Update once. Every view agrees. Only readers re-render.

The first post is on screen three times, through three different components. Like it from any of them: the mutation writes one Post record, and Relay notifies exactly the components whose fragments read the fields that changed. The render counters show it. Titles never re-render on a like.

Network latency250 msMake the server reject mutations

Fetching with Relay…

why it mattersNo prop drilling, no global state, no refetch. The mutation response has the same shape as the store, so Relay merges it and every subscriber sees it.
why it mattersOptimistic responses make the UI feel instant at any latency. Turn on server failure and Relay reverts the optimistic layer on its own.
why it mattersSubscriptions are per fragment, not per query. A like re-renders the LikeButtons and the widget, not the list or the titles.