What’s wrong with a global User module?

This post is inspired by a code review I’ve done at work today. We generally try to build our applications as a set of loosely coupled components, so when I saw a User schema in the root project directory, it got me thinking about its impact on the overall architecture of the system.

My first thought was that we, as web developers, are too used to thinking in terms of nouns instead of verbs. We think about database tables, records, associations. This may work fine for simple CRUD resources, but not for entire application. For more complex features, we should really start to think about the process. What are the features of the application? What can the end user do with it? This is what’s important. All the nouns are, in my opinion, just an implementation detail (do you think your consumer care about what database you use or how many tables you have?).

So what’s wrong with having a global User module?

In my opinion, it’s often added without any thought. It’s obvious that every application should have a User entity, right? But I think that doing so often leads to a bad design. I think it encourages you to just add fields when it’s necessary, instead of thinking about proper architecture and creating well separated modules. I’m pretty sure that once you create a global User schema, it will grow a lot and will be coupled with almost all parts of the system. This means that you’ll have no flexibility and the entire system will be really hard to understand. Have a look at the User modules/classes in your current projects. Is this code easy to understand? Are you comfortable when you change it?

Scientists show that we are capable of keeping only limited number things in our mind at any given time, so it’s extremely important to be able to think about different parts of the system in isolation. In order to do that, each part have to be as small and as loosely coupled with other parts as possible. And this is not possible when you have a huge User module which couples all the parts together.

A better way

On the other hand, let’s imagine that you have Authentication component. Inside it, you would probably have some User schema with the fields necessary for the authentication process. This schema would be private for this component. It shouldn’t ever be used by other components, so it wouldn’t be possible to add anything that’s not related with authentication. This guarantees that it would remain small, focused on doing one thing well and disconnected from the rest of the system. This would make it really easy to understand and change.

So if you wanted to, for example, add profile information, you couldn’t just add fields like name, email to the existing user schema. You’d have to create a separate component for this feature with a separate database table. This would result with two really small parts (Authenticationand Profiles) which are connected only by some user identifier. You can also use that identifier to reference the database tables to ensure data integrity if you want to. But on the application level, they would be a separate things.

If you run an e-commerce site, you would probably have some shipping information. So you’d create a Shipping context and keep the necessary information there.

Every time you wanted to add a new feature, you would just create a small component instead of adding fields to a huge User with all the information. Authentication, Reviews, Profiles, Payments, Shipping, … - they all are somehow connected to the user (as a person who uses the application), but they are all interested in different information and this information shouldn’t be kept in one place because it’s really hard to understand and change later (I’m talking about “one place” on the application level. As mentioned before, you might want to keep all the data in the same place - one database for example - to enforce the data integrity).

For comparison, with global User schema, it would probably already have all the fields connected with authentication, profiles, shipping etc. Now, imagine that you want to change one part of your system. If your User is public and every part of the system depends on it, how can you be sure that removing/changing one field won’t affect other parts? You can’t reason about one component of the system in isolation, you have to keep everything in your mind and this is almost impossible.

You are less flexible, too. Think about shipping. Imagine that you have to add an ability to have multiple shipping addresses associated with one user. If you have that information stored in the User struct, you have a lot of work to do. You have to create a separate schema/table, migrate the existing data, modify the users table in the database and make sure that you don’t break anything while doing that. On the other hand, when you start with seperate context with a function which returns the address, Shipping.fetch_shipping_info(user_id), you just have to return the list of addresses instead of one. There’s no data migrations necessary, the impact on the entire system is a lot lower, so there’s less risk involved.

That’s why I was talking about internal data structures as private. Other parts of the system are not allowed to use them. The only way to access the information is by using a clearly defined APIs.

Now let’s look at benefits from the inside and from the outside of the component.

Inside the component

Inside the Profiles component you may have aProfiles.fetch_profile(user_id) function that returns a map or struct with :name:email:phone_number fields. But does it mean that you have to have a schema and database table with this exact columns? Not at all!

Inside the Profiles context, you can make all the changes you want as long as you return :name:email:phone_number fields from the Profiles.fetch_profile(user_id) function.

You want to change the internal database schema? Or even change the database? Or drop the database and keep everything in memory? Or move the profiles feature to a separate service and retrieve the information via a HTTP call? As long as you maintain the interface, you’re good to go.

What’s important, that you can make all this changes and be sure that you won’t break anything in other parts of the system because you hide the implementation details behind the interface defined by Profiles.fetch_profile(user_id).

Outside of the component

From the outside, you don’t have to care about the implementation details to understand what this component does. You only care what the interface is. You provide user_id and get the information you need. It’s simple. It’s easy to understand. You don’t have too keep all the details in mind. You can understand the system better because it consists of loosely connected parts which communicate by usingclearly defined interfaces instead of details like schemas and database tables.


There’s another benefit of having the system built as a system of loosely coupled components. It is so easy to write the unit tests for the individual pieces of the system. You don’t have to worry about complicated setup because the interface of the component is really small and the component is isolated from the rest.

Your tests also become less fragile. Because you have a clean interface used to interact with the component, you can use this interface to test it. This is really a big deal because it allows you to refactor your code with confidence. How many times did you refactored your code and had to change both tests and the application code? This was because your tests were testing the implementation, not the behaviour of the component. The real power of refactoring is that you can change the tests or the application code while the other part remains stable. Only this gives you the confidence that you don’t break anything. If your tests passed before the change in the code and are still passing after, it means that everything is fine. If you are changing your tests and they still pass after the change, while the code is stable, you’re safe too. If you change both at the same time, you cannot have any confidence.

Think about this: tests are just another client of your application code. This means that they, too, should be decoupled from too much implementation details. They should be thought out and designed properly. Only then they can give you the confidence that you count for. Fortunately, having a good design of the application code often improves the design of the tests.

So why we choose nouns?

Coming back to the beginning — when modelling the system, let’s start to think about processes, features and actions instead of data structures or database tables.

But why do we often start with nouns? This may be caused by the misunderstanding of the Object Oriented programming which was, for a long time, the most popular programming paradigm. We was really focused on objects and missed the key idea. In the words of Alan Key, the father of object orientation:

Smalltalk is not only NOT its syntax or the class library, it is not even about classes. I’m sorry that I long ago coined the term “objects” for this topic because it gets many people to focus on the lesser idea.
The big idea is “messaging” — that is what the kernal of Smalltalk/Squeak is all about. (…) The key in making great and growable systems is much more to design how its modules communicate rather than what their internal properties and behaviors should be.


So don’t start with the properties. Even though there’s some real person in front of the device, this doesn’t mean that you should have some User entity with all of his/her properties. Think about how the components of the system should communicate to accomplish any given task. Only then figure out what data structures are necessary to implement this.

Common problems

Of course designing the system this way requires more initial work. It seems easier to just dump everything to the User schema. But remember that it’s only easy because when adding new features, you just think about one thing at a time.

It gets harder when you have to change things. To make the change you have to create the mental model of the entire system, understand the connections, wonder if anything will break. And unfortunately, most of our work is changing existing systems, not adding new features. So with a little more initial work, you’re be able to make your life a lot easier in the future. You’ll thank yourself later.

Another thing that may worry you is performance. You might think that splitting the system into isolated component would be slower because you would often have to make more queries to the database to get the necessary information (instead of just fetching the entire User). And while this statement might be true, the truth is that most of the time it doesn’t matter at all. Computers and database are really fast these days and they really can handle it. You might want to optimise things later, but as always - premature optimisation is the root of all evil. As someone smart once said: “First make it work, then make it elegant. Only then, if necessary, make it fast, while keeping it elegant”. So optimise only when it’s absolutely necessary and you have the data to prove that it is.


Think about verbs instead of nouns. Model the system around features, not data. Think before you start writing code. Creating software is a marathon, not a sprint. Don’t be afraid to sacrifice initial productivity for a better architecture. It will help you in the log run.