Fable
Two bridge keepers on two new bridges: one tests with a single person first, the other releases people in batches by headcount
Two new bridges were built outside the old city, completed on the same day. Old Shen, keeper of the east bridge, let two pole-carriers cross first, while he stood at the bridgehead watching the deck for cracks and listening for strange noises. Old Xu, keeper of the west bridge, held a ratio table: in the first hour, let through ten percent of pedestrians; in the second hour, thirty percent; in the third hour, sixty percent; and open the whole bridge before sunset.
The two pole carriers on the East Bridge were halfway across when the bridge deck shook. Old Shen immediately blew his whistle, called the people back, and had the bridge re-inspected. They found slight settlement in the third pier, and the craftsmen reinforced it overnight. The next day, Old Shen first let three carts pass. Once the bridge was stable, he began letting people cross. On the West Bridge, passage was allowed by proportion. By the third hour, 60% of the foot traffic had been let through, and the bridge deck began to sway. Old Xu saw that the plan said 60%, so he did not stop. As a result, the bridge deck swayed even more, and pedestrians started to run.
In the after-action review, the county magistrate asked Old Xu why he had not called a halt. Old Xu said the plan table had no step for deciding after observation, and proportional execution should not be interrupted. Old Shen said his logic was exactly the opposite—every step was try first, then release; if it fails the test, it is not released, and the observation result is the only pass. The magistrate wrote in the file: one decides by observation, the other advances by plan. They cannot be mixed.
In the end, both bridges were repaired. The magistrate ruled that on acceptance day, the new bridge must use Old Shen's method—apply the smallest load first, watch the key indicators, and don't proceed to the next step until they pass. For daily traffic, Old Xu's method could be used—advance in planned segments, because the bridge had already been verified as safe.
The new bridge-keeping apprentice asked a question: if the canary people got through fine, can we just release everything at once? Old Shen said no—the pressure when there are few people and the resonance when there are many are two different things. The canary only tells you that this version can start being released; it does not mean you can release it all in one go.
Later, a diagram was drawn in the bridge keeper's manual: release one or two cars first—observe—release ten percent—observe—release thirty percent—observe—release sixty percent—release all. Canary is the first step; gradual rollout is the whole path. The first step determines whether you can proceed; the later steps control how fast you proceed.
The last page of the manual had one line: a canary not failing does not mean you can release everything at once, and a canary release at 80% not failing does not mean observation is unnecessary. Both terms are saying the same thing—do not put everyone on the bridge all at once.