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The problem

1 s on screen = 100 ms

Three services care about every order. Why should the customer wait for all three?

The order service calls everyone

Each order calls email, then inventory, then analytics, one after another, before answering.

Live metrics for each version of the scene
Metric	Direct calls	Pub/sub	Change
target ≤ 400 ms	—	—	—
	—	—	—
	0	0	—

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When one event matters to many services, calling each in turn makes the user wait for all of them, and fail if any one fails.