Convention over Configuration

For a team to deliver consistency in the codebase, one should aim for a recipe that makes it easy to the right thing and hard to the wrong thing. Most software projects tend to put in place both formal rules and informal rules in the codebase that are recipes considered good for the project. These are often aimed at increasing the productivity of the developer and making it more predictable where and how to do things.

These recipes are often not formalized to the level of being enforced by something like build time tools. During times of high pressure and stress, the rules can easily be broken due to the “in the interest of time”-principle. This is obviously something that makes the codebase inconsistent and hard to maintain over time.

At Dolittle we believe in formalizing the conventions, putting it front and center and making it hard to the wrong thing. This helps in governing the recipes and forces the team to deliver more consistently and stripping away the ability to take shortcuts. Dolittle itself throughout is relying heavily on this and enforcing some of the things hard while other things might be more configurable conventions.

Structure

An example of an enforced convention is how Dolittle looks at application structure. It expects an application with one or more bounded context in it and within a bounded context you can have modules which in turn can have features in it. This hierarchy gives the developers a vocabulary at the same time a strong structure. For instance, a C# developer will not be allowed to put things in arbitrary namespaces as the Dolittle tooling is expecting artifacts to be in certain expected places based on the topology of the bounded context.

Discovering

A very strong principle at the heart of Dolittle is the notion of discovery. The code that we write can all be discovered either at build-time or at runtime. This is a very powerful concept enabling one to much easier adhere to the Open / Closed principle. Meaning that we don’t have to open up code to get new things in place, it will be discovered by how the conventions are and configured.

You find examples of this in the Dolittle SDKs where things like event processors and command handlers are discovered and automatically registered for the concrete command to be handled or event to be processed.

If you’re a C# developer, there are great ways to discover either implementations of types or instances of types.

These can simply be taken dependencies on in a constructor and you can start iterating over them.

Inversion of Control conventions

The inversion of control principle speaks about inverting the control of creation and lifecycle of objects and leaving this responsibility to a container that governs this. A container holds binding definitions internally that holds information about which concrete implementation to provide and which lifecycle it should be in (transient, per thread, singleton and so forth). These definitions could be setup manually, putting a lot of responsibility on the developer to do this manually and might even be error-prone. Often the binding is actually a recipe - a pattern the developers do all the time, something that can be automated by leveraging discovery mechanism. For instance a very common pattern is to represents the dependencies as interfaces. Concrete implementations are often then named almost exactly the same, only differing with a prefix or a postfix. The pattern is then consistently applied and is something that can be discovered and bindings can be put in place automatically. Example of this is an interface called IFoo bound towards Foo.

Another benefit of doing ths definitions programatically through discovery mechanism is the ability to then apply something like lifecycle cross-cuttingly throughout the system.