Discuss queues, NoSQL, and concurrency

Explain the role of message queues in decoupling services and smoothing traffic spikes; describe delivery semantics (at-least-once, at-most-once, exactly-once) and how you would achieve them. Compare push vs. pull consumption models for event delivery, including trade-offs for latency, throughput, backpressure, and client complexity. When would you choose a NoSQL datastore over a relational database? Discuss data models, consistency models, scaling, and query patterns. Define a deadlock in the context of concurrent or distributed systems and outline techniques to prevent, avoid, detect, and recover from it.