What is a check-then-act race condition, and how do you fix it correctly?
⚡ Short Answer
Check-then-act (e.g. 'if not present, put') is two steps that aren't atomic — two threads both pass the check and both act, causing duplicates/lost updates. Fix with an atomic compound operation (putIfAbsent, computeIfAbsent, compareAndSet) or a lock.
☕Coffee Chat Question
Concept Made Simple
“What is a check-then-act race condition, and how do you fix it correctly?”
🧠Mind Map Answer
Remember It Faster
if (!map.containsKey(k)) map.put(k, v); — two threads can both see 'absent' and both put. The check and the act must be one atomic step.
⌨️Hands-on Keyboard
Learn by Doing
// BAD: check-then-act race
if (!cache.containsKey(k)) cache.put(k, load(k));
// GOOD: atomic
cache.computeIfAbsent(k, this::load);🔥What If?
Think Beyond the Expected
A lazy-init cache occasionally loads the same key twice under load — root cause?
Classic check-then-act race: two threads find the key absent and both call the expensive load(). computeIfAbsent makes the check-and-insert atomic per key, so load() runs once — and on ConcurrentHashMap it's done under the bin lock.
😂Real World
Duplicate inserts, double-charging, and 'singleton created twice' bugs are almost always non-atomic check-then-act; the fix is an atomic method or a unique DB constraint as a backstop.
🎯Interviewer's Expectation
Keywords they're listening for:
⚠️Common Mistakes
- ✗containsKey then put on a shared map
- ✗get-then-put for increments
- ✗Assuming synchronizedMap makes compound ops atomic
✅Best Practices
- ✓Use atomic compound operations
- ✓Back critical invariants with DB constraints
- ✓Make lazy init idempotent
🔁Follow-up Questions
- 1Why isn't synchronizedMap enough for check-then-act?
- 2How does computeIfAbsent guarantee single execution?
- 3How do DB unique constraints act as a safety net?
🧩Related Technologies
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Plain-language foundations
I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner. Topic: volatile & Memory Model (Multithreading) Interview question: "What is a check-then-act race condition, and how do you fix it correctly?" Please: 1. Explain the core idea in simple, plain language, using an everyday analogy. 2. Define any technical terms you use. 3. Walk through one small, concrete example. 4. Finish with a single sentence I can easily remember. Keep the tone friendly and assume I'm new to this topic.
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