applications, databases, and, particularly, on the on-going operation of the mission critical IS.
Maximize the benefits of modern technology and methods.
The five basic steps
Iteratively migrate the computing environment.
Iteratively migrate the legacy application.
Iteratively migrate the legacy data.
Iteratively migrate the legacy user and system interface.
Iteratively cut-over from the legacy to the target components.
General Migration Method
Iteratively install the target environment.
Iteratively analyze the legacy IS.
Iteratively decompose the legacy IS.
Iteratively design the target applications and interfaces.
Iteratively design the target database.
Iteratively migrate the legacy applications.
Iteratively migrate the legacy interfaces.
Iteratively cut-over the target IS.
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
The six phases for complete system transformation
1. Source Code Inventory and Assessment
2. Project Planning and Estimation
3. Legacy Analysis
4. User Interface Transformation
5. Business Logic Transformation
6. Data Transformation
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