Replacing a legacy system in a complex enterprise IT environment is (obviously) not an easy task. The most important key success factor is data migration from the old system to the new one.

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Data Migration
Migration Approaches .................................................................1
Big bang migration.....................................................................1
Incremental migration................................................................2
Parallel migration (in conjunction with big
bang/incremental) ...................................................................3
Migration Approaches
Replacing a legacy system in a complex enterprise IT environment is (obviously) not an easy task. The
most important key success factor is data migration from the old system to the new one.
Several general types of migration are possible. The most commonly used approaches are:
 big bang migration
 incremental migration
 parallel migration
Big bang migration
In big bang migration the data is moved to the new target environment in one operational process.
The business must allow the project team sufficient time in order to move the data from the source
system(s) to the target system(s). Enough time also needs to be available in the case that a “roll-
back/fallback” needs to be performed for some unexpected reason (the “roll-back/fallback” can take as
long as the migration itself).
One challenge facing the project team is normally the ability to convince the business to agree to a
large enough timeframe for the preparation, extract, loading, verification and testing of the data in the
target system. The best time period to perform this type of migration is over a weekend (or in a period
when activity on the legacy system is usually low).
During the migration period, under normal circumstances, the source system(s) will be unavailable for
access.
Business teams and technical as well as operational staff will need to be on-hand to assist with the
verification and testing stage once the migration is complete. Someone in authority, together with the
migration/project team will need to be available to sign-off and open the new system to users once
he/she has been convinced that the data has been moved correctly and the new system is ready for use.
A big bang migration has the following advantages and disadvantages over other migration
approaches:

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Advantages Disadvantages
No need to run two systems simultaneously, post
migration.
Systems will be unavailable for update purposes
during the migration window.
Legacy (source) system can be de-commissioned
earlier.
Once data has been migrated, and the target
system is being used, if any problems are detected
it is more difficult to make corrections.
This is the “normal method” used for a
migration.
Putting “all your eggs in one basket” approach –
i.e. If it breaks it all breaks.
No need to keep systems in sync. “Roll-back/Fallback” procedures are difficult and
time-consuming to put into place.
Parallel running the systems post-migration for a
period of time is possible (if required to do so –
but can be expensive to do so).
Incremental migration
The incremental migration approach works by migrating portions of the database at one time. This can
be achieved in many ways (for example, in a billing system: by bill day, customer segment or any
measurable or selectable attribute).
Incremental migration mitigates the risks associated with the big bang type migration: if a problem is
found it is often far easier to roll back or correct a smaller subset of data, than the complete set or full
base.
The incremental migration method does also usually have a much bigger business overhead associated
with it: for a period of time the external systems (and indeed operational teams) need to integrate with
the two systems (legacy and new). For instance mediation has to “know” where to send CDRs to,
revenue assurance will have to review two systems rather than just one and DWH would have to
receive data from two sources, etc.
New customers and subscribers should ideally be entered onto the new target system whilst the
migration is in progress. This can lead to “migration on demand”: for example, if the new subscriber is
attached to an old customer, the customer should be migrated immediately in the new system. There is
also the issue whereby the opposite is true, and a subscriber on the target system needs to be migrated
back. Migration on demand causes many issues and problems in the interim period. Procedural as well
as technical solutions should be put into place to mitigate this issue as far as possible.
The key to an incremental migration is to thoroughly understand the business and data landscape, to
identify what slices and dices can be used for the migration process and have procedural as well as
technical solutions in place for any issues identified.
An integration migration has the following advantages and disadvantages over other migration
approaches:
Advantages Disadvantages
Easier to correct data as smaller volume of data
moved at a time
Temporary measures are required to enable the
business to function during cut-over period.
This can be quite costly.

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Advantages Disadvantages
Roll-back if necessary should be easier to
perform, as a smaller volume of data has been
moved.
Staff has to learn new procedures that are in
place during the migration process only.
Parallel migration (in conjunction with big bang/incremental)
A parallel migration is basically either a big bang migration or incremental migration, followed by a
period of dual-running of the systems in parallel.
This approach has the effect of running two systems side by side, and effort has to be put into keeping
the two systems in sync.
This method is not recommended, from an operational perspective unless there is a low level of
certainty that the target system would perform satisfactorily, either from a functionality or performance
standpoint. In this case, however, it would be unwise to start the migration process at all.
A parallel migration has the following advantages and disadvantages over other migration approaches:
Advantages Disadvantages
The ability to ensure that the target system
performs correctly prior to de-commissioning the
source system.
Same expense as either big bang or incremental
method with the additional expense of keeping
two systems in sync.
Easy to revert to using the original system, with
the knowledge that no updates are required.
Staff has to learn new procedures that are in
place during the migration process only.