Fintech Mindset to Software Design

If you’re creating a line of business or enterprise-type software, I think one of the most valuable skills you can have isn’t technical. Rather it’s understanding how the business domain you are in works. One way is following how money flows through a system by having a fintech mindset.

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Revenue & Cost

I was on the Azure Devops Podcast, where I mentioned that a big influence on my career was working with an Accountant. No surprise, this has affected how I look at a line of business and enterprise-type systems, how they can be decomposed, and how money flows through the system.

At a high level, I’m thinking about Revenue and Cost.

For context, I’m going to talk about project-based work to illustrate.

How does the company generate revenue? How does the software system we are building fit into how the company generates revenue?

How does the company incur costs to generate that revenue? How do we keep track of that in the system, similar to how we generate revenue.

We generally have customers, sales, and invoicing on the revenue side.

Revenue Side

There is some type of sales process involved with a CRM that ultimately leads us to a sale and invoice a customer to get paid.

On the cost side, we may have employees with a salary or work per hour. We pay them at some interval (weekly, bi-weekly, monthly) based on their salary or hours worked (timesheet).

Employee Cost Side

Other costs can occur, such as materials or services from outside vendors. We will create purchase orders and get receipts.

Vendor Cost Side

Follow the Money

When thinking about decomposing a system and understanding a domain, I help to follow the money. This can lead to understanding the workflows and processes that generate revenue and cost.

In the case of project base work, the heart is in the execution. The actual project. There are many ways to “sell” a project, such as Time & Materials, Fixed Price, etc. The same goes for costs, some employees might be Salary (fixed) some may be hourly.

Regardless, the point is the execution of the project is where the complexity lies. This is likely to be the heart of the system you’re building.

Execution

For example, if you’ve worked on a software project before or done any project management, you can relate that this is the heart of complexity.

So if you were writing a system to manage the full lifecycle of project-based work, the execution of the project would be the core. Many boundaries would act purely in a supporting role, such as CRM, Employee, Payroll, Vendors, and Invoicing. You don’t necessarily need to write your own; this is usually pretty generic, and you can buy it off the shelf or software as a service.

And this makes sense because you’re not about to write a CRM or Accounting system. You can integrate with those to push/pull the relevant data required.

On the other hand, Timesheets, Sales, and Purchasing may be something you choose to write because there isn’t anything specific enough for the context/niche of what you’re building.

Core and Supporting Boundaries

Fintech Mindset

I’ve written software for Distribution, Accounting, Manufacturing, and various forms of Transportation, and they have all of this in common. Have a fintech mindset by understanding revenue and costs and how each is made.

Here’s a screenshot of QuickBooks, an accounting system typically used for small businesses. It gives a good example of how everything is related and how money flows.

Quickbooks

In writing line of business and enterprise type systems, I’ve always found the best developers are the ones that have equally great technical development ability as well as domain knowledge. Having a fintech mindset and understanding how money flows helps.

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SOLID? Nope, just Coupling and Cohesion

How do we avoid writing spaghetti code so our systems don’t turn into a hot mess? For me Coupling and Cohesion. Some people focus on things like SOLID principles and Clean Architecture. While I don’t necessarily have a problem with that if you’re pragmatic, I don’t ever really think about either of those explicitly.

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Coupling and Cohesion

With over 20 years of professional software development experience, I’m mostly thinking about coupling and cohesion as a guide to software design. I’m not explicitly thinking about SOLID principles or clean architecture.

yin yang

Coupling and cohesion are like the yin-yang of software design. They push and pull against each other. You’re striving for high cohesion and low coupling, however, you’re trying to find a balance. We’re always fighting against coupling.

There are many different forms of coupling but to narrow it down for simplicity’s sake:

“degree of interdependence between software modules”

ISO/IEC/IEEE 24765:2010 Systems and software engineering — Vocabulary

At a micro level, this could be the interdependence between classes or functions, and at the macro level, this could be the interdependence between services.

When you’re thinking about spaghetti code you’re likely actually referring to a system that has a high degree of coupling at the macro and micro levels. This makes changes difficult to make or they break other parts of the system because there’s a high degree of coupling. Check out my post on Write Stable Code using Coupling Metrics for more on coupling.

Cohesion refers to:

“degree to which the elements inside a module belong together”

Structured Design: Fundamentals of a Discipline of Computer Program and Systems Design

You can look at cohesion both from a micro and macro level. From a micro level, this can be viewed as how do all the methods relate in a class? Or how do all the functions of a module relate? From a macro level, how do all the features relate to a service? Check out my post on Highly COHESIVE Software Design to tame Complexity for more on cohesion.

But what does “belong together” mean? For me, this is about functional cohesion. Grouping-related options of a task. Not grouping based on data (which is informational cohesion). Yes, data is important, however, data is required by the functionality that is exposed.

Business Capabilities

Speaking of functionality, let’s jump to the Single Responsibility Principle for a second as this might clarify why functional cohesion is important.

When you write a software module, you want to make sure that when changes are requested, those changes can only originate from a single person, or rather, a single tightly coupled group of people representing a single narrowly defined business function. You want to isolate your modules from the complexities of the organization as a whole, and design your systems such that each module is responsible (responds to) the needs of just that one business function.

https://blog.cleancoder.com/uncle-bob/2014/05/08/SingleReponsibilityPrinciple.html

This might seem a bit more clear how the single responsibility principle at its root addresses coupling and cohesion.

If we focus on business capabilities and group them together, we’ll end up with a service. That’s why I always define a service as:

A Service is the authority of a set of business capabilities.

At a macro level, we’re trying to have a high degree of cohesion by grouping by business capabilities. Behind those capabilities is data ownership.

Unfortunately, many are still focused purely on data and what I call “entity services”. This was a screenshot from a post on Reddit. The question was if this data model diagram should be implemented as a service per “entity”.

entity services

This type of thinking is based on informational cohesion, not functional cohesion. The focus solely on data and not thinking at all about functionality and behavior leads to design generally around CRUD.

As an example, you’ll often have “Manager”, and “Repository” classes that look similar to this.

IProductRepository

The Product “entity” is really just a data model with a bunch of properties. There is no behavior exposed, they are just data buckets.

So where is the actual functionality of a product? In a large system, for example in a distribution domain, a product isn’t just a product.

A product means different things to different people within that domain.

Service Boundaries by Functional Cohesion

Sales are thinking about the Sale Price of a product and are customer-centric. Purchasing/Procurement is thinking about the cost and is vendor centeric. The warehouse is concerned about shipping and receiving. They all have different business capabilities, and because those capabilities are different, they care about different data behind those capabilities.

A product isn’t just a product that has to live within a single boundary. It’s a concept that can live within multiple different service boundaries.

Organizing related business capacities (features) into services allows us to decide how we want to handle technical concerns within each service or to a specific feature. Dependencies shared concerns such as data access, validation, etc. All of these decisions can be localized and made per service.

Services with features with high functional cohesion

Loose Coupling

So you’ve defined boundaries based on functional cohesion, but they must have some interdependence between each other?

Tight Coupling between Services

Having a free for all where any service can be coupled to any other service is still a hot mess of spaghetti. In other words, tight coupling. The system will still be hard to change and fragile to change.

You want to remove the tight coupling by having service boundaries be independent and not directly coupled to other services. One way of achieving this is through loose coupling provided by messaging.

Loose Coupling between Services

Because you’re grouping by business capabilities, and the data behind those capabilities, each service should have all the data it needs. This means that you aren’t coupling services because you need to fetch data from them to perform an action.

Services together need to create workflow and exchange information. As an example from the diagram above, if the Warehouse has a “Quantity on Hand”, you might think that Sales would need that so it knows if it can sell a given product. However, Sales actually has its own concept called ATP (Available to Promise) which is a business function that is the projected amount of inventory it can sell. This consists of what’s in stock in the Warehouse, not allocated existing orders (Invoicing), as well as purchase orders and expected receipts (Purchasing).

Sales can maintain their ATP by consuming events from other services. It does not need to make directly coupled to an API to make calls at runtime to calculate ATP for a given product. It maintains and owns the ATP for all products based on events it’s consuming that are published from other services. When Invoicing publishes an OrderInvoiced event, it can subtract an amount from ATP. If the warehouse does a stock count and publishes an InventoryAdjusted event, Sales will update the ATP accordingly.

Responsibility

From a lower-level code perspective, the coupling can be challenging when we have to deal with many different technical concerns. Web, Authorization, Validation, Data Access, and Business Logic. But as mentioned earlier, each feature/capability can define how each of these concerns is handled.

While you can share between features, such as a domain model, this naturally starts limiting the coupling between a set of features.

One approach to handle the common concerns is by using the pipes & filters pattern. Specifically, the Russian doll model which the request passes through a filter that can call (or not) the next filter. This allows you to separate various concerns and create a pipeline for a request.

Pipes & Filters

For more check out my post on Separating Concerns with Pipes & Filters

Coupling and Cohesion

Building a large system without any boundaries (low cohesion) and a high degree of coupling is ultimately building a big turd pile of a system.

Big Turd Pile

It will be hard to add new functionality, and hard to change existing functionality without breaking and causing regressions.

Focusing on having highly cohesive services that provide specific business capabilities and loosely coupling between those services to provide workflow and business processes will allow you to build a system that is more resilient to change.

Smaller Turd Piles

Decomposing a large system into smaller independent logical service boundaries allows you to make different decisions that are localized to an individual service. No system will be perfect, especially over time and it will need to evolve. Focus on coupling and cohesion as a guide.

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Is an ANEMIC Domain Model really that BAD?

Is an anemic domain model a bad thing? Most would probably call it an anti-pattern, and it should be avoided. But is that really true? Well, it depends on what your intent is. Are you trying to create a domain model? Or are you really just trying to create a data model?

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Example

In this post, I’m going to be using the example of a food delivery service. You order food from the restaurant and it’s delivered to you.

There is the concept of a shipment which is the delivery of your food order. Every shipment has two stops. The first stop is the Pickup which is the restaurant, and the second stop is the Delivery which is your house/apartment/work or wherever you’re getting the food delivered to.

Conceptual Model

Each stop (Pickup and Delivery) goes through a progression from being In-Transit, Arrived, and Departed.

Stop Progression

The first Stop must go through this full progression before the second Stop can start. This makes sense because the food delivery driver must first Arrive at the Restaurant (the first stop). Once they pick up the food, they depart. Once they have departed the First Stop, then they can Arrive at your house/apartment/work for the delivery. Once they deliver the food, the second stop is now Departed.

Anemic Domain Model

An anemic domain model is an object model that contains a data structure but no behavior. In the case of my shipment example, this means it has an object for the Shipment and the Stops, but no behavior within the objects to make the stop progressions.

While there are a few methods for setting data, this isn’t behavior. We need logic to control the Status on a stop. For example, we can’t change the Stop Status on the Delivery Stop to Arrived if we haven’t Departed the Pickup Stop.

There is no behavior, my object model of Shipment and Stops are just data buckets.

So where does the logic live? In an anemic domain model, generally, you’ll find these in a “Service” or “Manager”. So in this example, you’d likely have a ShipmentService or ShipmentManager that contains the logic.

In the ShipmentService above, we do have logic in various methods for validating that the current state is valid before we perform a state change. Also, note that most of these methods are taking the shipment as a parameter however sometimes I see data access done directly within these methods.

Regardless of where the data access is, the point is that the logic resides separately from the data. The object method of Shipment and Stops are just data buckets and contain no actual logic. All of the logic is separated and contained within inside the ShipmentService which then mutates the Shipment and Stops.

So what’s the issue with this?

It depends on what your intent is. If you’re intent is to make an object-oriented domain model, then this isn’t it. You’ve made an object method that represents the data structure. You’ve made a data model. You think you have a domain model, but you really just have a data model.

Now I suspect most people aren’t trying to create a domain model, what they’re really doing is just trying to create some separation or layering (more on this later in this post). What you’re really closer to are Transaction Scripts.

Entity Services depend on an ORM for data access. The ORM provides returns Entities that are really a one-to-one mapping of a table structure if you’re using a relational database.

Anemic Domain Model

Spectrum

Now I actually view most anemic domain models closer to Transaction Scripts than I do a Domain Model. Anemic domain models are kind of this in-between phase of going a bit further than a Transaction Script but not all the way to a Domain Model.

Transaction Script to Anemic Domain Model, to Domain Model

Transaction scripts are much more procedural and often times has many more mixed concerns, such as data access, validation, etc.

The intent of a transaction script is to handle a single request. It contains everything. It would contain the data access to get out the Entities/Domain Objects. It would contain all the validation logic and also state changes.

Transaction Script

Because you have many mixed concerns, people want to separate these concerns and land with Anemic Domain Models.

Where this starts falling apart is when you need to duplicate logic or state changes that exist in multiple different transaction scripts. When you start seeing this duplication, you then want to start having transaction scripts call other transaction scripts so you can reuse them and stop duplicating code.

Transaction Script calling another Transaction Script

This becomes difficult because if they both contain their own data access logic, then there’s often no simple way of sharing the same underlying transaction.

Once people hit this point, this is where I believe they start going down the Anemic Domain Model path. Instead of duplicating code in different transaction scripts, that’s shared within a single class that turns into the Service (eg, ShipmentService).

Domain Model

The alternative is to have a domain model that exposes behaviors and encapsulates data. Don’t separate the behaviors from the data like you are with an anemic domain model. Keep both behaviors and data within the same object model.

Domain Model

Similar to a transaction script, it can be the one delegating what to do. It can provide the data access to retrieve and/or build our domain model (often an Aggregate Root). It can then call the relevant public methods on the domain model to perform the requested command/action.

The constructor takes a list of Stops. The only way a Stop can be mutated is by calling methods on the ShipmentAggregateRoot. We’re hiding (encapsulating) the data and only exposing behaviors.

To invoke our domain model, we have command handlers that get our Aggregate Root via a Repository, call the appropriate method, then save the aggregate.

Similar to a transaction script, it’s handling a single request, but now instead of having mixed concerns within it, it’s simply delegating to data access and our aggregate root.

Anemic Domain Model

Is an anemic domain model a terrible thing and an anti-pattern that you should avoid? Well, it depends on what your intent is! If you’re trying to create a domain model because you have a lot of complexity, then yes it’s an anti-pattern. However, if you don’t have a lot of complexity that warrants a domain model, a transaction script might be better served for simplicity.

If you’re using transaction scripts and things start getting more and more complex or you start duplicating logic across transaction scripts, then start thinking about building a domain model that hides (encapsulates) data and exposes behaviors.

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