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

Merging C
ONTEXTS
: S
HARED
 K
ERNEL
 
 C
ONTINUOUS
 I
NTEGRATION
If your 
SHARED KERNEL
is expanding, you may be lured by the advantages of full unification of the
two 
BOUNDED CONTEXTS
. This is not just a matter of resolving the model differences. You are going
to be changing team structures and ultimately the language people speak.
Start by preparing the people and the teams.
Be sure that all the processes needed for 
CONTINUOUS INTEGRATION
(shared code ownership,
frequent integration, and so on) are in place on 
each team
, separately. Harmonize
integration procedures on the two teams so that everyone is doing things in the same way.
1.
Start circulating team members between teams. This will create a pool of people who
understand both models, and will begin to connect the people of the two teams.
2.
Clarify the distillation of each model individually. (See Chapter 15.)
3.
At this point, confidence should be high enough to begin merging the core domain into the
SHARED KERNEL
. This can take several iterations, and sometimes temporary translation layers
are needed between the newly shared parts and the not-yet-shared parts. Once into merging
the 
CORE DOMAIN
, it is best to go pretty fast. It is a high-overhead phase, fraught with errors,
4.

---

and should be shortened as much as possible, taking priority over most new development.
But don't take on more than you can handle.
To merge the 
CORE
models, you have a few choices. You can stick with one model and modify the
other to be compatible with it, or you can create a new model of the subdomain and adapt both
contexts to use it. Watch out if the two models have been tailored to address distinct user needs.
You may need the specialized power of both original models. This calls for developing a deeper
model that can supersede both original models. Developing a deeper unifying model is very
difficult, but if you are committed to the full merger of the two 
CONTEXTS
, you no longer have the
option of multiple dialects. There will be a reward in terms of the clarity of integration of the
resulting model and code. Be careful that it doesn't come at the cost of your ability to address the
specialized needs of your users.
As the 
SHARED KERNEL
grows, increase the integration frequency to daily and finally to
CONTINUOUS INTEGRATION
.
5.
As the 
SHARED KERNEL
approaches the point of encompassing all of the two former 
BOUNDED
CONTEXTS
, you will find yourself with either one large team or two smaller teams that have a
shared code base that they 
INTEGRATE CONTINUOUSLY
, and that trade members back and forth
frequently.