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Ensure

As explained in Clojure: The Essential Reference

ensure is a Clojure function that validates a ref's value remains unchanged during the current transaction.

ensure validates that a ref’s value remains unchanged during the current transaction. It is used inside dosync when a transaction must observe a ref without allowing that value to change concurrently. section 14.4

Guarding a transaction with ensure

(def votes {"honeypot" (ref 0) "candidate-0" (ref 0) "candidate-1" (ref 0)}) (defn batch [prefs] (future (dosync (ensure (votes "honeypot")) (doseq [color prefs :while (< @(votes "honeypot") 5)] (update votes color commute inc))))) Running two batches produces a result such as: {:total-votes 115, :winner "candidate-1", :fraud? true} Here, ensure checks the honeypot ref while the transaction processes vote preferences. section 14.4

The practical shape of ensure is:

  • Pass it the ref whose value must remain unchanged during the transaction. section 14.4
  • Place it within dosync alongside coordinated ref operations such as commute. section 14.4

How Clojure: The Essential Reference covers it

How deep it goes
Defines transaction-time ref validation and demonstrates it in one concurrent voting example.
Examples
Demonstrates ensure on a honeypot ref while concurrent batches update vote counters.
What it assumes
Assumes familiarity with refs, dosync, dereferencing, and commute in Clojure’s STM.
What you can do after
Apply it via Clojure transactions when a ref’s observed value must remain stable.
How it connects
Connects ensure with refs, dosync, commute, transactions, and software transactional memory.
Cover of Clojure: The Essential Reference
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This explainer was generated by Manning's knowledge graph from the text of Clojure: The Essential Reference. The section links point to the parts of the book it draws on.