---
layout: docs
page_title: lifecycle Block - Job Specification
description: |-
  The "lifecycle" block configures when a task is run within the lifecycle of a
  task group
---

# `lifecycle` Block

<Placement groups={['job', 'group', 'task', 'lifecycle']} />

The `lifecycle` block is used to express task dependencies in Nomad by
configuring when a task is run within the lifecycle of a task group.

Main tasks are tasks that do not have a `lifecycle` block. Lifecycle task hooks
specify when other tasks are run in relation to the main tasks.
There are three different lifecycle hooks, indicating when a task is started:

- prestart tasks are started immediately
- poststart tasks are started after the main tasks are running
- poststop tasks are started after the main tasks are dead

Tasks can be run with an additional parameter indicating whether they are ephemeral
tasks or "sidecar" tasks that are expected to run for the duration of the main tasks.
The absence of the sidecar flag indicates that the task is ephemeral
and should not be restarted if it completes successfully.

Learn more about [Nomad's task dependencies][learn-taskdeps].

## `lifecycle` Parameters

- `hook` `(string: <required>)` - Specifies when a task should be run within
  the lifecycle of a group. The following hooks are available:

  - `prestart` - Will be started immediately. The main tasks will not start until
    all `prestart` tasks with `sidecar = false` have completed successfully.
  - `poststart` - Will be started once all main tasks are running.
  - `poststop` - Will be started once all main tasks have stopped successfully
    or exhausted their failure [retries](/nomad/docs/job-specification/restart).

- `sidecar` `(bool: false)` - Controls whether a task is ephemeral or
  long-lived within the task group. If a lifecycle task is ephemeral
  (`sidecar = false`), the task will not be restarted after it completes successfully. If a
  lifecycle task is long-lived (`sidecar = true`) and terminates, it will be
  restarted as long as the allocation is running.

[learn-taskdeps]: /nomad/tutorials/task-deps

## Lifecycle Examples

The following include examples of archetypal lifecycle patterns.

### Init Task Pattern

Init tasks are useful for performing initialization steps that can't be more easily
accomplished using [`template`](/nomad/docs/job-specification/template) or
[`artifact`](/nomad/docs/job-specification/artifact), like waiting on other services
or performing complex initialization.

In the following example, the init task will block the main task from starting
until the upstream database service is listening on the expected port:

```hcl
  task "wait-for-db" {
    lifecycle {
      hook = "prestart"
      sidecar = false
    }

    driver = "exec"
    config {
      command = "sh"
      args = ["-c", "while ! nc -z db.service.local.consul 8080; do sleep 1; done"]
    }
  }

  task "main-app" {
    ...
  }
```

### Companion Sidecar Pattern

Companion or sidecar tasks run alongside the main task to perform an auxiliary
task. Common examples include proxies and log shippers. These tasks benefit from
running in the same task group because of tighter filesystem and networking
coupling.

```hcl
  task "fluentd" {
    lifecycle {
      hook = "poststart"
      sidecar = true
    }

    driver = "docker"
    config {
      image = "fluentd/fluentd"
    }

    template {
      destination = "local/fluentd.conf"
      data = ...
    }
  }

  task "main-app" {
    ...
  }
```

### Cleanup Task Pattern

Poststop tasks run after the main tasks have stopped. They are useful for performing
post-processing that isn't available in the main tasks or for recovering from
failures in the main tasks.

The example below shows a chatbot which posts a notification when the main tasks
have stopped:

```hcl
  task "main-app" {
    ...
  }

  task "announce" {
    lifecycle {
      hook = "poststop"
    }

    driver = "docker"
    config {
      image = "alpine/httpie"
      command = "http"
      args = [
        "POST",
        "https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX",
        "text='All done!'"
      ]
    }
  }
```