» 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.

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, the agent must be configured with enable_script_checks set to true in order to enable script checks.

  • HTTP + Interval - These checks make an HTTP GET request every Interval (e.g. every 30 seconds) to the specified URL. The status of the service depends on the HTTP response code: any 2xx code is considered passing, a 429 Too Many Requests 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 the check interval, with a max of 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.

  • TCP + Interval - These checks make an TCP connection attempt every Interval (e.g. every 30 seconds) to the specified IP/hostname and port. 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 equal to the check interval, with a max of 10 seconds. It is possible to configure a custom TCP check timeout value by specifying the timeout field in the check definition.

  • Time to Live (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, warn, fail, and 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 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. The state of the check will be updated at the given interval by probing the configured endpoint. 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.

  • 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:

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

A HTTP check:

{
  "check": {
    "id": "api",
    "name": "HTTP API on port 5000",
    "http": "https://localhost:5000/health",
    "tls_skip_verify": false,
    "method": "POST",
    "header": {"x-foo":["bar", "baz"]},
    "interval": "10s",
    "timeout": "1s"
  }
}

A TCP check:

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

A TTL check:

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

A Docker check:

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

A gRPC check:

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

An alias check for a local service:

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

Each type of definition must include a name and may optionally provide an id and notes field. The id must be unique per agent otherwise only the last defined check with that id will be registered. If the id is not set and the check is embedded within a service definition a unique check id is generated. Otherwise, id will be set to name. If names might conflict, unique IDs should be provided.

The notes field is opaque to Consul but can be used to provide a human-readable description of the current state of the check. With a script check, the field is set to any output generated by the script. Similarly, an external process updating a TTL check via the HTTP interface can set the notes value.

Checks may also contain a token field to provide an ACL token. This token is used for any interaction with the catalog for the check, including anti-entropy syncs 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.

Script, TCP, HTTP, Docker, and gRPC checks must include an interval field. This field is parsed by Go's time package, and has the following formatting specification:

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 "┬Ás"), "ms", "s", "m", "h".

In Consul 0.7 and later, checks that are associated with a service may also contain an optional deregister_critical_service_after field, which is a timeout in the same Go time format as interval and ttl. If a check is in the critical state for more than this configured value, then its associated service (and all of its associated checks) will automatically be deregistered. 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 should generally be configured with a timeout that's much, much longer than any expected recoverable outage for the given service.

To configure a check, either provide it as a -config-file option to the agent or place it inside the -config-dir of the agent. The file must end in a ".json" or ".hcl" extension to be loaded by Consul. Check definitions can also be updated by sending a SIGHUP to the agent. Alternatively, the check can be registered dynamically using the HTTP API.

» Check Scripts

A check script is generally free to do anything to determine the status of the check. The only limitations placed are that the exit codes must obey this convention:

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

This is the only convention that Consul depends on. Any output of the script will be captured and stored in the notes field so that it can be viewed by human operators.

In Consul 0.9.0 and later, the agent must be configured with enable_script_checks set to true in order to enable script checks.

» 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:

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

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:

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

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.

» Multiple Check Definitions

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

{
  "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",
      "script": "/bin/check_cpu",
      "interval": "10s"
    },
    ...
  ]
}