Deno KV Watch

Deno KV watch allows you to detect changes to your KV database, making it easier to build real-time applications, newsfeeds, chat, and more.

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 https://raw.githubusercontent.com/denoland/deno-docs/main/by-example/kv-watch.ts

Additional resources: