---
layout: docs
page_title: check Block - Job Specification
description: |-
  The "check" block declares service check definition for a service registered into the Nomad or Consul service provider.
---

# `check` Block

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

The `check` block instructs Nomad to register a check associated with a [service][service]
into the Nomad or Consul service provider.

```hcl
job "example" {
  datacenters = ["dc1"]

  group "cache" {
    network {
      port "db" { to = 6379 }
    }

    service {
      provider = "nomad"
      name     = "redis"
      port     = "db"
      check {
        name     = "redis_probe"
        type     = "tcp"
        interval = "10s"
        timeout  = "1s"
      }
    }

    task "redis" {
      driver = "docker"
      config {
        image = "redis:7"
        ports = ["db"]
      }
    }
  }
}
```

### `check` Parameters

- `address_mode` `(string: "host")` - Same as `address_mode` on `service`.
  Unlike services, checks do not have an `auto` address mode as there's no way
  for Nomad to know which is the best address to use for checks. Consul needs
  access to the address for any HTTP or TCP checks. See
  [below for details.](#using-driver-address-mode) Unlike `port`, this setting
  is _not_ inherited from the `service`.
  If the service `address` is set and the check `address_mode` is not set, the
  service `address` value will be used for the check address.

- `args` `(array<string>: [])` - Specifies additional arguments to the
  `command`. This only applies to script-based health checks.

- `check_restart` - See [`check_restart` block][check_restart_block].

- `command` `(string: <varies>)` - Specifies the command to run for performing
  the health check. The script must exit: 0 for passing, 1 for warning, or any
  other value for a failing health check. This is required for script-based
  health checks. Only supported in the Consul service provider.

  ~> **Caveat:** The command must be the path to the command on disk, and no
  shell exists by default. That means operators like `||` or `&&` are not
  available. Additionally, all arguments must be supplied via the `args`
  parameter. To achieve the behavior of shell operators, specify the command
  as a shell, like `/bin/bash` and then use `args` to run the check.

- `grpc_service` `(string: <optional>)` - What service, if any, to specify in
  the gRPC health check. gRPC health checks require Consul 1.0.5 or later.

- `grpc_use_tls` `(bool: false)` - Use TLS to perform a gRPC health check. May
  be used with `tls_skip_verify` to use TLS but skip certificate verification.
  May be used with `tls_server_name` to specify the ServerName to use for SNI
  and validation of the certificate presented by the server being checked.

- `initial_status` `(string: <enum>)` - Specifies the starting status of the
  service. Valid options are `passing`, `warning`, and `critical`. Omitting
  this field (or submitting an empty string) will result in the Consul default
  behavior, which is `critical`. Only supported in the Consul service provider.
  In the Nomad service provider, the initial status of a check is `pending`
  until Nomad produces an initial check status result.

- `success_before_passing` `(int:0)` - The number of consecutive successful checks
  required before Consul will transition the service status to [`passing`][consul_passfail].
  Only supported in the Consul service provider.

- `failures_before_critical` `(int:0)` - The number of consecutive failing checks
  required before Consul will transition the service status to [`critical`][consul_passfail].
  Only supported in the Consul service provider.

- `interval` `(string: <required>)` - Specifies the frequency of the health checks
  that Consul or Nomad service provider will perform. This is specified using a label
  suffix like "30s" or "1h". This must be greater than or equal to "1s".

- `method` `(string: "GET")` - Specifies the HTTP method to use for HTTP
  checks. Must be a valid HTTP method.

- `body` `(string: "")` - Specifies the HTTP body to use for HTTP checks.

- `name` `(string: "service: <name> check")` - Specifies the name of the health
  check. If the name is not specified Nomad generates one based on the service name.

- `path` `(string: <varies>)` - Specifies the path of the HTTP endpoint which
  will be queried to observe the health of a service. Nomad will automatically
  add the IP of the service and the port, so this is just the relative URL to
  the health check endpoint. This is required for HTTP-based health checks.

- `expose` `(bool: false)` - Specifies whether an [Expose Path](/nomad/docs/job-specification/expose#path-parameters)
  should be automatically generated for this check. Only compatible with
  Connect-enabled task-group services using the default Connect proxy. If set, check
  [`type`](#type) must be `http` or `grpc`, and check `name` must be set.
  Only supported in the Consul service provider.

- `port` `(string: <varies>)` - Specifies the label of the port on which the
  check will be performed. Note this is the _label_ of the port and not the port
  number unless `address_mode = driver`. The port label must match one defined
  in the [`network`][network] block. If a port value was declared on the
  `service`, this will inherit from that value if not supplied. If supplied,
  this value takes precedence over the `service.port` value. This is useful for
  services which operate on multiple ports. `grpc`, `http`, and `tcp` checks
  require a port while `script` checks do not. Checks will use the host IP and
  ports by default. In Nomad 0.7.1 or later numeric ports may be used if
  `address_mode="driver"` is set on the check.

- `protocol` `(string: "http")` - Specifies the protocol for the HTTP-based
  health checks. Valid options are `http` and `https`.

- `task` `(string: "")` - Specifies the task associated with this
  check. Scripts are executed within the task's environment, and
  `check_restart` blocks will apply to the specified task. Inherits
  the [`service.task`][service_task] value if not set. Must be unset
  or equivalent to `service.task` in task-level services.

- `timeout` `(string: <required>)` - Specifies how long to wait for a health check
  query to succeed. This is specified using a label suffix like "30s" or "1h". This
  must be greater than or equal to "1s"

  ~> **Caveat:** Script checks use the task driver to execute in the task's
  environment. For task drivers with namespace isolation such as `docker` or
  `exec`, setting up the context for the script check may take an unexpectedly
  long amount of time (a full second or two), especially on busy hosts. The
  timeout configuration must allow for both this setup and the execution of
  the script. Operators should use long timeouts (5 or more seconds) for script
  checks, and monitor telemetry for
  `client.allocrunner.taskrunner.tasklet_timeout`.

- `type` `(string: <required>)` - This indicates the check types supported by
  Nomad. For Consul service checks, valid options are `grpc`, `http`, `script`,
  and `tcp`. For Nomad service checks, valid options are `http` and `tcp`.

- `tls_server_name` `(string: "")` - Indicates the ServerName to use for SNI and
  validation of the certificate presented by the server being checked, when
  performing TLS enabled checks (`https` and `grpc` with `grpc_use_tls`). If left
  unspecified, the ServerName will be inferred from the address of the server
  being checked unless the address is an IP address. There are two common cases
  where this is beneficial:

    - When the check address contains an IP, `tls_server_name` can be specified
      for SNI. Note: setting `tls_server_name` will also override the hostname
      used to verify the certificate presented by the server being checked.

    - When the check address contains a hostname which isn't be present in the
      SAN (Subject Alternative Name) field of the certificate presented by the
      server being checked. Note: setting `tls_server_name` will also override
      the hostname used for SNI.

  This field is only supported in the Consul service provider. 

- `tls_skip_verify` `(bool: false)` - Skip verification of certificates for
  `https` and `grpc` with `grpc_use_tls` checks . Only supported in the Consul
  service provider.

- `on_update` `(string: "require_healthy")` - Specifies how checks should be
  evaluated when determining deployment health (including a job's initial
  deployment). This allows job submitters to define certain checks as readiness
  checks, progressing a deployment even if the Service's checks are not yet
  healthy. Checks inherit the Service's value by default. The check status is
  not altered in the service provider and is only used to determine the check's
  health during an update.

  - `require_healthy` - In order for Nomad to consider the check healthy during
    an update it must report as healthy.

  - `ignore_warnings` - If a Service Check reports as warning, Nomad will treat
    the check as healthy. The Check will still be in a warning state in Consul.

  - `ignore` - Any status will be treated as healthy.

  ~> **Caveat:** `on_update` is only compatible with certain
  [`check_restart`][check_restart_block] configurations. `on_update = "ignore_warnings"` requires that `check_restart.ignore_warnings = true`.
  `check_restart` can however specify `ignore_warnings = true` with `on_update = "require_healthy"`. If `on_update` is set to `ignore`, `check_restart` must
  be omitted entirely.

#### `header` Block

HTTP checks may include a `header` block to set HTTP headers. The `header`
block parameters have lists of strings as values. Multiple values will cause
the header to be set multiple times, once for each value.

```hcl
service {
  # ...
  check {
    type     = "http"
    port     = "lb"
    path     = "/_healthz"
    interval = "5s"
    timeout  = "2s"
    header {
      Authorization = ["Basic ZWxhc3RpYzpjaGFuZ2VtZQ=="]
    }
  }
}
```

### HTTP Health Check

This example shows a service with an HTTP health check. This will query the
service on the IP and port registered with Nomad at `/_healthz` every 5 seconds,
giving the service a maximum of 2 seconds to return a response, and include an
Authorization header. Any non-2xx code is considered a failure.

```hcl
service {
  check {
    type     = "http"
    port     = "lb"
    path     = "/_healthz"
    interval = "5s"
    timeout  = "2s"
    header {
      Authorization = ["Basic ZWxhc3RpYzpjaGFuZ2VtZQ=="]
    }
  }
}
```

### Multiple Health Checks

This example shows a service with multiple health checks defined. All health
checks must be passing in order for the service to register as healthy.

```hcl
service {
  check {
    name     = "HTTP Check"
    type     = "http"
    port     = "lb"
    path     = "/_healthz"
    interval = "5s"
    timeout  = "2s"
  }

  check {
    name     = "HTTPS Check"
    type     = "http"
    protocol = "https"
    port     = "lb"
    path     = "/_healthz"
    interval = "5s"
    timeout  = "2s"
    method   = "POST"
  }

  check {
    name      = "Postgres Check"
    type      = "script"
    command   = "/usr/local/bin/pg-tools"
    args      = ["verify", "database", "prod", "up"]
    interval  = "5s"
    timeout   = "2s"
    on_update = "ignore_warnings"
  }
}
```

### gRPC Health Check

gRPC health checks use the same host and port behavior as `http` and `tcp`
checks, but gRPC checks also have an optional gRPC service to health check. Not
all gRPC applications require a service to health check.

```hcl
service {
  check {
    type            = "grpc"
    port            = "rpc"
    interval        = "5s"
    timeout         = "2s"
    grpc_service    = "example.Service"
    grpc_use_tls    = true
    tls_skip_verify = true
  }
}
```

In this example Consul would health check the `example.Service` service on the
`rpc` port defined in the task's [network resources][network] block. See
[Using Driver Address Mode](#using-driver-address-mode) for details on address
selection.

### Script Checks with Shells

Note that script checks run inside the task. If your task is a Docker container,
the script will run inside the Docker container. If your task is running in a
chroot, it will run in the chroot. Please keep this in mind when authoring check
scripts.

This example shows a service with a script check that is evaluated and interpolated in a shell; it
tests whether a file is present at `${HEALTH_CHECK_FILE}` environment variable:

```hcl
service {
  check {
    type    = "script"
    command = "/bin/bash"
    args    = ["-c", "test -f ${HEALTH_CHECK_FILE}"]
  }
}
```

Using `/bin/bash` (or another shell) is required here to interpolate the `${HEALTH_CHECK_FILE}` value.

The following examples of `command` fields **will not work**:

```hcl
# invalid because command is not a path
check {
  type    = "script"
  command = "test -f /tmp/file.txt"
}

# invalid because path will not be interpolated
check {
  type    = "script"
  command = "/bin/test"
  args    = ["-f", "${HEALTH_CHECK_FILE}"]
}
```

### Healthiness versus Readiness Checks

Multiple checks for a service can be composed to create healthiness and readiness
checks by configuring [`on_update`][on_update] for the check.

```hcl
service {
  # This is a healthiness check that will be used to verify the service
  # is responsive to tcp connections and behaving as expected.
  check {
    name     = "connection_tcp"
    type     = "tcp"
    port     = 6379
    interval = "10s"
    timeout  = "2s"
  }

  # This is a readiness check that is used to verify that, for example, the
  # application has elected a leader by making a request to its /leader endpoint.
  # Failures of this check are ignored during deployments.
  check {
    name      = "leader_elected"
    type      = "http"
    path      = "/leader"
    interval  = "10s"
    timeout   = "2s"
    on_update = "ignore"
  }
}
```

For checks registered into the Nomad service provider, the status information will
indicate `Mode = readiness` for readiness checks and `Mode = healthiness` for health
checks.

### Check status on CLI

For checks registered into the Nomad service provider, the status information of
checks can be viewed per-allocation. The `alloc status` command now includes
summary information for Nomad service checks.

```shell-session
$ nomad alloc status <allocation-id>
```

<CodeBlockConfig hideClipboard>

```plaintext
Nomad Service Checks:
Service   Task     Name          Mode         Status
database  task     db_tcp_probe  readiness    success
web       (group)  healthz       healthiness  failure
web       (group)  index-page    healthiness  success
```

</CodeBlockConfig>

The `alloc checks` command can be used for viewing complete check status information
for all checks in an allocation.

```shell-session
$ nomad alloc checks <allocation-id>
```

<CodeBlockConfig hideClipboard>

```plaintext
Status of 3 Nomad Service Checks

ID         =  d8651d93a50b9e28375a7beb9418c418
Name       =  db_tcp_probe
Group      =  example.group[0]
Task       =  task
Service    =  database
Status     =  success
Mode       =  readiness
Timestamp  =  2022-08-22T10:41:23-05:00
Output     =  nomad: tcp ok

ID          =  0413b61bda7014f02671675d7e146373
Name        =  index-page
Group       =  example.group[0]
Task        =  (group)
Service     =  web
Status      =  success
StatusCode  =  200
Mode        =  healthiness
Timestamp   =  2022-08-22T10:41:23-05:00
Output      =  nomad: http ok

ID         =  c3cce3f0c97975f84bbf39bdd50deaea
Name       =  healthz
Group      =  example.group[0]
Task       =  (group)
Service    =  web
Status     =  failure
Mode       =  healthiness
Timestamp  =  2022-08-22T10:41:23-05:00
Output     =  nomad: Get "http://:9999/": dial tcp :9999: connect: connection refused
```

</CodeBlockConfig>

---

<sup>
  <small>1</small>
</sup>
<small>
  {' '}
  Script checks are not supported for the QEMU driver since the Nomad client
  does not have access to the file system of a task for that driver.
</small>

[check_restart_block]: /nomad/docs/job-specification/check_restart
[consul_passfail]: /consul/docs/discovery/checks#success-failures-before-passing-critical
[network]: /nomad/docs/job-specification/network 'Nomad network Job Specification'
[service]: /nomad/docs/job-specification/service
[service_task]: /nomad/docs/job-specification/service#task-1
[on_update]: /nomad/docs/job-specification/service#on_update
