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Humble Object pattern

Separating hard-to-test and easy-to-test behavior

Basic idea

  • Problem: some behavior (like how data is represented on a screen) is hard to test
  • Solution: split the behavior into two parts
    • Humble Object containing the hard-to-test stuff, stripped down to the bare essence
    • Other, easily testable part which contains everything stripped from the Humble Object

Example: showing data in a UI

  • Create a view model that describes as much as possible about how the data will be shown
    • Dates already converted to correct format
    • Flags describing if elements should be disabled, hidden, ...
  • Humble Object is a view which does nothing more than showing contents of view model on the screen

Values as boundaries

(also called "Functional Core, Imperative Shell")

See this talk: Boundaries (some more details here )

Same idea as Humble Object pattern, but taken to a more architectural level:

  • Functional Core: all decision logic sits here, no mutation, input and output are just values (see below)
    • Pure functions: output depends on input only, not on some other kind of state
    • Very easy and efficient to unit test
  • Imperative Shell: layer around the core, holds dependencies, takes care of interaction with the outside world (DB, web, screen, keyboard, ...)
    • Contains the slow code that can fail
    • Hard to test, but there shouldn't be much to test (a few integration or end-to-end tests should give you all the confidence you need)
    • Straightforward mapping between values returned from core and the effect on DB/screen/...
      • Core could return a complete representation of the new state, or some representation of changes to be made on top of the previous state (print a line on screen, perform a specific update on a specific DB row, ...)
  • System can consist of multiple shells (each with their own core) that communicate with each other

The definition of a value in this context: anything immutable

  • Could be a simple integer
  • Could be an object with immutable state, plus methods whose result depends on that state (and thus doesn't change either)
  • Every mutating operation should return a new object rather than mutating an existing object's state
    • Example: Java's String class

Additional benefit of these values (in addition to testability): values like these are easy to pass as messages. This way, values can be used as boundaries between classes, subsystems, processes and even different machines.

Some limitations to this "values as boundaries" approach:

  • Doesn't make sense for systems where the main complexity sits in the interaction with the outside world (this would leave us with a core that's more or less empty)
  • Challenging if the business logic requires a lot of back-and-forth with a DB or similar
    • "To perform action A, we need to retrieve X, then based on X we decide what Y to retrieve, and then we calculate how to update some Z based on X and Y"
    • If we extract the X->'what Y to retrieve' and X,Y->'which Z to update in which way' logic to the core (because that is the decision logic), it becomes harder to get the full picture of the business logic because it is split across multiple core functions that are tied together by the imperative shell
    • You can imagine this gets worse and worse when more back-and-forth is needed