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Deno KV watch

Warning: This is an unstable API that is subject to change or removal at anytime.
Deno KV watch allows you to detect changes to your KV database, making it easier to build real-time applications, newsfeeds, chat, and more.

Edit on Github
Open the default database
const kv = await Deno.openKv();
Set "counter" value to 0, then increment every second.
await kv.set(["counter"], new Deno.KvU64(0n));
setInterval(() => {
  kv.atomic().sum(["counter"], 1n).commit();
}, 1000);
Listen for changes to "counter" and log value.
for await (const [entry] of kv.watch([["counter"]])) {
  console.log(`Counter: ${entry.value}`);
}
You can also create a stream reader from kv.watch, which returns a ReadableStream.
const stream = kv.watch([["counter"]]).getReader();
while (true) {
  const counter = await stream.read();
  if (counter.done) {
    break;
  }
  console.log(`Counter: ${counter.value[0].value}`);
}
To use server-sent events, let's create a server that responds with a stream. Each time a change to "counter" is detected, send the updated value to the client.
Deno.serve((_req) => {
  const stream = kv.watch([["counter"]]).getReader();
  const body = new ReadableStream({
    async start(controller) {
      while (true) {
        if ((await stream.read()).done) {
          return;
        }
        const data = await kv.get(["counter"]);
        controller.enqueue(
          new TextEncoder().encode(`Counter: ${data.value}\n`),
        );
      }
    },
    cancel() {
      stream.cancel();
    },
  });
  return new Response(body, {
    headers: {
      "Content-Type": "text/event-stream",
    },
  });
});

Run this example locally using the Deno CLI:

deno run --unstable-kv https://docs.deno.com/learn/examples/kv-watch.ts

Additional resources