nats.js/jetstream/examples/07_consume_jobs.js

/*
 * Copyright 2023 The NATS Authors
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import { connect, delay } from "@nats-io/transport-deno";
import { SimpleMutex } from "@nats-io/nats-core/internal";
import { jetstream } from "@nats-io/jetstream";
import { setupStreamAndConsumer } from "./util.js";

// create a connection
const nc = await connect();

// make a stream and fill with messages, and create a consumer
// create a stream with a random name with some messages and a consumer
const { stream, consumer } = await setupStreamAndConsumer(nc, 100);

// retrieve an existing consumer
const js = jetstream(nc);
const c = await js.consumers.get(stream, consumer);

const messages = await c.consume({ max_messages: 10 });

// this example controls parallel processing of the messages
// by only allowing 5 concurrent messages to be processed
// and then only allowing additional processing as others complete
const rl = new SimpleMutex(5);

async function schedule(m) {
  // pretend to do work
  await delay(1000);
  m.ack();
  console.log(`${m.seq}`);
}

for await (const m of messages) {
  await rl.lock();
  schedule(m)
    .catch((err) => {
      console.log(`failed processing: ${err.message}`);
      m.nak();
    })
    .finally(() => {
      rl.unlock();
    });
}