Idempotency Thinking Training

Format: Given an operation, determine whether it's idempotent; if not, figure out how to make it idempotent.

What is Idempotency: Executing the same operation once or ten times should produce the same result.

Exercise: Determine whether the following operations are idempotent:

Operation	Idempotent?	Why?
Set username to "Alice"	Yes	Setting it 10 times still results in "Alice"
Add $100 to account balance	No	Adding 10 times results in $1000
Send welcome email	?	You decide
Delete post with ID 123	?	You decide
Create a new order	?	You decide
Update user age to 25	?	You decide
Like a post	?	You decide

Why It Matters: When the network is unstable, requests may be sent twice. If "place order" is not idempotent, the user might be charged twice.

Solution Concepts:

• Idempotency Key: Each request carries a unique ID; the server checks if it's already been processed
• "Set" is naturally idempotent compared to "increment": balance = 200 vs balance += 100