ParentBased is a decorator

[pellared]

ParentBased sampler is decorator (and not a composite).

The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies.

What problems can the Composite design pattern solve?
[...]
A part-whole hierarchy should be represented as tree structure

ParentBased does not produce a tree. Composite aims to solve a different (but kind of similar) problem.

decorator pattern [...] that allows behavior to be added to an individual object, dynamically, without affecting the behavior of other objects from the same class.

What problems can it solve?
Responsibilities should be added to (and removed from) an object dynamically at run-time.[5]

A flexible alternative to subclassing for extending functionality should be provided.

The key feature of the ParentBased is how it changes the "behavior" of the wrapped sampler. We decorate the existing sampler so that it handles the sampling based on the parent's span.

References:

• https://en.wikipedia.org/wiki/Decorator_pattern
• https://en.wikipedia.org/wiki/Composite_pattern
• https://sourcemaking.com/design_patterns/composite
• https://sourcemaking.com/design_patterns/decorator
• https://stackoverflow.com/questions/2233952/difference-between-the-composite-pattern-and-decorator-pattern

Related issue open-telemetry/opentelemetry-go#4565

[yurishkuro]

[MrAlias]

I am not in favor of using explicitly object-oriented language to describe things in the specification given many languages that implement it are not object-oriented. Also, the implementation of this sampler can quite literally be a decision hierarchy tree matching what you define as "composite". I think the current definition is correct.

[yurishkuro]

Decorator is a design pattern that means "modifies behavior of underlying thing without changing its interface". A function can decorate another function, there's nothing inherently object-oriented about it.

In contrast, "composite" means containing more than one other things, and doesn't imply anything about the resulting interface. In the context of ParentBased sampler decorator is a better term.

[tigrannajaryan]

This is a modification to a Stable doc. I believe we are allowed to do since it does not change any contract or behavior, it merely uses a more correct terminology.