Refs and Transactions

Refs are for coordinated, synchronous state changes across multiple values. Clojure’s STM (Software Transactional Memory) lets you update multiple refs as one atomic transaction—without explicit locks.

Use refs when:

• you must update multiple pieces of state together (invariants across refs)
• you want atomicity and consistency across those updates

Create and Read

1(def checking (ref 100))
2(def savings  (ref 200))
3
4@checking ; => 100

Update with dosync + alter

All ref updates must happen inside a transaction:

1(dosync
2  (alter checking - 10)
3  (alter savings  + 10))

If the transaction conflicts with another transaction, it may be retried automatically.

ref-set and commute

• ref-set replaces the ref’s value (still inside dosync).
• commute is an optimization for commutative updates (like adding to a counter) where the STM can reorder safely.

Important: Avoid Side Effects Inside dosync

Because transactions can retry, doing I/O inside dosync can repeat side effects.

The common pattern is:

• compute the new values transactionally
• perform side effects (logging, I/O) outside the transaction using the committed result

Knowledge Check: Refs / STM