Race Conditions

In one line

Two threads interleave on shared state and the result depends on timing — the bug class every LLD question secretly tests.

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Think of it like this

Two people grabbing for the last seat on a bus at the exact same moment. If there's no rule for who checks and who sits first, both might think they got it — and you end up with a mess (double booking).

Key ideas

01
Classic shape: check-then-act (isFree → occupy) or read-modify-write (balance++).
02
The bug is invisible: works 99.99% in dev, corrupts state under load.
03
Fix by construction: atomic claim operations (compute, CAS, DB row locks, synchronized block).
04
Describe races as interleavings out loud: 'thread A reads free=true, thread B reads free=true, both claim'.
05
Interview narration: 'this is a race; I make the claim atomic' — graders look for that sentence.

Java / Spring map

→
synchronized, ConcurrentHashMap.compute, AtomicReference, SELECT…FOR UPDATE.

Code & diagrams

lost update: two threads, one counterdiagram

Rendering diagram…

RaceDemo.javajava

The bug, then the fix — side by side.

// BUGGY — check then act, two threads can both pass
class BuggySpotClaim {
  private boolean free = true;
  public boolean claim() {
    if (free) {                 // T1 and T2 both see true
      free = false;             // then both claim
      return true;
    }
    return false;
  }
}

// FIX — the check and the set are one synchronized claim
class FixedSpotClaim {
  private boolean free = true;
  public synchronized boolean claim() {
    if (!free) return false;
    free = false;
    return true;
  }
}
// Or with atomics:
class AtomicSpotClaim {
  private final AtomicBoolean free = new AtomicBoolean(true);
  public boolean claim() { return free.compareAndSet(true, false); }  // one CAS
}

Explain without notes

Replay the interleaving that double-claims with the buggy version on a whiteboard.

Practice

Find a check-then-act in the Vending Machine and fix it — state exactly which method was the race.

Trade-offs

↔
Locks pessimistically serialize; CAS retries under contention — both acceptable until contention is extreme.

Completion checklist

I see 'check-then-act' as an immediate race alert in any design.

Back to phase

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