What are traits in Scala, and how are they different from classes?

Traits: Interface-like constructs that can define abstract and concrete methods/fields. Support multiple inheritance of type. Mixed in via with.

Classes: Define objects with state and behavior. Single inheritance; one superclass.

Key Differences: Traits enable composition; classes define core logic. Traits can be partially implemented; classes hold primary behavior. A class extends one class and mixes in multiple traits.

Architectural Logic (Why Spark Uses Traits): Spark's codebase relies heavily on traits for cross-cutting concerns (Serializable, Logging, Configurable). This keeps core classes focused while allowing mix-ins for logging, metrics, and configuration—reducing coupling and improving testability.

Scalability Trade-offs: Trait composition avoids deep inheritance hierarchies that become hard to maintain. Overuse of traits with conflicting method implementations can cause linearization complexity.

Example:

trait Logging {
  def log(msg: String): Unit = println(s"[LOG] $msg")
}
trait Timestamp {
  def timestamp: Long = System.currentTimeMillis
}
class DataProcessor extends Logging with Timestamp {
  def process(): Unit = log(s"Processed at $timestamp")
}