open-consul/website/content/docs/discovery/checks.mdx

---
layout: docs
page_title: Monitor Services - Check Definitions
description: >-
  One of the primary roles of the agent is management of system- and
  application-level health checks. A health check is considered to be
  application-level if it is associated with a service. A check is defined in a
  configuration file or added at runtime over the HTTP interface.
---

# Checks

One of the primary roles of the agent is management of system-level and application-level health
checks. A health check is considered to be application-level if it is associated with a
service. If not associated with a service, the check monitors the health of the entire node.
Review the [health checks tutorial](/consul/tutorials/developer-discovery/service-registration-health-checks?utm_source=docs)
to get a more complete example on how to leverage health check capabilities in Consul.

A check is defined in a configuration file or added at runtime over the HTTP interface. Checks
created via the HTTP interface persist with that node.

There are several different kinds of checks:

- Script + Interval - These checks depend on invoking an external application
  that performs the health check, exits with an appropriate exit code, and potentially
  generates some output. A script is paired with an invocation interval (e.g.
  every 30 seconds). This is similar to the Nagios plugin system. The output of
  a script check is limited to 4KB. Output larger than this will be truncated.
  By default, Script checks will be configured with a timeout equal to 30 seconds.
  It is possible to configure a custom Script check timeout value by specifying the
  `timeout` field in the check definition. When the timeout is reached on Windows,
  Consul will wait for any child processes spawned by the script to finish. For any
  other system, Consul will attempt to force-kill the script and any child processes
  it has spawned once the timeout has passed.
  In Consul 0.9.0 and later, script checks are not enabled by default. To use them you
  can either use :

  - [`enable_local_script_checks`](/docs/agent/config/cli-flags#_enable_local_script_checks):
    enable script checks defined in local config files. Script checks defined via the HTTP
    API will not be allowed.
  - [`enable_script_checks`](/docs/agent/config/cli-flags#_enable_script_checks): enable
    script checks regardless of how they are defined.

  ~> **Security Warning:** Enabling script checks in some configurations may
  introduce a remote execution vulnerability which is known to be targeted by
  malware. We strongly recommend `enable_local_script_checks` instead. See [this
  blog post](https://www.hashicorp.com/blog/protecting-consul-from-rce-risk-in-specific-configurations)
  for more details.

- `HTTP + Interval` - These checks make an HTTP `GET` request to the specified URL,
  waiting the specified `interval` amount of time between requests (eg. 30 seconds).
  The status of the service depends on the HTTP response code: any `2xx` code is
  considered passing, a `429 Too ManyRequests` is a warning, and anything else is
  a failure. This type of check
  should be preferred over a script that uses `curl` or another external process
  to check a simple HTTP operation. By default, HTTP checks are `GET` requests
  unless the `method` field specifies a different method. Additional header
  fields can be set through the `header` field which is a map of lists of
  strings, e.g. `{"x-foo": ["bar", "baz"]}`. By default, HTTP checks will be
  configured with a request timeout equal to 10 seconds.

  It is possible to configure a custom HTTP check timeout value by
  specifying the `timeout` field in the check definition. The output of the
  check is limited to roughly 4KB. Responses larger than this will be truncated.
  HTTP checks also support TLS. By default, a valid TLS certificate is expected.
  Certificate verification can be turned off by setting the `tls_skip_verify`
  field to `true` in the check definition. When using TLS, the SNI will be set
  automatically from the URL if it uses a hostname (as opposed to an IP address);
  the value can be overridden by setting `tls_server_name`.

  Consul follows HTTP redirects by default. Set the `disable_redirects` field to
  `true` to disable redirects.

- `TCP + Interval` - These checks make a TCP connection attempt to the specified
  IP/hostname and port, waiting `interval` amount of time between attempts
  (e.g. 30 seconds). If no hostname
  is specified, it defaults to "localhost". The status of the service depends on
  whether the connection attempt is successful (ie - the port is currently
  accepting connections). If the connection is accepted, the status is
  `success`, otherwise the status is `critical`. In the case of a hostname that
  resolves to both IPv4 and IPv6 addresses, an attempt will be made to both
  addresses, and the first successful connection attempt will result in a
  successful check. This type of check should be preferred over a script that
  uses `netcat` or another external process to check a simple socket operation.
  By default, TCP checks will be configured with a request timeout of 10 seconds.
  It is possible to configure a custom TCP check timeout value by specifying the
  `timeout` field in the check definition.

- `UDP + Interval` - These checks direct the client to periodically send UDP datagrams
  to the specified IP/hostname and port. The duration specified in the `interval` field sets the amount of time 
  between attempts, such as `30s` to indicate 30 seconds. The check is logged as healthy if any response from the UDP server is received. Any other result sets the status to `critical`.
  The default interval for, UDP checks is `10s`, but you can configure a custom UDP check timeout value by specifying the
  `timeout` field in the check definition. If any timeout on read exists, the check is still considered healthy.

- `Time to Live (TTL)` ((#ttl)) - These checks retain their last known state
  for a given TTL. The state of the check must be updated periodically over the HTTP
  interface. If an external system fails to update the status within a given TTL,
  the check is set to the failed state. This mechanism, conceptually similar to a
  dead man's switch, relies on the application to directly report its health. For
  example, a healthy app can periodically `PUT` a status update to the HTTP endpoint;
  if the app fails, the TTL will expire and the health check enters a critical state.
  The endpoints used to update health information for a given check are: [pass](/api-docs/agent/check#ttl-check-pass),
  [warn](/api-docs/agent/check#ttl-check-warn), [fail](/api-docs/agent/check#ttl-check-fail),
  and [update](/api-docs/agent/check#ttl-check-update). TTL checks also persist their
  last known status to disk. This allows the Consul agent to restore the last known
  status of the check across restarts. Persisted check status is valid through the
  end of the TTL from the time of the last check.

- `Docker + Interval` - These checks depend on invoking an external application which
  is packaged within a Docker Container. The application is triggered within the running
  container via the Docker Exec API. We expect that the Consul agent user has access
  to either the Docker HTTP API or the unix socket. Consul uses `$DOCKER_HOST` to
  determine the Docker API endpoint. The application is expected to run, perform a health
  check of the service running inside the container, and exit with an appropriate exit code.
  The check should be paired with an invocation interval. The shell on which the check
  has to be performed is configurable which makes it possible to run containers which
  have different shells on the same host. Check output for Docker is limited to
  4KB. Any output larger than this will be truncated. In Consul 0.9.0 and later, the agent
  must be configured with [`enable_script_checks`](/docs/agent/config/cli-flags#_enable_script_checks)
  set to `true` in order to enable Docker health checks.

- `gRPC + Interval` - These checks are intended for applications that support the standard
  [gRPC health checking protocol](https://github.com/grpc/grpc/blob/master/doc/health-checking.md).
  The state of the check will be updated by probing the configured endpoint, waiting `interval`
  amount of time between probes (eg. 30 seconds). By default, gRPC checks will be configured
  with a default timeout of 10 seconds.
  It is possible to configure a custom timeout value by specifying the `timeout` field in
  the check definition. gRPC checks will default to not using TLS, but TLS can be enabled by
  setting `grpc_use_tls` in the check definition. If TLS is enabled, then by default, a valid
  TLS certificate is expected. Certificate verification can be turned off by setting the
  `tls_skip_verify` field to `true` in the check definition.
  To check on a specific service instead of the whole gRPC server, add the service identifier after the `gRPC` check's endpoint in the following format `/:service_identifier`.

- `H2ping + Interval` - These checks test an endpoint that uses http2
  by connecting to the endpoint and sending a ping frame. TLS is assumed to be configured by default.
  To disable TLS and use h2c, set `h2ping_use_tls` to `false`. If the ping is successful
  within a specified timeout, then the check is updated as passing.
  The timeout defaults to 10 seconds, but is configurable using the `timeout` field. If TLS is enabled a valid
  certificate is required, unless `tls_skip_verify` is set to `true`.
  The check will be run on the interval specified by the `interval` field.

- `Alias` - These checks alias the health state of another registered
  node or service. The state of the check will be updated asynchronously, but is
  nearly instant. For aliased services on the same agent, the local state is monitored
  and no additional network resources are consumed. For other services and nodes,
  the check maintains a blocking query over the agent's connection with a current
  server and allows stale requests. If there are any errors in watching the aliased
  node or service, the check state will be critical. For the blocking query, the
  check will use the ACL token set on the service or check definition or otherwise
  will fall back to the default ACL token set with the agent (`acl_token`).

## Check Definition

A script check:
=======

Review the [service health checks tutorial](/consul/tutorials/developer-discovery/service-registration-health-checks?utm_source=docs)
to get a more complete example on how to leverage health check capabilities in Consul.

## Registering a health check

There are three ways to register a service with health checks:

1. Start or reload a Consul agent with a service definition file in the
   [agent's configuration directory](/docs/agent#configuring-consul-agents).
1. Call the
   [`/agent/service/register`](/api-docs/agent/service#register-service)
   HTTP API endpoint to register the service.
1. Use the
   [`consul services register`](/commands/services/register)
   CLI command to register the service.

When a service is registered using the HTTP API endpoint or CLI command,
the checks persist in the Consul data folder across Consul agent restarts.

## Types of checks

This section describes the available types of health checks you can use to
automatically monitor the health of a service instance or node.

-> **To manually mark a service unhealthy:** Use the maintenance mode
  [CLI command](/commands/maint) or
  [HTTP API endpoint](/api-docs/agent#enable-maintenance-mode)
  to temporarily remove one or all service instances on a node
  from service discovery DNS and HTTP API query results.

### Script check ((#script-interval))

Script checks periodically invoke an external application that performs the health check,
exits with an appropriate exit code, and potentially generates some output.
The specified `interval` determines the time between check invocations.
The output of a script check is limited to 4KB.
Larger outputs are truncated.

By default, script checks are configured with a timeout equal to 30 seconds.
To configure a custom script check timeout value,
specify the `timeout` field in the check definition.
After reaching the timeout on a Windows system,
Consul waits for any child processes spawned by the script to finish.
After reaching the timeout on other systems,
Consul attempts to force-kill the script and any child processes it spawned.

Script checks are not enabled by default.
To enable a Consul agent to perform script checks,
use one of the following agent configuration options:

- [`enable_local_script_checks`](/docs/agent/config/cli-flags#_enable_local_script_checks):
  Enable script checks defined in local config files.
  Script checks registered using the HTTP API are not allowed.
- [`enable_script_checks`](/docs/agent/config/cli-flags#_enable_script_checks):
  Enable script checks no matter how they are registered.

  ~> **Security Warning:**
  Enabling non-local script checks in some configurations may introduce
  a remote execution vulnerability known to be targeted by malware.
  We strongly recommend `enable_local_script_checks` instead.
  For more information, refer to
  [this blog post](https://www.hashicorp.com/blog/protecting-consul-from-rce-risk-in-specific-configurations).

The following service definition file snippet is an example
of a script check definition:

<CodeTabs heading="Script Check">

```hcl
check = {
  id = "mem-util"
  name = "Memory utilization"
  args = ["/usr/local/bin/check_mem.py", "-limit", "256MB"]
  interval = "10s"
  timeout = "1s"
}
```

```json
{
  "check": {
    "id": "mem-util",
    "name": "Memory utilization",
    "args": ["/usr/local/bin/check_mem.py", "-limit", "256MB"],
    "interval": "10s",
    "timeout": "1s"
  }
}
```

</CodeTabs>

#### Check script conventions

A check script's exit code is used to determine the health check status:

- Exit code 0 - Check is passing
- Exit code 1 - Check is warning
- Any other code - Check is failing

Any output of the script is captured and made available in the
`Output` field of checks included in HTTP API responses,
as in this example from the [local service health endpoint](/api-docs/agent/service#by-name-json).

### HTTP check ((#http-interval))

HTTP checks periodically make an HTTP `GET` request to the specified URL,
waiting the specified `interval` amount of time between requests.
The status of the service depends on the HTTP response code: any `2xx` code is
considered passing, a `429 Too ManyRequests` is a warning, and anything else is
a failure. This type of check
should be preferred over a script that uses `curl` or another external process
to check a simple HTTP operation. By default, HTTP checks are `GET` requests
unless the `method` field specifies a different method. Additional request
headers can be set through the `header` field which is a map of lists of
strings, such as `{"x-foo": ["bar", "baz"]}`.

By default, HTTP checks are configured with a request timeout equal to 10 seconds.
To configure a custom HTTP check timeout value,
specify the `timeout` field in the check definition.
The output of an HTTP check is limited to approximately 4KB.
Larger outputs are truncated.
HTTP checks also support TLS. By default, a valid TLS certificate is expected.
Certificate verification can be turned off by setting the `tls_skip_verify`
field to `true` in the check definition. When using TLS, the SNI is implicitly
determined from the URL if it uses a hostname instead of an IP address.
You can explicitly set the SNI value by setting `tls_server_name`.

Consul follows HTTP redirects by default.
To disable redirects, set the `disable_redirects` field to `true`.

The following service definition file snippet is an example
of an HTTP check definition:

<CodeTabs heading="HTTP Check">

```hcl
check = {
  id = "api"
  name = "HTTP API on port 5000"
  http = "https://localhost:5000/health"
  tls_server_name =  ""
  tls_skip_verify = false
  method = "POST"
  header = {
     Content-Type = ["application/json"]
  }
  body = "{\"method\":\"health\"}"
  disable_redirects = true
  interval = "10s"
  timeout = "1s"
}
```

```json
{
  "check": {
    "id": "api",
    "name": "HTTP API on port 5000",
    "http": "https://localhost:5000/health",
    "tls_server_name": "",
    "tls_skip_verify": false,
    "method": "POST",
    "header": { "Content-Type": ["application/json"] },
    "body": "{\"method\":\"health\"}",
    "interval": "10s",
    "timeout": "1s"
  }
}
```

</CodeTabs>

### TCP check ((#tcp-interval))

TCP checks periodically make a TCP connection attempt to the specified IP/hostname and port, waiting `interval` amount of time between attempts.
If no hostname is specified, it defaults to "localhost".
The health check status is `success` if the target host accepts the connection attempt,
otherwise the status is `critical`. In the case of a hostname that
resolves to both IPv4 and IPv6 addresses, an attempt is made to both
addresses, and the first successful connection attempt results in a
successful check. This type of check should be preferred over a script that
uses `netcat` or another external process to check a simple socket operation.

By default, TCP checks are configured with a request timeout equal to 10 seconds.
To configure a custom TCP check timeout value,
specify the `timeout` field in the check definition.

The following service definition file snippet is an example
of a TCP check definition:

<CodeTabs heading="TCP Check">

```hcl
check = {
  id = "ssh"
  name = "SSH TCP on port 22"
  tcp = "localhost:22"
  interval = "10s"
  timeout = "1s"
}
```

```json
{
  "check": {
    "id": "ssh",
    "name": "SSH TCP on port 22",
    "tcp": "localhost:22",
    "interval": "10s",
    "timeout": "1s"
  }
}
```

</CodeTabs>

### UDP check ((#udp-interval))

UDP checks periodically direct the Consul agent to send UDP datagrams
to the specified IP/hostname and port,
waiting `interval` amount of time between attempts.
The check status is set to `success` if any response is received from the targeted UDP server.
Any other result sets the status to `critical`.

By default, UDP checks are configured with a request timeout equal to 10 seconds.
To configure a custom UDP check timeout value,
specify the `timeout` field in the check definition.
If any timeout on read exists, the check is still considered healthy.

The following service definition file snippet is an example
of a UDP check definition:

<CodeTabs heading="UDP Check">

```hcl
check = {
  id = "dns"
  name = "DNS UDP on port 53"
  udp = "localhost:53"
  interval = "10s"
  timeout = "1s"
}
```

```json
{
  "check": {
    "id": "dns",
    "name": "DNS UDP on port 53",
    "udp": "localhost:53",
    "interval": "10s",
    "timeout": "1s"
  }
}
```

</CodeTabs>

### Time to live (TTL) check ((#ttl))

TTL checks retain their last known state for the specified `ttl` duration.
If the `ttl` duration elapses before a new check update
is provided over the HTTP interface,
the check is set to `critical` state.

This mechanism relies on the application to directly report its health.
For example, a healthy app can periodically `PUT` a status update to the HTTP endpoint.
Then, if the app is disrupted and unable to perform this update
before the TTL expires, the health check enters the `critical` state.
The endpoints used to update health information for a given check are: [pass](/api-docs/agent/check#ttl-check-pass),
[warn](/api-docs/agent/check#ttl-check-warn), [fail](/api-docs/agent/check#ttl-check-fail),
and [update](/api-docs/agent/check#ttl-check-update). TTL checks also persist their
last known status to disk. This persistence allows the Consul agent to restore the last known
status of the check across agent restarts. Persisted check status is valid through the
end of the TTL from the time of the last check.

To manually mark a service unhealthy,
it is far more convenient to use the maintenance mode
[CLI command](/commands/maint) or
[HTTP API endpoint](/api-docs/agent#enable-maintenance-mode)
rather than a TTL health check with arbitrarily high `ttl`.

The following service definition file snippet is an example
of a TTL check definition:

<CodeTabs heading="TTL Check">

```hcl
check = {
  id = "web-app"
  name = "Web App Status"
  notes = "Web app does a curl internally every 10 seconds"
  ttl = "30s"
}
```

```json
{
  "check": {
    "id": "web-app",
    "name": "Web App Status",
    "notes": "Web app does a curl internally every 10 seconds",
    "ttl": "30s"
  }
}
```

</CodeTabs>

### Docker check ((#docker-interval))

These checks depend on periodically invoking an external application that
is packaged within a Docker Container. The application is triggered within the running
container through the Docker Exec API. We expect that the Consul agent user has access
to either the Docker HTTP API or the unix socket. Consul uses `$DOCKER_HOST` to
determine the Docker API endpoint. The application is expected to run, perform a health
check of the service running inside the container, and exit with an appropriate exit code.
The check should be paired with an invocation interval. The shell on which the check
has to be performed is configurable, making it possible to run containers which
have different shells on the same host.
The output of a Docker check is limited to 4KB.
Larger outputs are truncated.
The agent must be configured with [`enable_script_checks`](/docs/agent/config/cli-flags#_enable_script_checks)
set to `true` in order to enable Docker health checks.

The following service definition file snippet is an example
of a Docker check definition:

<CodeTabs heading="Docker Check">

```hcl
check = {
  id = "mem-util"
  name = "Memory utilization"
  docker_container_id = "f972c95ebf0e"
  shell = "/bin/bash"
  args = ["/usr/local/bin/check_mem.py"]
  interval = "10s"
}
```

```json
{
  "check": {
    "id": "mem-util",
    "name": "Memory utilization",
    "docker_container_id": "f972c95ebf0e",
    "shell": "/bin/bash",
    "args": ["/usr/local/bin/check_mem.py"],
    "interval": "10s"
  }
}
```

</CodeTabs>

### gRPC check ((##grpc-interval))

gRPC checks are intended for applications that support the standard
[gRPC health checking protocol](https://github.com/grpc/grpc/blob/master/doc/health-checking.md).
The state of the check will be updated by periodically probing the configured endpoint,
waiting `interval` amount of time between attempts.

By default, gRPC checks are configured with a timeout equal to 10 seconds.
To configure a custom Docker check timeout value,
specify the `timeout` field in the check definition.

gRPC checks default to not using TLS.
To enable TLS, set `grpc_use_tls` in the check definition.
If TLS is enabled, then by default, a valid TLS certificate is expected.
Certificate verification can be turned off by setting the
`tls_skip_verify` field to `true` in the check definition.
To check on a specific service instead of the whole gRPC server, add the service identifier after the `gRPC` check's endpoint in the following format `/:service_identifier`.

The following service definition file snippet is an example
of a gRPC check for a whole application:

<CodeTabs heading="gRPC Check">

```hcl
check = {
  id = "mem-util"
  name = "Service health status"
  grpc = "127.0.0.1:12345"
  grpc_use_tls = true
  interval = "10s"
}
```

```json
{
  "check": {
    "id": "mem-util",
    "name": "Service health status",
    "grpc": "127.0.0.1:12345",
    "grpc_use_tls": true,
    "interval": "10s"
  }
}
```

</CodeTabs>

The following service definition file snippet is an example
of a gRPC check for the specific `my_service` service

<CodeTabs heading="gRPC Specific Service Check">

```hcl
check = {
  id = "mem-util"
  name = "Service health status"
  grpc = "127.0.0.1:12345/my_service"
  grpc_use_tls = true
  interval = "10s"
}
```

```json
{
  "check": {
    "id": "mem-util",
    "name": "Service health status",
    "grpc": "127.0.0.1:12345/my_service",
    "grpc_use_tls": true,
    "interval": "10s"
  }
}
```

</CodeTabs>

### H2ping check ((#h2ping-interval))

H2ping checks test an endpoint that uses http2 by connecting to the endpoint
and sending a ping frame, waiting `interval` amount of time between attempts.
If the ping is successful within a specified timeout,
then the check status is set to `success`.

By default, h2ping checks are configured with a request timeout equal to 10 seconds.
To configure a custom h2ping check timeout value,
specify the `timeout` field in the check definition.

TLS is enabled by default.
To disable TLS and use h2c, set `h2ping_use_tls` to `false`.
If TLS is not disabled, a valid certificate is required unless `tls_skip_verify` is set to `true`.

The following service definition file snippet is an example
of an h2ping check definition:

<CodeTabs heading="H2ping Check">

```hcl
check = {
  id = "h2ping-check"
  name = "h2ping"
  h2ping = "localhost:22222"
  interval = "10s"
  h2ping_use_tls = false
}
```

```json
{
  "check": {
    "id": "h2ping-check",
    "name": "h2ping",
    "h2ping": "localhost:22222",
    "interval": "10s",
    "h2ping_use_tls": false
  }
}
```

</CodeTabs>

### Alias check

These checks alias the health state of another registered
node or service. The state of the check updates asynchronously, but is
nearly instant. For aliased services on the same agent, the local state is monitored
and no additional network resources are consumed. For other services and nodes,
the check maintains a blocking query over the agent's connection with a current
server and allows stale requests. If there are any errors in watching the aliased
node or service, the check state is set to `critical`.
For the blocking query, the check uses the ACL token set on the service or check definition.
If no ACL token is set in the service or check definition,
the blocking query uses the agent's default ACL token
([`acl.tokens.default`](/docs/agent/config/config-files#acl_tokens_default)).

~> **Configuration info**: The alias check configuration expects the alias to be
registered on the same agent as the one you are aliasing. If the service is
not registered with the same agent, `"alias_node": "<node_id>"` must also be
specified. When using `alias_node`, if no service is specified, the check will
alias the health of the node. If a service is specified, the check will alias
the specified service on this particular node.

The following service definition file snippet is an example
of an alias check for a local service:

<CodeTabs heading="Alias Check">

```hcl
check = {
  id = "web-alias"
  alias_service = "web"
}
```

```json
{
  "check": {
    "id": "web-alias",
    "alias_service": "web"
  }
}
```

</CodeTabs>

## Check definition

This section covers some of the most common options for check definitions.
For a complete list of all check options, refer to the
[Register Check HTTP API endpoint documentation](/api-docs/agent/check#json-request-body-schema).

-> **Casing for check options:**
   The correct casing for an option depends on whether the check is defined in
   a service definition file or an HTTP API JSON request body.
   For example, the option `deregister_critical_service_after` in a service
   definition file is instead named `DeregisterCriticalServiceAfter` in an
   HTTP API JSON request body.

#### General options

- `name` `(string: <required>)` - Specifies the name of the check.

- `id` `(string: "")` - Specifies a unique ID for this check on this node.
  
  If unspecified, Consul defines the check id by:
  - If the check definition is embedded within a service definition file,
     a unique check id is auto-generated.
  - Otherwise, the `id` is set to the value of `name`.
    If names might conflict, you must provide unique IDs to avoid
    overwriting existing checks with the same id on this node.

- `interval` `(string: <required for interval-based checks>)` - Specifies
  the frequency at which to run this check.
  Required for all check types except TTL and alias checks.

  The value is parsed by Go's `time` package, and has the following
  [formatting specification](https://golang.org/pkg/time/#ParseDuration):

  > A duration string is a possibly signed sequence of decimal numbers, each with
  > optional fraction and a unit suffix, such as "300ms", "-1.5h" or "2h45m".
  > Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".

- `service_id` `(string: <required for service health checks>)` - Specifies
  the ID of a service instance to associate this check with.
  That service instance must be on this node.
  If not specified, this check is treated as a node-level check.
  For more information, refer to the
  [service-bound checks](#service-bound-checks) section.

- `status` `(string: "")` - Specifies the initial status of the health check as
   "critical" (default), "warning", or "passing". For more details, refer to
   the [initial health check status](#initial-health-check-status) section.
  
  -> **Health defaults to critical:** If health status it not initially specified,
     it defaults to "critical" to protect against including a service
     in discovery results before it is ready. 

- `deregister_critical_service_after` `(string: "")` - If specified,
  the associated service and all its checks are deregistered
  after this check is in the critical state for more than the specified value.
  The value has the same formatting specification as the [`interval`](#interval) field.

  The minimum timeout is 1 minute,
  and the process that reaps critical services runs every 30 seconds,
  so it may take slightly longer than the configured timeout to trigger the deregistration.
  This field should generally be configured with a timeout that's significantly longer than
  any expected recoverable outage for the given service.

- `notes` `(string: "")` - Provides a human-readable description of the check.
  This field is opaque to Consul and can be used however is useful to the user.
  For example, it could be used to describe the current state of the check.

- `token` `(string: "")` - Specifies an ACL token used for any interaction
  with the catalog for the check, including
  [anti-entropy syncs](/docs/architecture/anti-entropy) and deregistration.

  For alias checks, this token is used if a remote blocking query is necessary to watch the state of the aliased node or service.

#### Success/failures before passing/warning/critical

To prevent flapping health checks and limit the load they cause on the cluster,
a health check may be configured to become passing/warning/critical only after a
specified number of consecutive checks return as passing/critical.
The status does not transition states until the configured threshold is reached.

- `success_before_passing` - Number of consecutive successful results required
  before check status transitions to passing. Defaults to `0`. Added in Consul 1.7.0.

- `failures_before_warning` - Number of consecutive unsuccessful results required
  before check status transitions to warning. Defaults to the same value as that of
  `failures_before_critical` to maintain the expected behavior of not changing the
  status of service checks to `warning` before `critical` unless configured to do so.
  Values higher than `failures_before_critical` are invalid. Added in Consul 1.11.0.

- `failures_before_critical` - Number of consecutive unsuccessful results required
  before check status transitions to critical. Defaults to `0`. Added in Consul 1.7.0.

This feature is available for all check types except TTL and alias checks.
By default, both passing and critical thresholds are set to 0 so the check
status always reflects the last check result.

<CodeTabs heading="Flapping Prevention Example">

```hcl
checks = [
  {
    name = "HTTP TCP on port 80"
    tcp = "localhost:80"
    interval = "10s"
    timeout  = "1s"
    success_before_passing =  3
    failures_before_warning =  1
    failures_before_critical =  3
  }
]
```

```json
{
  "checks": [
    {
      "name": "HTTP TCP on port 80",
      "tcp": "localhost:80",
      "interval": "10s",
      "timeout": "1s",
      "success_before_passing": 3,
      "failures_before_warning": 1,
      "failures_before_critical": 3
    }
  ]
}
```

</CodeTabs>

## Initial health check status

By default, when checks are registered against a Consul agent, the state is set
immediately to "critical". This is useful to prevent services from being
registered as "passing" and entering the service pool before they are confirmed
to be healthy. In certain cases, it may be desirable to specify the initial
state of a health check. This can be done by specifying the `status` field in a
health check definition, like so:

<CodeTabs heading="Status Field Example">

```hcl
check = {
  id = "mem"
  args = ["/bin/check_mem", "-limit", "256MB"]
  interval = "10s"
  status = "passing"
}
```

```json
{
  "check": {
    "id": "mem",
    "args": ["/bin/check_mem", "-limit", "256MB"],
    "interval": "10s",
    "status": "passing"
  }
}
```

</CodeTabs>

The above service definition would cause the new "mem" check to be
registered with its initial state set to "passing".

## Service-bound checks

Health checks may optionally be bound to a specific service. This ensures
that the status of the health check will only affect the health status of the
given service instead of the entire node. Service-bound health checks may be
provided by adding a `service_id` field to a check configuration:

<CodeTabs heading="Status Field Example">

```hcl
check = {
  id = "web-app"
  name = "Web App Status"
  service_id = "web-app"
  ttl = "30s"
}
```

```json
{
  "check": {
    "id": "web-app",
    "name": "Web App Status",
    "service_id": "web-app",
    "ttl": "30s"
  }
}
```

</CodeTabs>

In the above configuration, if the web-app health check begins failing, it will
only affect the availability of the web-app service. All other services
provided by the node will remain unchanged.

## Agent certificates for TLS checks

The [enable_agent_tls_for_checks](/docs/agent/config/config-files#enable_agent_tls_for_checks)
agent configuration option can be utilized to have HTTP or gRPC health checks
to use the agent's credentials when configured for TLS.

## Multiple check definitions

Multiple check definitions can be defined using the `checks` (plural)
key in your configuration file.

<CodeTabs heading="Multiple Checks Example">

```hcl
checks = [
  {
    id       = "chk1"
    name     = "mem"
    args     = ["/bin/check_mem", "-limit", "256MB"]
    interval = "5s"
  },
  {
    id       = "chk2"
    name     = "/health"
    http     = "http://localhost:5000/health"
    interval = "15s"
  },
  {
    id       = "chk3"
    name     = "cpu"
    args     = ["/bin/check_cpu"]
    interval = "10s"
  },
  ...
]
```

```json
{
  "checks": [
    {
      "id": "chk1",
      "name": "mem",
      "args": ["/bin/check_mem", "-limit", "256MB"],
      "interval": "5s"
    },
    {
      "id": "chk2",
      "name": "/health",
      "http": "http://localhost:5000/health",
      "interval": "15s"
    },
    {
      "id": "chk3",
      "name": "cpu",
      "args": ["/bin/check_cpu"],
      "interval": "10s"
    },
    ...
  ]
}
```

</CodeTabs>