In the previous article I used a term “compilation tiers”, but I didn’t define it. But before the definition, I have to describe all the possibilities of the split. To make it easier let’s assume that we split our big solution into two smaller ones: solution one and solution two. Now we have following possibilities:
Solution two depends on solution one
Image is worth 1000 words, so here it is: 
The most interesting part of above picture is the green arrow. It represents dependencies, usually called artifacts. The artifacts can be following:
- ==packages== like Nuget packages/Maven packages/npm packages/etc.
- ==plain files== like simple code files/dll files/protobuf schema/etc.
We can define above as: Solution one ==produces== some kind of artifacts, that are needed by solution two during or before ==the build==. It is not a run-time dependency. It is a compilation dependency.
Solution one depends on solution two
As in old joke: replace one with two and start from the beginning of the article.
Solution one and two doesn’t have any relationship
Again 1000 words: 
How above can happen? I wrote about this in the previous article. In most cases, there is a run-time dependency instead of build dependency. The second option is simpler: projects just don’t relate to each other at all. For example database migration tool and mobile app. They just don’t have any relation.
Definition
We can define compilation tier as:
Compilation tier is a subset of all projects without artifact dependencies to each other. The only artifact dependency can be to previous compilation tier.
Making above simpler: ==You can compile solutions from one compilation tier in parallel==.
Real life
Find first seam is quite easy. First split probably also. But month or two pass away and You will start to worry if You did a good job. Are you compilation tiers correct? To find out you will have to create a checklist:
- You don’t have circular dependencies between tiers. After You introduce package manager it can be easy to create such. Because all packages already exist in artifact repository.
- You can compile everything from zero. Very similar to above.
- Most changes are in last tiers. If you do a lot of changes in first tiers, it suggests that every time You will have to compile everything.
- You didn’t create too many tiers. I think a number between 5 and 10 is maximum. In most projects, it should be below 5.
Next time
Next time I will try to introduce contracts (interfaces) between solutions. But if you wish something else please let me know in comments.