The clearest signal is polling. If you look at your traffic and see consumers hitting the same endpoint every thirty seconds to check whether anything changed, you are paying for their impatience in bandwidth and compute, and they are paying for it in latency. That is an event waiting to be published.
The inversion is the point: instead of the consumer asking “has anything changed?” over and over, the producer says “this happened” the moment it does. Payment settled, file processed, order shipped, risk threshold crossed. The consumer subscribes and reacts.
Webhooks are the accessible entry point and where most providers should start. They are APIs in reverse — an HTTP POST going the other direction — and any developer who understands HTTP already understands them. That accessibility is exactly why they won while heavier event technologies stayed niche, and it is why the humble webhook is still quietly handling an enormous share of the automation on the web.
Streams, queues and brokers come next, when volume, ordering, replay or fan-out demand them. Adopting them is a genuine sign of maturity in an organization’s API journey — but only if you bring the governance with you. The most common mistake is letting the event-driven side grow up ungoverned because it arrived later and looks different.
