--- 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 [service health checks tutorial](https://learn.hashicorp.com/tutorials/consul/service-registration-health-checks) 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: ```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" } } ``` #### 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: ```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" } } ``` ### 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: ```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" } } ``` ### 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: ```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" } } ``` ### OSService check OSService checks periodically direct the Consul agent to monitor the health of a service running on the host operating system as either a Windows service (Windows) or a SystemD service (Unix). The check is logged as `healthy` if the service is running. If it is stopped or not running, the status is `critical`. All other results set the status to `warning`, which indicates that the check is not reliable because an issue is preventing the check from determining the health of the service. The following service definition file snippet is an example of an OSService check definition: ```hcl check = { id = "myco-svctype-svcname-001" name = "svcname-001 Windows Service Health" service_id = "flash_pnl_1" os_service = "myco-svctype-svcname-001" interval = "10s" } ``` ```json { "check": { "id": "myco-svctype-svcname-001", "name": "svcname-001 Windows Service Health", "service_id": "flash_pnl_1", "os_service": "myco-svctype-svcname-001", "interval": "10s" } } ``` ### 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: ```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" } } ``` ### 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: ```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" } } ``` ### 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: ```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" } } ``` The following service definition file snippet is an example of a gRPC check for the specific `my_service` service ```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" } } ``` ### 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: ```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 } } ``` ### 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": ""` 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: ```hcl check = { id = "web-alias" alias_service = "web" } ``` ```json { "check": { "id": "web-alias", "alias_service": "web" } } ``` ## 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: )` - 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: )` - 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: )` - 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. ```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 } ] } ``` ## 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: ```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" } } ``` 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: ```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" } } ``` 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. ```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" }, ... ] } ```