Observer cuts coupling through a one-to-many relationship that notifies subscribers of state changes. The Subject just notifies, unaware of who listens. EventEmitter, RxJS, and event-driven architecture all extend from here. It differs from broker-mediated pub/sub.
The difference between splitting code by technical layers (horizontal) and by features or domains (vertical). Trade-offs and selection criteria for each approach.
Strategy encapsulates a family of algorithms behind a single interface and swaps them at runtime. It extends by adding a strategy instead of editing an if/switch. In languages with first-class functions, a separate Strategy class collapses into a single function.
Facade provides a simple entry point to a complex subsystem, cutting the coupling between the client and the internals. A service layer or a library SDK is effectively a Facade. It differs from Adapter (interface conversion) and Mediator (two-way coordination).
Layered architecture separates code into horizontal layers by technical responsibility. A summary of the four-layer structure, dependency direction rules, and how DIP decouples layers.
Decorator replaces the subclass explosion of inheritance with composition. It stacks wrappers that keep the same interface to add behavior at runtime. java.io, Python function decorators, and web middleware chains are all variations on the same intent.
Cache patterns split along two axes — who fills the cache on a miss, and where writes go. The five canonical patterns (cache-aside, read-through, write-through, write-back, refresh-ahead) are combinations of choices along those two axes.