Concurrency Utilities

Concurrency in Clojure starts with immutable values, then adds a small set of explicit tools for “change over time.” That is different from the Java habit of mutating objects and protecting them with locks.

Use this section as a quick refresher on when to reach for atoms, refs, agents, futures, and related utilities—especially when you are reviewing concurrent code and want to sanity-check its shape.

In this section

• Atoms
  Atoms are Clojure’s go-to for synchronous, independent state: atomic updates with swap! and reset! (think AtomicReference).
• Refs and Transactions
  Refs use Software Transactional Memory (STM) for coordinated, synchronous updates to multiple pieces of shared state.
• Agents
  Agents manage asynchronous, independent state changes: queue actions with send/send-off and handle failures explicitly.
• Futures and Promises
  Simple async tools on the JVM: futures run a computation; promises are one-time values you deliver later (both deref with @).