Atoms, Refs, Agents, and Vars

Clojure separates values (usually immutable data) from references (places where state can change).

Here are the core tools you will see in real projects:

• Var: a namespace-level reference (defined with def/defn). Vars support redefinition at the REPL and can be dynamically bound in limited cases.
• Atom: independent, synchronous updates to a single value via swap! (CAS-style).
• Ref: coordinated updates across multiple refs using STM transactions (dosync + alter).
• Agent: asynchronous, serialized updates to an agent’s value via send.

Java mental model: atoms feel like AtomicReference + “update with a function”; refs feel like transactional coordination; agents feel like a single-threaded work queue for state updates.

This section helps you choose a tool based on the kind of state and coordination your problem actually has.

In this section

• Clojure Concurrency Primitives: Atoms, Refs, Agents, and Vars
  Explore Clojure's concurrency primitives—atoms, refs, agents, and vars—and learn how they simplify state management in concurrent programming.
• Atoms in Clojure: Managing State with Compare-and-Swap
  Explore the concept of atoms in Clojure, a mechanism for managing synchronous, independent state changes using compare-and-swap operations. Learn how to create and update atoms safely without locks.
• Clojure Refs and Software Transactional Memory (STM): A Guide for Java Developers
  Explore Clojure's Refs and Software Transactional Memory (STM) for coordinated, synchronous state changes, and learn how STM simplifies concurrency management compared to traditional Java approaches.
• Clojure Agents: Managing Asynchronous State Changes
  Explore how Clojure agents facilitate asynchronous, independent state changes, allowing actions to be performed in the background without blocking the main thread. Learn to use `send` and `send-off` for dispatching actions to agents and retrieving their state with `deref`.
• Vars in Clojure: Dynamic Bindings and Thread-Local State
  Explore the concept of Vars in Clojure, focusing on their role in dynamic bindings and managing thread-local state for Java developers transitioning to Clojure.