Domain-Driven Design: Tackling Complexity in the Heart of Software

6.
Phasing Out a Legacy System
All good things must come to an end, even legacy computer software. But it doesn't happen on its
own. These old systems can be so woven into the business and other systems that extricating
them can take many years. Fortunately, it doesn't have to be done all at once.
The possibilities are too various for me to do more than scratch the surface here. But I'll discuss a
common case: An old system that is used daily in the business has been supplemented recently by
a handful of more modern systems that communicate with the legacy system through an
ANTICORRUPTION LAYER
.
One of the first steps should be to decide on a testing strategy. Automated unit tests should be
written for new functionality in the new systems, but phasing out legacy introduces special testing
needs. Some organizations run new and old in parallel for some period of time.
In any given iteration:
Identify specific functionality of the legacy that could be added to one of the favored systems
within a single iteration.
1.
Identify additions that will be required in the 
ANTICORRUPTION LAYER
.
2.
Implement.
3.
Deploy.
4.
Sometimes it will be necessary to spend more than one iteration writing equivalent functionality to
a unit that can be phased out of the legacy, but still plan the new functions in small, iteration-sized
units, only waiting multiple iterations for deployment.
Deployment is another point at which too much variation exists to cover all the bases. It would be
nice for development if these small, incremental changes could be rolled out to production, but
usually it is necessary to organize bigger releases. The users must be trained to use the new

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software. A parallel period sometimes must be completed successfully. Many logistical problems
will have to be worked out.
Once it is finally running in the field:
Identify any unnecessary parts of the 
ANTICORRUPTION LAYER
and remove them.
5.
Consider excising the now-unused modules of the legacy system, though this may not turn
out to be practical. Ironically, the better designed the legacy system is, the easier it will be to
phase it out. But badly designed software is hard to dismantle a little at a time. It may be
possible to just ignore the unused parts until a later time when the remainder has been
phased out and the whole thing can be switched off.
6.
Repeat this over and over. The legacy system should become less involved in the business, and
eventually it will be possible to see the light at the end of the tunnel and finally switch off the old
system. Meanwhile, the 
ANTICORRUPTION LAYER
will alternately shrink 
and 
swell as various
combinations increase or decrease the interdependence between the systems. All else being equal,
of course, you should migrate first those functions that lead to smaller 
ANTICORRUPTION LAYERS
. But
other factors are likely to dominate, and you may have to live with some hairy translations during
some transitions.