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mediumSystem Design

What is the difference between optimistic and pessimistic locking?

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01

Understand the problem

Compare locking strategies.

locking
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

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03

Study the solution

Step 1: Outline use cases and constraints

Gather requirements and scope the problem. Ask questions to clarify use cases and constraints. Discuss assumptions.

Use cases

We'll scope the problem to handle only the following use cases

  • User needs to understand locking to make architecture decisions

Solution ready — 2 min read

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04

Read the code

Optimistic update (version check)
-- Optimistic locking with a version column (compare-and-set).
-- 1. Read the current row + its version:
SELECT id, stock, version FROM products WHERE id = 42;   -- version = 7

-- 2. Write only if nobody changed it in the meantime:
UPDATE products
SET stock = stock - 1, version = version + 1
WHERE id = 42 AND version = 7;

-- If 0 rows were updated, someone else won the race -> re-read and retry.
05

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