An example of "Clean Architecture" in Go to demonstrate developing a testable application that can be run on AppEngine with Google Cloud Storage or with traditional hosting and MongoDB for storage (but not limited to either).
There are a number of different application architectures that are all simlar variations on the same theme which is to have clean separation of concerns and dependencies that follow the best practices of "the dependency invesion principle":
A. High-level modules should not depend on low-level modules. Both should depend on abstractions.
B. Abstractions should not depend on details. Details should depend on abstractions.
Variations of the approach include:
- The Clean Architecture advocated by Robert Martin ('Uncle Bob')
- Ports & Adapters or Hexagonal Architecture by Alistair Cockburn
- Onion Architecture by Jeffrey Palermo
From more in-depth practical application of many of the ideas I can strongly recommend the excellent book Implementing Domain-Driven Design by Vaughn Vernon that goes into far greater detail.
Besides the clean codebase, the approaches also bring other advantages - significant parts of the system can be unit tested quickly and easily without having to fire up the full web stack, something that is often difficult when the dependencies go the wrong way (if you need a database and a web-server running to make your tests work, you're doing it wrong).
I'd used it before in the world of .NET but forgot about it after moving to coding more in Python. After switching languages again (yeah, right) to the wonderful world of go I came across a blog post which re-ignited my interest in it: Applying The Clean Architecture to Go applications
It's a great read but I found the example a little overly-complex with too much of the focus on relational database model parts and at the same time it was light on some issues I wanted to resolve such as switching between different storage types and web UI or framework (and Go has so many of those to chose from!).
I've also been looking for a way to make my application usable both standalone and on AppEngine as well as being easier to test, so this seemed like a good opportunity to do some experimenting and this is what I came up with as a prototype which I've hopefully simplified to show the techniques.
We've all heard of n-tier or layered architecture, especially if you've come from the world of .NET or Java and it's unfair that it gets a bad rep. Probably because it was often implemented so poorly with the typical mistake of everything relying on the database layer at the bottom which made software rigid, difficult to test and closely tied to the vendor's database implementation (hardly surprising that they promoted it so hard).
Reversing the dependencies though has a wonderful transformative effect on your code. Here is my interpretation of the layers or rings implemented using the Go language (or 'Golang' for Google).
At the center of the dependencies is the domain. These are the business objects or entities and should represent and encapsulate the fundamental business rules such as "can a closed customer account create a new order?". There is usually a single root object that represents the system and which has the factory methods to create other objects (which in turn may have their own methods to create others). This is where the domain-driven design lives.
Looking at this should give you an idea of the business model for the application and what the system is working with. This package allows code such as unit tests to excercise the core parts of the app for testing to ensure that basic rules are enforced.
The application level rules and use-cases orchestrate the domain model and add richer rules and logic including persistence. I prefer the term engine for this package because it is the engine of what the app actually does. The rules implemented at this level should not affect the domain model rules but obviously may depend on them. The rules of the application also shouldn't rely on the UI or the persistence frameworks being used.
Why would the business rules change depending on what UI framework is the new flavour of the month or if we want to change from an RDBMS to MongoDB or some cloud datastore? Those are implementation details that pull the levers of the use cases or are used by the engine via abstract interfaces.
These are concerned with converting data from a form that the use-cases handle to whatever the external framework and drivers use. A use-case may expect a request struct with a set of parameters and return a response struct with the results. The public facing part of the app is more likely to expect to send requests as JSON or http form posts and return JSON or rendered HTML. The database may return results in a structure that needs to be adapted into something the rest of the app can use.
These are the ports that allow the system to talk to 'outside things' which could be a database for persistence or a web server for the UI. None of the inner use cases or domain entities should know about the implementation of these layers and they may change over time because ... well, we used to store data in SQL, then MongoDB and now cloud datastores. Changing the storage should not change the application or any of the business rules. I tend to call these "providers" because ... well, .NET.
Within app folder ...
Install the AppEngine SDK for Go:
goapp serve
Start mongodb and build / run the go app as normal:
mongod --config /usr/local/etc/mongod.conf
go run app.go
Not yet added
ginkgo watch -cover domain
go tool cover -html=domain/domain.coverprofile
Go has build tags to control which code is included and when running on AppEngine
the appengine tag is automatically applied. This provides an easy way to include
or exclude code that will only work on one platform or the other. i.e. there is no
point building the appengine provider into a standalone build and some code can't
be executed on appengine classic - this provides a way to keep things separated.
Surely it's needed for such a thing? No it isn't. While DI can be a useful tool, very often it takes over a project and becomes an entangled part of the application architecture masquerading as the framework. Seriously, you don't need it and it often comes with a huge cost in terms of complexity and runtime performance. Whatever a DI framework does, you can do yourself with some factories - what we used before the world went DI crazy and thought Spring was a good idea (oh, how we laugh about it now).
The Query spec provides a way to pass a query definition to the providers in a storage agnostic way without depending on any database specific querying language. This was attempted in .NET with Linq to mixed results - you often ended up coding for the specifics of certain databases (usually SQL server) but in this case the query language is much simpler and designed to be more lightweight as it only has to provide some filtering capability for what is going to be a NoSQL database or a SQL database being used in a non-relational way.
I picked AppEngine Datastore and MongoDB because they are kind of similar in that they are both NoSQL stores but are pretty different in how connections and state are maintained. The MongoDB storage has the connection passed in through the factory setup. The Datastore has no permanent connection and uses the context from each request.
There's a lot missing. Some obvious things would be to pass request information such as authenticated user, request IP etc... in a standard struct embedded within each interactor request. The responses should also return errors that the web ui adapter could use in the response.
A console adapter could be created to demonstrate the ability to use the engine app logic and the storage without the web ui. Speaking of which ... some unit tests would also show how the majority of the system can be tested without having to fire up a web server or a database. Test storage instances can be used to test the engine and test engine instances can help test the web handlers.
Why do I separate the imports in Go? I just like it ... I divide the imports into:
Standard packages (e.g. strings or fmt)
Extensions to packages (e.g. net/http or encoding/json)
3rd party packages (e.g. google.golang.org/appengine/datastore)
Application packages (e.g. github.com/captaincodeman/clean-go/domain)