#Queue

Sending one notification is easy. As notifications scale up, preventing duplicates, respecting preferences, and handling retries becomes a completely different engineering problem.

Emergency Room Logic: Critical patients first. How Priority Queue works with O(1) access.

FIFO (First In First Out). From Roller Coaster lines to Message Queues (Kafka). Circular Queue, Blocking Queue for thread safety, and Deque for sliding windows.

Pringles can (Stack) vs Restaurant line (Queue). The most basic data structures, but without them, you can't understand recursion or message queues.