When a user posts content, three services (feed, notifications, analytics) must each process it. They are currently called in series, adding latency. Replace with a pub-sub fan-out.
Fan-out (publish-subscribe) publishes one event to a topic and delivers it to all independent subscribers simultaneously, in parallel. The dual benefit: total latency equals the slowest subscriber, not the sum of all, and adding a new consumer requires no change to the publisher — the system grows without regression. Use it when multiple services need to react to the same event without being coupled to each other; the trade-off is that delivery order across different subscribers is not guaranteed.
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