triggerdotdev--trigger.dev/docs/queue-concurrency.mdx

---
title: "Concurrency & Queues"
description: "Configure what you want to happen when there is more than one run at a time."
---

When you trigger a task, it isn't executed immediately. Instead, the task [run](/runs) is placed into a queue for execution. By default, each task gets its own queue with unbounded concurrency—meaning the task runs as soon as resources are available, subject only to the overall concurrency limits of your environment. If you need more control (for example, to limit concurrency or share limits across multiple tasks), you can define a custom queue as described later in this document.

Controlling concurrency is useful when you have a task that can't be run concurrently, or when you want to limit the number of runs to avoid overloading a resource.

It's important to note that only actively executing runs count towards concurrency limits. Runs that are delayed or waiting in a queue do not consume concurrency slots until they begin execution.

## Default concurrency

By default, all tasks have an unbounded concurrency limit, limited only by the overall concurrency limits of your environment. This means that each task could possibly "fill up" the entire
concurrency limit of your environment.

Each individual queue has a maximum concurrency limit equal to your environment's base concurrency limit. If you don't explicitly set a queue's concurrency limit, it will default to your environment's base concurrency limit.

<Note>
  Your environment has a base concurrency limit and a burstable limit (default burst factor of 2.0x
  the base limit). Individual queues are limited by the base concurrency limit, not the burstable
  limit. For example, if your base limit is 10, your environment can burst up to 20 concurrent runs,
  but any single queue can have at most 10 concurrent runs. If you're a paying customer you can
  request higher limits by [contacting us](https://www.trigger.dev/contact).
</Note>

## Setting task concurrency

You can set the concurrency limit for a task by setting the `concurrencyLimit` property on the task's queue. This limits the number of runs that can be executing at any one time:

```ts /trigger/one-at-a-time.ts
// This task will only run one at a time
export const oneAtATime = task({
  id: "one-at-a-time",
  queue: {
    concurrencyLimit: 1,
  },
  run: async (payload) => {
    //...
  },
});
```

This is useful if you need to control access to a shared resource, like a database or an API that has rate limits.

## Sharing concurrency between tasks

As well as putting queue settings directly on a task, you can define a queue and reuse it across multiple tasks. This allows you to share the same concurrency limit:

```ts /trigger/queue.ts
export const myQueue = queue({
  name: "my-queue",
  concurrencyLimit: 1,
});

export const task1 = task({
  id: "task-1",
  queue: myQueue,
  run: async (payload: { message: string }) => {
    // ...
  },
});

export const task2 = task({
  id: "task-2",
  queue: myQueue,
  run: async (payload: { message: string }) => {
    // ...
  },
});
```

In this example, `task1` and `task2` share the same queue, so only one of them can run at a time.

## Setting the concurrency when you trigger a run

When you trigger a task you can override the concurrency limit. This is really useful if you sometimes have high priority runs.

The task:

```ts /trigger/override-concurrency.ts
const paidQueue = queue({
  name: "paid-users",
  concurrencyLimit: 10,
});

export const generatePullRequest = task({
  id: "generate-pull-request",
  queue: {
    //normally when triggering this task it will be limited to 1 run at a time
    concurrencyLimit: 1,
  },
  run: async (payload) => {
    //todo generate a PR using OpenAI
  },
});
```

Triggering from your backend and overriding the concurrency:

```ts app/api/push/route.ts
import { generatePullRequest } from "~/trigger/override-concurrency";

export async function POST(request: Request) {
  const data = await request.json();

  if (data.branch === "main") {
    //trigger the task, with a different queue
    const handle = await generatePullRequest.trigger(data, {
      // Set the paid users queue
      queue: "paid-users",
    });

    return Response.json(handle);
  } else {
    //triggered with the default (concurrency of 1)
    const handle = await generatePullRequest.trigger(data);
    return Response.json(handle);
  }
}
```

## Concurrency keys and per-tenant queuing

If you're building an application where you want to run tasks for your users, you might want a separate queue for each of your users (or orgs, projects, etc.).

You can do this by using `concurrencyKey`. It creates a separate queue for each value of the key.

Your backend code:

```ts app/api/pr/route.ts
import { generatePullRequest } from "~/trigger/override-concurrency";

export async function POST(request: Request) {
  const data = await request.json();

  if (data.isFreeUser) {
    //free users can only have 1 PR generated at a time
    const handle = await generatePullRequest.trigger(data, {
      queue: "free-users",
      concurrencyKey: data.userId,
    });

    //return a success response with the handle
    return Response.json(handle);
  } else {
    //trigger the task, with a different queue
    const handle = await generatePullRequest.trigger(data, {
      queue: "paid-users",
      concurrencyKey: data.userId,
    });

    //return a success response with the handle
    return Response.json(handle);
  }
}
```

## Concurrency and subtasks

When you trigger a task that has subtasks, the subtasks will not inherit the concurrency settings of the parent task. Unless otherwise specified, subtasks will run on their own queue

```ts /trigger/subtasks.ts
export const parentTask = task({
  id: "parent-task",
  run: async (payload) => {
    //trigger a subtask
    await subtask.triggerAndWait(payload);
  },
});

// This subtask will run on its own queue
export const subtask = task({
  id: "subtask",
  run: async (payload) => {
    //...
  },
});
```

## Waits and concurrency

With our [task checkpoint system](/how-it-works#the-checkpoint-resume-system), tasks can wait at various waitpoints (like waiting for subtasks to complete, delays, or external events). The way this system interacts with the concurrency system is important to understand.

Concurrency is only released when a run reaches a waitpoint and is checkpointed. When a run is checkpointed, it transitions to the `WAITING` state and releases its concurrency slot back to both the queue and the environment, allowing other runs to execute or resume.

This means that:

- Only actively executing runs count towards concurrency limits
- Runs in the `WAITING` state (checkpointed at waitpoints) do not consume concurrency slots
- You can have more runs in the `WAITING` state than your queue's concurrency limit
- When a waiting run resumes (e.g., when a subtask completes), it must re-acquire a concurrency slot

For example, if you have a queue with a `concurrencyLimit` of 1:

- You can only have exactly 1 run executing at a time
- You may have multiple runs in the `WAITING` state that belong to that queue
- When the executing run reaches a waitpoint and checkpoints, it releases its slot
- The next queued run can then begin execution

<Note>
  We sometimes refer to the parent task as the "parent" and the subtask as the "child". Subtask and
  child task are used interchangeably. We apologize for the confusion.
</Note>

### Waiting for a subtask on a different queue

When a parent task triggers and waits for a subtask on a different queue, the parent task will checkpoint and release its concurrency slot once it reaches the wait point. This prevents environment deadlocks where all concurrency slots would be occupied by waiting tasks.

```ts /trigger/waiting.ts
export const parentTask = task({
  id: "parent-task",
  queue: {
    concurrencyLimit: 1,
  },
  run: async (payload) => {
    //trigger a subtask and wait for it to complete
    await subtask.triggerAndWait(payload);
    // The parent task checkpoints here and releases its concurrency slot
    // allowing other tasks to execute while waiting
  },
});

export const subtask = task({
  id: "subtask",
  run: async (payload) => {
    //...
  },
});
```

When the parent task reaches the `triggerAndWait` call, it checkpoints and transitions to the `WAITING` state, releasing its concurrency slot back to both its queue and the environment. Once the subtask completes, the parent task will resume and re-acquire a concurrency slot.

### Waiting for a subtask on the same queue

When a parent task and subtask share the same queue, the checkpointing behavior ensures that recursive task execution can proceed without deadlocks, up to the queue's concurrency limit.

```ts /trigger/waiting-same-queue.ts
export const myQueue = queue({
  name: "my-queue",
  concurrencyLimit: 1,
});

export const parentTask = task({
  id: "parent-task",
  queue: myQueue,
  run: async (payload) => {
    //trigger a subtask and wait for it to complete
    await subtask.triggerAndWait(payload);
  },
});

export const subtask = task({
  id: "subtask",
  queue: myQueue,
  run: async (payload) => {
    //...
  },
});
```

When the parent task checkpoints at the `triggerAndWait` call, it releases its concurrency slot back to the queue, allowing the subtask to execute. Once the subtask completes, the parent task will resume.

However, you can only have recursive waits up to your queue's concurrency limit. If you exceed this limit, you will receive a `RECURSIVE_WAIT_DEADLOCK` error:

```ts /trigger/deadlock.ts
export const myQueue = queue({
  name: "my-queue",
  concurrencyLimit: 1,
});

export const parentTask = task({
  id: "parent-task",
  queue: myQueue,
  run: async (payload) => {
    await subtask.triggerAndWait(payload);
  },
});

export const subtask = task({
  id: "subtask",
  queue: myQueue,
  run: async (payload) => {
    await subsubtask.triggerAndWait(payload); // This will cause a deadlock
  },
});

export const subsubtask = task({
  id: "subsubtask",
  queue: myQueue,
  run: async (payload) => {
    //...
  },
});
```

This results in a `RECURSIVE_WAIT_DEADLOCK` error because the queue can only support one level of recursive waiting with a concurrency limit of 1:

![Recursive task deadlock](/images/recursive-task-deadlock-min.png)

### Mitigating recursive wait deadlocks

To avoid recursive wait deadlocks when using shared queues:

1. **Increase the queue's concurrency limit** to allow more levels of recursive waiting
2. **Use different queues** for parent and child tasks to eliminate the possibility of deadlock
3. **Design task hierarchies** to minimize deep recursive waiting patterns

Remember that the number of recursive waits you can have on a shared queue is limited by that queue's concurrency limit.