Explain common concurrency patterns and their implementations.

Producer-Consumer: producers add to queue, consumers process. Use blocking queue (Java BlockingQueue, Python queue.Queue). Thread Pool: reuse threads for tasks, avoid creation overhead (ExecutorService, ThreadPoolExecutor). Future/Promise: represent async result, chain with then/map. Actor Model: isolated actors communicate via messages (Akka, Erlang). Read-Write Lock: multiple readers OR single writer. Semaphore: limit concurrent access to resource. Barrier: synchronization point for multiple threads. Fork-Join: divide work, process in parallel, combine results. Choose based on: task nature, coordination needs, performance requirements.