The Function Call That Can Run Twice

Subtitle: It reads like local code. It is a network.

• Left column - Local call:
• Two outcomes only: it ran, or it did not
• If it dies, your process dies with it
• No third state to reason about
• Costs nanoseconds, not milliseconds
• Reading a field just reads memory

• Right column - Remote call:
• Third outcome: it ran, reply was lost
• A timeout is absence of news, not a fact
• 'Never arrived' looks exactly the same
• Retry on timeout = at-least-once
• So 'charge this card' can run twice

• Simple difference:
• Local: it ran, or it died with you.
• Remote: it ran, died, or ran unseen.

• The latency lie:
• The syntax hides a round trip
• user.name can cost 40ms, not 40ns
• Forty calls in a loop = forty trips
• The innocuous line becomes the outage

• Exactly-once does not exist:
• No protocol gives it over a network
• Libraries do at-least-once plus dedup
• The dedup window is the real guarantee
• Ask its size before you trust it

The one-clause fix:

- Give the request a key the server remembers, so the second arrival returns the first result instead of doing the work again.

• Honest test - add the latency:
• Inject 200ms into every remote call
• Run one ordinary page path
• See which harmless line times out

Taped rule strip: The resemblance is the defect.