Transactional Scripts (aka Services) and Domain Model
I often see people falling for the trap of “Transactional scripts” so I decided that it is probably worth writing about this so more programmers would be able to identify flaws in their design and improve.
Let’s start with Transactional Scripts. What are they?
I would say that this is Procedural Programming disguised as OOP. In practice, it looks something like this: you have a bunch of objects that have only values but only few methods (probably setters and getters) and a bunch of “Services”* that manipulate those objects to get them into desired state (and persist) according your business process. Let rephrase that to emphasize what’s happening - you have multiple scenarios that hold your business logic but your “model” objects don’t have any.
Example for completeness:
Domain oriented model on the other hand, does encourage keeping closely related processes and business objects or entities together. If some specific process is related to object, they will probably be in the same class. Most importantly, code is shaped after actual business language and flow, instead of being placed somewhere predefined.
For comparison, let’s take a look here:
According to Fowler, using Transactional Scripts is justifiable for small projects. For larger ones you want to use Domain Driven Design.
The problem with Transactional Scripts is that while they are easy to start with, maintenance costs rumps up quite quickly when your project size increases. I would say this happens due to distributed logic among different parts of system - let’s say you have special “corporate” order type which needs to be executed using different rules than for the regular customer. You will end up writing all of that logic under separate Transactional Script that has to be incorporated into existing ones. This may not be so trivial task when you have around 100 or more places to update.
Additional flow might break existing scripts’ functionality, new script may not be able to reuse same code for other scripts since it’s hard to extract from current ones. DDD’ish approach would probably solve the same problem using plain polymorphism.
It’s a lot harder to apply SOLIDprinciples while using Transactional Scripts. Also, these scripts tend to have circular dependencies between each other for different modules. Business logic isn’t encapsulated by business entities.
It’s really hard to come up with decent examples here because we want to have example short so it would be easier to follow what is this example about. However, while it is easy to understand it will negate main point of showing why such transactional scripts are harder to maintain compared to proper Domain Model. Also, having really big examples here wouldn’t look nice and main text would be hard to read.
If you can notice any of these problems in your projects, it could mean that Transactional Scripts aren’t working well for you:
- Adding additional step in our flow requires major restructuring
- Changing “domain” object’s rules ripples through multiple files
- Different feature modules have cyclic dependencies between them
- Code cannot be changed since it is used in MANY files
- Entity’s lifecycle cannot be described or can be only partly
- Big project cannot be broken into smaller modules (SOAish) because of spaghetti dependencies
In the end this sums up to that that Transactional Scripts don’t follow OOP principles and thus results in code that’s harder to maintain. Remember that part where you have to keep your business logic (Transactional Scripts) for your Entities that are saved in database? That’s basically means that you are going to have low cohesion[link]. Let me elaborate on this point next.
There is nice interpretation about orthogonality and cohesionin “The Pragmatic Programmer” book. Orthogonality in software projects basically means that changing one piece of system won’t impact another one because they are “orthogonal” (independent) to each other.
What does cohesion have to do with this? Cohesion shows how much module or class depends on itself to accomplish its task and how few other components it uses. Since Transactional Scripts approach to handling business logic always results in low cohesion (classes will always heavily depend on other classes) it basically means that your project in the end will be a lot harder to maintain than DDD’ish approach.
One may think that just using plain old good OO Design and SOLID principles would mitigate this problem and this is totally true. But, basically, Domain Driven Design is good practices applied in your business domain context :).
I hope I made my reasoning clear why do I prefer DDD approach to tackling business logic and I hope it will help you to refine your design or help choosing better approach too :).
*They are often called “Services” but actually “Services” are different things in Domain Driven Design. What do people usually call “Services” in software actually are “Transactional Scripts”.