Sketchplanations

Explaining one thing a week in a sketch

The two generals problem

This is a fun thought-experiment highlighting the challenges of communicating over an unreliable channel — where your message may or may not get through, and where you won’t know if it has.

The situation is that two armies, either side of a hostile valley, are trying to coordinate an attack, and they want to attack at the same time to be sure to succeed. The trouble is, to coordinate they have to send brave messengers through the hostile valley to the other camp and the messengers may or may not be captured along the way.

To coordinate the attack you have to know that your message got through which you’d know if you received a confirmation. But how would the general sending the confirmation know that their confirmation got through unless they received a confirmation of the confirmation. And on…

Also see: Metcalfe’s Law, idempotence.

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