Frameworks are supposed to help you build things more quicker and hide a lot of complexity around cross-cutting and infrastructural concerns. They are supposed to make it easier for inexperienced developers to join a running projection. But frameworks also introduce a lot of magic, and that magic is going to backfire at some point. At least, that's my experience. And when it backfires, your code is so entangled with that framework, that you can't get rid of it anymore. So, instead of building and using frameworks, build and use libraries. That's easier said then done, and I'll can talk about many great and not so great examples. So let me share some of the practices I use to build composable libraries myself. I'll talk about principles of package design and scoping, keeping your NuGet package dependencies in check, and how to use layered APIs to increase usability without hiding the magic.

1. Dennis Doomen | @ddoomen | Aviva Solutions | The Continuous Improver

3. …and badly designed libraries

13. Library
Consumer
Bunch of classes that can
be used directly
X
No inheritance
Uses composition
over inheritance
internally
Convenience classes
that don’t hide the
magic
Main Package
Shared Package
Contract
Contract
Only depend on
more abstract
packages…
Stable Package
…or depend on more
stable packages
Auxiliary
Package
Classes that are not
used together do not
belong togetherOptional
Dependency
Dependency
Package
Consumers should
not be faced with
optional
dependencies

25. public interface IEventStore
{
IDisposable Subscribe(
long? lastCheckpoint,
Func<Transaction[]>, Task> handler,
string subscriptionId);
}
public delegate IDisposable CreateSubscription(
long? lastCheckpoint,
Func<Transaction[]>, Task> handler,
string subscriptionId);

26. e.g. LibLog, TinyIoc, FluidCaching

31. Command Handlers
Commands
Domain Model
Event Store
Events
App
Persistent
Projections
RavenDB/Nhibernate/Raw SQL
Projectors
Events
API Controller
Projections
Events

32. Library
Consumer
LiquidProjections
LiquidProjections.Abstractions
Transaction
Delegates
LiquidProjections.
NEventStore
NHibernate
LiquidProjections
NHibernate
RavenDB
LiquidProjections
RavenDB
LiquidProjections.
Testing

34. • Liquid Projections – The Example
https://github.com/liquidprojections
• Principal for Successful Package Management
http://www.continuousimprover.com/2016/10/princi
ples-for-successful-package.html
• The magic of hiding your NuGet dependencies
http://www.continuousimprover.com/2016/05/the-
magic-of-hiding-your-nuget.html
• To DRY or not to DRY, it is a matter of boundaries
http://www.continuousimprover.com/2016/05/the-
magic-of-hiding-your-nuget.html
• 12 tips to write unit tests that don’t cripple your code
base
http://www.continuousimprover.com/2015/11/12-
tips-to-write-unit-tests-that-dont.html