Why Abstraction is Really Important

Abstraction
Abstraction is one of the key elements of good software design.
It helps encapsulate behavior. It helps decouple software elements. It helps having more self-contained modules. And much more.

Abstraction makes the application extendable in much easier way. It makes refactoring much easier.
When developing with higher level of abstraction, you communicate the behavior and less the implementation.

General
In this post, I want to introduce a simple scenario that shows how, by choosing a simple solution, we can get into a situation of hard coupling and rigid design.

Then I will briefly describe how we can avoid situation like this.

Case study description
Let’s assume that we have a domain object called RawItem.

public class RawItem {
    private final String originator;
    private final String department;
    private final String division;
    private final Object[] moreParameters;
    
    public RawItem(String originator, String department, String division, Object... moreParameters) {
        this.originator = originator;
        this.department = department;
        this.division = division;
        this.moreParameters = moreParameters;
    }
}

The three first parameters represent the item’s key.
I.e. An item comes from an originator, a department and a division.
The “moreParameters” is just to emphasize the item has more parameters.

This triplet has two basic usages:
1. As key to store in the DB
2. As key in maps (key to RawItem)

Storing in DB based on the key
The DB tables are sharded in order to evenly distribute the items.
Sharding is done by a hash key modulo function.
This function works on a string.

Suppose we have N shards tables: (RAW_ITEM_REPOSITORY_00, RAW_ITEM_REPOSITORY_01,..,RAW_ITEM_REPOSITORY_NN),
then we’ll distribute the items based on some function and modulo:

String rawKey = originator + "_"  + department + "_" + division;
// func is String -> Integer function, N = # of shards
// Representation of the key is described below
int shard = func(key)%N;

Using the key in maps
The second usage for the triplet is mapping the items for fast lookup.
So, when NOT using abstraction, the maps will usually look like:

Map<String, RawItem> mapOfItems = new HashMap<>();
// Fill the map...

“Improving” the class
We see that we have common usage for the key as string, so we decide to put the string representation in the RawItem.

// new member
private final String key;

// in the constructor:
this.key = this.originator + "_" + this.department + "_"  + this.division;

// and a getter
public String getKey() {
  return key;
}

Assessment of the design
There are two flows here:
1. Coupling between the sharding distribution and the items’ mapping
2. The mapping key is strict. any change forces change in the key, which might introduce hard to find bugs

And then comes a new requirement
Up until now, the triplet: originator, department and division made up a key of an item.
But now, a new requirement comes in.
A division can have subdivision.
It means that, unlike before, we can have two different items from the same triplet. The items will differ by the subdivision attribute.

Difficult to change
Regarding the DB distribution, we’ll need to keep the concatenated key of the triplet.
We must keep the modulo function the same. So distribution will remain using the triplets, but the schema will change and hava ‘subdivision’ column as well.
We’ll change the queries to use the subdivision together with original key.

In regard to the mapping, we’ll need to do a massive refactoring and to pass an ItemKey (see below) instead of just String.

Abstraction of the key
Let’s create ItemKey

public class ItemKey {
    private final String originator;
    private final String department;
    private final String division;
    private final String subdivision;

    public ItemKey(String originator, String department, String division, String subdivision) {
        this.originator = originator;
        this.department = department;
        this.division = division;
        this.subdivision = subdivision;
    }

    public String asDistribution() {
        return this.originator + "_" + this.department + "_"  + this.division;
    }
}

And,

Map<ItemKey, RawItem> mapOfItems = new HashMap<>();
// Fill the map...
    // new constructor for RawItem
    public RawItem(ItemKey itemKey, Object... moreParameters) {
        // fill the fields
    }

Lesson Learned and conclusion
I wanted to show how a simple decision can really hurt.

And, how, by a small change, we made the key abstract.
In the future the key can have even more fields, but we’ll need to change only the inner implementation of it.
The logic and mapping usage should not be changed.

Regarding the change process,
I haven’t described how to do the refactoring, as it really depends on how the code looks like and how much is it tested.
In our case, some parts were easy, while others were really hard. The hard parts were around code that was looking deep in the implementation of the key (string) and the item.

This situation was real
We actually had this flow in our design.
Everything was fine for two years, until we had to change the key (add the subdivision).
Luckily all of our code is tested so we could see what breaks and fix it.
But it was painful.

There are two abstraction that we could have initially implement:
1. The more obvious is using a KEY class (as describe above). Even if it only has one String field
2. Any map usage need to be examined whether we’ll benefit by hiding it using abstraction

The second abstraction is harder to grasp and to fully understand and implement.

So,
do abstraction, tell a story and use the interfaces and don’t get into details while telling it.

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The Single Responsibility Principle

Introduction
In this post I would like to cover the Single Responsibility Principle (SRP).
I think that this is the basis of any clean and well designed system.

What is SRP?
The term was introduced by Robert C. Martin.
It is the ‘S’ from the SOLID principles, which are the basis for OOD.
http://en.wikipedia.org/wiki/SOLID_(object-oriented_design)
Here’s the PDF paper for SRP by Robert C. Martin https://docs.google.com/file/d/0ByOwmqah_nuGNHEtcU5OekdDMkk/

From Wikipedia:

…In object-oriented programming, the single responsibility principle states that every class should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility….

From Clean Code:

A class or module should have one, and only one, reason to change.

So if a class (or module) needs to be modified for more than one reason, it does more than one thing. I.e. has more than one responsibility.

Why SRP?

  • Organize the code
    Let’s imagine a car mechanic who owns a repair shop.
    He has many many tools to work with. The tools are divided into types; Pliers, Screw-Drivers (Phillips / Blade), Hammers, Wrenches (Tubing / Hex) and many more.

    How would it be easier to organize the tools?
    Few drawers with different types in each one of them?
    Or, many small drawers, each containing a specific type?

    Now, imagine the drawer as the module. This is why many small modules (classes) are more organized then few large ones.

  • Less fragile
    When a class has more than one reason to be changed, it is more fragile.
    A change in one location might lead to some unexpected behavior in totally other places.
  • Low Coupling
    More responsibilities lead to higher coupling.
    The couplings are the responsibilities.
    Higher coupling leads to more dependencies, which is harder to maintain.
  • Code Changes
    Refactoring is much easier for a single responsibility module.
    If you want to get the shotgun effect, let your classes have more responsibilities.
  • Maintainability
    It’s obvious that it is much easier to maintain a small single purpose class, then a big monolithic one.
  • Testability
    A test class for a ‘one purpose class’ will have less test cases (branches).
    If a class has one purpose it will usually have less dependencies, thus less mocking and test preparing.
    The “self documentation by tests” becomes much clearer.
  • Easier Debugging
    Since I started doing TDD and test-first approach, I hardly debug. Really.
    But, there come times when I must debug in order to understand what’s going on.
    In a single responsibility class, finding the bug or the cause of the problem, becomes a much easier task.

What needs to have single responsibility?
Each part of the system.

  • The methods
  • The classes
  • The packages
  • The modules

How to Recognize a Break of the SRP?

  • Class Has Too Many Dependencies
    A constructor with too many input parameters implies many dependencies (hopefully you do inject dependencies).

    Another way too see many dependencies is by the test class.
    If you need to mock too many objects, it usually means breaking the SRP.

  • Method Has Too Many Parameters
    Same as the class’s smell. Think of the method’s parameters as dependencies.
  • The Test Class Becomes Too Complicated
    If the test has too many variants, it might suggest that the class has too many responsibilities.
    It might suggest that some methods do too much.
  • Class / Method is Long
    If a method is long, it might suggest it does too much.
    Same goes for a class.
    My rule of thumb is that a class should not exceed 200-250 LOC. Imports included 😉
  • Descriptive Naming
    If you need to describe what your class / method / package is using with the AND world, it probably breaks the SRP.
  • Class With Low Cohesion
    Cohesion is an important topic of its own and should have its own post.
    But Cohesion and SRP are closely related and it is important to mention it here.
    In general, if a class (or module) is not cohesive, it probably breaks the SRP.

    A hint for a non-cohesive class:
    The class has two fields. One field is used by some methods. The other field is used by the other methods.

  • Change In One Place Breaks Another
    If a change in the code to add a new feature or simply refactor broke a test which seems unrelated, it might suggest a breaking the SRP.
  • Shotgun Effect
    If a small change makes a big ripple in your code. If you need to change many locations it might suggest, among other smells, that the SRP is broken.
  • Unable to Encapsulate a Module
    I will explain using Spring, but the concept is important (not the implementation).
    Suppose you use the @Configuration or XML configuration.
    If you can’t encapsulate the beans in that configuration, it should give you a hint of too much responsibility.
    The Configuration should hide any inner bean and expose minimal interfaces.
    If you need to change the Configuration due to more than one reason, then, well, you know…

How to make the design compliant with the Single Responsibility Principle
The suggestions below can apply to other topics of the SOLID principles.
They are also good for any Clean Code suggestion.
But here they are aimed for the Single Responsibility Principle.

  • Awareness
    This is a general suggestion for clean code.
    We need to be aware of our code. We need to take care.
    As for SRP, we need to try and catch as early as we can a class that is responsible for too much.
    We need to always look for a ‘too big method’.
  • Testable Code
    Write your code in a way that everything can be tested.
    Then, you will surly want that your tests be simple and descriptive.
  • TDD
    (I am not going to add anything here)
  • Code Coverage Metrics
    Sometimes, when a class does too much, it won’t have 100% coverage at first shot.
    Check the code quality metrics.
  • Refactoring and Design Patterns
    For SRP, we’ll mostly do extract-method, extract-class, move-method.
    We’ll use composition and strategy instead of conditionals.
  • Clear Modularization of the System
    When using a DI injector (Spring), I think that Configuration class (or XML) can pinpoint the modules design. And modules’ single responsibility.
    I prefer to have several small to medium size of configuration files (XML or Java) than having one big file / class.
    It helps see the responsibility of the module and easier to maintain.
    I think that the configuration approach of injection has an advantage of annotation approach. Simply because the Configuration approach put the modules in the spotlight.

Conclusion
As I mentioned in the beginning of this post, I think that Single-Responsibility-Principle is the basis of a good design.
If you have this principle in your mind while designing and developing, you will have a simpler more readable code.
Better design will be followed.

One Final Note
As always, one needs to be careful on how to apply practices, code and design.
Sometimes we might do over-work and make simple things over complex.
So a common sense must be applied at any refactor and change.