Fetching Data and Caching and Revalidation covered reads. Writes go the other way: the browser pushes. A Server Action is an async function that lives on the server and that React 7,897 calls from the browser as if it were local — no route to invent, no fetch to write, no JSON to parse — and the updated page comes back in the same response as the function's return value.
Every listing, log and screenshot below came from a Next.js 16.3.5 10,514 project on Node.js 25.8.0 2,131 with React 19.3.0 and zod 4.6.5, extending the notes app of Revalidating Tags with a session and an owner per note. React's form Hooks are Forms; this is how Next.js wires them.
One sentence governs the rest. A Server Action compiles to a public HTTP endpoint. 'use server' does not hide the function; it publishes it under an ID. Anyone who learns that ID can POST to it, with any arguments, without loading your page.
