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Vertical vs. Horizontal Decomposition of Responsibility

  • Palo Alto, CA
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OOPoop

Objects responsible for too many things are a problem. Because their complexity is high, they are difficult to maintain and extend. Decomposition of responsibility is what we do in order to break these overly complex objects into smaller ones. I see two types of this refactoring operation: vertical and horizontal. And I believe the former is better than the latter.

Once Upon a Time in America (1984) by Sergio Leone
Once Upon a Time in America (1984) by Sergio Leone

Let's say this is our code (it is Ruby):

class Log
  def initialize(path)
    @file = IO.new(path, 'a')
  end
  def put(text)
    line = Time.now.strftime("%d/%m/%Y %H:%M ") + text
    @file.puts line
  end
end

Obviously, objects of this class are doing too much. They save log lines to the file and also format them—an obvious violation of a famous single responsibility principle. An object of this class would be responsible for too many things. We have to extract some functionality out of it and put that into another object(s). We have to decompose its responsibility. No matter where we put it, this is how the Log class will look after the extraction:

class Log
  def initialize(path)
    @file = IO.new(path, 'a')
  end
  def put(line)
    @file.puts line
  end
end

Now it only saves lines to the file, which is perfect. The class is cohesive and small. Let's make an instance of it:

log = Log.new('/tmp/log.txt')

Next, where do we put the lines with formatting functionality that were just extracted? There are two approaches to decompose responsibility: horizontal and vertical. This one is horizontal:

class Line
  def initialize(text)
    @line = text
  end
  def to_s
    Time.now.strftime("%d/%m/%Y %H:%M ") + text
  end
end

In order to use Log and Line together, we have to do this:

log.put(Line.new("Hello, world"))

See why it's horizontal? Because this script sees them both. They both are on the same level of visibility. We will always have to communicate with both of them when we want to log a line. Both objects of Log and Line are in front of us. We have to deal with two classes in order to log a line:

PlantUML SVG diagram

To the contrary, this decomposition of responsibility is vertical:

class TimedLog
  def initialize(log)
    @origin = log
  end
  def put(text)
    @origin.put(Time.now.strftime("%d/%m/%Y %H:%M ") + text)
  end
end

Class TimedLog is a decorator, and this is how we use them together:

log = TimedLog.new(log)

Now, we just put a line in the log:

log.put("Hello, world")

The responsibility is decomposed vertically. We still have one entry point into the log object, but the object "consists" of two objects, one wrapped into another:

PlantUML SVG diagram

In general, I think horizontal decomposition of responsibility is a bad idea, while vertical is a much better one. That's because a vertically decomposed object decreases complexity, while a horizontally decomposed one actually makes things more complex because its clients have to deal with more dependencies and more points of contact.