On the subject of libraries on large code bases. I think one should be careful on deciding what goes into a library and what goes into an API/Service. Both allow you to share code. But there is a difference on who controls the deployment in prod. For example if you need to change a business rule, you can change the code, and when the API/service is deployed the rule change takes effect. However in a library you make the change and deploy the library but it is up to the apps that include the library to decide when the change is deployed in prod. As the owner of the library, you no longer have control on when it is deployed in prod.