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Queues & Concurrency

You can use queues to run many workflows at once with managed concurrency. Queues provide flow control, letting you manage how many workflows run at once or how often workflows are started.

To create a queue, specify its name:

import { DBOS, WorkflowQueue } from "@dbos-inc/dbos-sdk";

const queue = new WorkflowQueue("example_queue");

You can then enqueue any workflow by passing the queue as an argument to DBOS.startWorkflow. Enqueuing a function submits it for execution and returns a handle to it. Queued tasks are started in first-in, first-out (FIFO) order.

const queue = new WorkflowQueue("example_queue");

class Tasks {
  @DBOS.workflow()
  static async processTask(task) {
    // ...
  }
}

async function main() {
  const task = ...
  const handle = await DBOS.startWorkflow(Tasks, {queueName: queue.name}).processTask(task)
}

Queue Example​

Here's an example of a workflow using a queue to process tasks in parallel:

import { DBOS, WorkflowQueue } from "@dbos-inc/dbos-sdk";

const queue = new WorkflowQueue("example_queue");

async function taskFunction(task) {
    // ...
}
const taskWorkflow = DBOS.registerWorkflow(taskFunction, {"name": "taskWorkflow"});

async function queueFunction(tasks) {
  const handles = []
  
  // Enqueue each task so all tasks are processed concurrently.
  for (const task of tasks) {
    handles.push(await DBOS.startWorkflow(taskWorkflow, { queueName: queue.name })(task))
  }

  // Wait for each task to complete and retrieve its result.
  // Return the results of all tasks.
  const results = []
  for (const h of handles) {
    results.push(await h.getResult())
  }
  return results
}
const queueWorkflow = DBOS.registerWorkflow(queueFunction, {"name": "queueWorkflow"})

Managing Concurrency​

You can control how many workflows from a queue run simultaneously by configuring concurrency limits. This helps prevent resource exhaustion when workflows consume significant memory or processing power.

Worker Concurrency​

Worker concurrency sets the maximum number of workflows from a queue that can run concurrently on a single DBOS process. This is particularly useful for resource-intensive workflows to avoid exhausting the resources of any process. For example, this queue has a worker concurrency of 5, so each process will run at most 5 workflows from this queue simultaneously:

import { DBOS, WorkflowQueue } from "@dbos-inc/dbos-sdk";

const queue = new WorkflowQueue("example_queue", { workerConcurrency: 5 });

Global Concurrency​

Global concurrency limits the total number of workflows from a queue that can run concurrently across all DBOS processes in your application. For example, this queue will have a maximum of 10 workflows running simultaneously across your entire application.

warning

Worker concurrency limits are recommended for most use cases. Take care when using a global concurrency limit as any PENDING workflow on the queue counts toward the limit, including workflows from previous application versions

import { DBOS, WorkflowQueue } from "@dbos-inc/dbos-sdk";

const queue = new WorkflowQueue("example_queue", { concurrency: 10 });

Rate Limiting​

You can set rate limits for a queue, limiting the number of functions that it can start in a given period. Rate limits are global across all DBOS processes using this queue. For example, this queue has a limit of 50 with a period of 30 seconds, so it may not start more than 50 functions in 30 seconds:

const queue = new WorkflowQueue("example_queue", { rateLimit: { limitPerPeriod: 50, periodSec: 30 } });

Rate limits are especially useful when working with a rate-limited API, such as many LLM APIs.

Setting Timeouts​

You can set a timeout for an enqueued workflow by passing a timeoutMS argument to DBOS.startWorkflow. When the timeout expires, the workflow and all its children are cancelled. Cancelling a workflow sets its status to CANCELLED and preempts its execution at the beginning of its next step.

Timeouts are start-to-completion: a workflow's timeout does not begin until the workflow is dequeued and starts execution. Also, timeouts are durable: they are stored in the database and persist across restarts, so workflows can have very long timeouts.

Example syntax:

const queue = new WorkflowQueue("example_queue");

async function taskFunction(task) {
    // ...
}
const taskWorkflow = DBOS.registerWorkflow(taskFunction, {"name": "taskWorkflow"});

async function main() {
  const task = ...
  const timeout = ... // Timeout in milliseconds
  const handle = await DBOS.startWorkflow(taskWorkflow, {queueName: queue.name, timeoutMS: timeout})(task);
}

Deduplication​

You can set a deduplication ID for an enqueued workflow as an argument to DBOS.startWorkflow. At any given time, only one workflow with a specific deduplication ID can be enqueued in the specified queue. If a workflow with a deduplication ID is currently enqueued or actively executing (status ENQUEUED or PENDING), subsequent workflow enqueue attempt with the same deduplication ID in the same queue will raise a DBOSQueueDuplicatedError exception.

For example, this is useful if you only want to have one workflow active at a time per user—set the deduplication ID to the user's ID.

Example syntax:

const queue = new WorkflowQueue("example_queue");

async function taskFunction(task) {
    // ...
}
const taskWorkflow = DBOS.registerWorkflow(taskFunction, {"name": "taskWorkflow"});

async function main() {
  const task = ...
  const dedup: string = ...
  try {
    const handle = await DBOS.startWorkflow(taskWorkflow, {queueName: queue.name, enqueueOptions: {deduplicationID: dedup}})(task);
  } catch (e) {
    // Handle DBOSQueueDuplicatedError
  }
}

Priority​

You can set a priority for an enqueued workflow as an argument to DBOS.startWorkflow. Workflows with the same priority are dequeued in FIFO (first in, first out) order. Priority values can range from 1 to 2,147,483,647, where a low number indicates a higher priority. If using priority, you must set usePriority: true on your queue.

tip

Workflows without assigned priorities have the highest priority and are dequeued before workflows with assigned priorities.

Example syntax:

const queue = new WorkflowQueue("example_queue", {usePriority: true});

async function taskFunction(task) {
    // ...
}
const taskWorkflow = DBOS.registerWorkflow(taskFunction, {"name": "taskWorkflow"});

async function main() {
  const task = ...
  const priority: number = ...
  const handle = await DBOS.startWorkflow(taskWorkflow, {queueName: queue.name, enqueueOptions: {priority: priority}})(task);
}

In-Order Processing​

You can use a queue with concurrency=1 to guarantee sequential, in-order processing of events. Only a single event will be processed at a time. For example, this app processes events sequentially in the order of their arrival:

import { DBOS, WorkflowQueue } from "@dbos-inc/dbos-sdk";
import express from "express";

const serialQueue = new WorkflowQueue("in_order_queue", { concurrency: 1 });
const app = express();

class Tasks {
  @DBOS.workflow()
  static async processTask(task){
    // ... process task
  }
}

app.get("/events/:event", async (req, res) => {
  await DBOS.startWorkflow(Tasks, {queueName: serialQueue.name}).processTask(req.params);
  await res.send("Workflow Started!");
});

// Launch DBOS and start the server
async function main() {
  await DBOS.launch();
  app.listen(3000, () => {});
}

main().catch(console.log);

Enqueue with DBOSClient​

DBOSClient provides a way to programmatically interact with your DBOS application from external code. Among other things, this allows you to enqueue workflows from outside your DBOS application.

Since DBOSClient is designed to be used from outside your DBOS application, workflow and queue metadata must be specified explicitly.

Example:

import { DBOSClient } from "@dbos-inc/dbos-sdk";

const client = await DBOSClient.create({systemDatabaseUrl: process.env.DBOS_SYSTEM_DATABASE_URL});

type ProcessTask = typeof Tasks.processTask;
await client.enqueue<ProcessTask>(
    {
        workflowName: 'processTask',
        workflowClassName: 'Tasks',
        queueName: 'example_queue',
    }, 
    task);