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Configuring health checks

Reviewed on 13 December 2023Published on 05 June 2023

Health check overview

Load Balancers should only forward traffic to “healthy” backend servers. To monitor the health of a backend server, health checks regularly attempt to communicate with backend servers to ensure that servers are listening. Backend servers that fail a given number of health checks (by default, 3) are marked as Down and the Load Balancer stops forwarding traffic to them.

In this document, we explain some of the different settings available when setting up health checks for your Load Balancer.

Health check types

Health checks are available in many different types. By default, the health check type is set to be the same as that of the backend protocol, that is to say TCP or HTTP. However, this can be overridden.

The following health check types are available:

HTTP

When selecting HTTP health checks, you are prompted to select values for the following fields:

Method: Specifies the HTTP method the Load Balancer uses to send health check requests to the backend server. The following options are available:

  • GET: The Load Balancer sends a simple request to retrieve information. This is the most common choice for a health check method.
  • POST: The Load Balancer submits data for processing to a specified resource endpoint.
  • PUT: The Load Balancer submits data to update a specified resource.

While GET is a commonly-used method as it is considered a “safe” choice with minimal side effects on the backend server, POST or PUT may be suitable for checking the health of applications running on backend servers that rely heavily on form submissions, data updates or write operations.

Path: The destination URL path that the Load Balancer will send the health check request to, e.g. /health. Ensure that you implement the correct logic on your backend server application so that a request can be received at this path.

Expected code: The HTTP status code that the Load Balancer expects to receive from the backend server in order to consider it healthy. Common choices include 200 (successful request) or 2xx, eg 201 (successfully created). Ensure that you implement the correct logic on your backend server application so that the appropriate response code is sent.

Host header value: This is an optional field which specifies the domain name or IP address to which the request should be sent, e.g. www.example.com or 98.94.76.20. If a value is specified, the Load Balancer will use this value in the Host header field of its request. This header is important for backend servers hosting multiple domains or several virtual hosts. The server can use the Host header to determine which website (virtual host) the request is intended for. You can leave this field empty if you do not need this functionality.

MYSQL

For MYSQL health checks, you are prompted to select the database username. The MYSQL health check sends two MySQL packets to the server: first a Client Authentication packet and then a QUIT packet to properly close the MySQL session. The received MySQL Handshake Initialization packet (and/or Error packet) is then parsed. This basic but useful test does not produce errors or aborted connections on the server. However, bear in mind that it does not check database presence or database consistency (an external check e.g. with xinetd would be required for this).

Tip

The user specified for the health check must be unlocked and authorized without a password. To create a basic limited user in MySQL with optional resource limits:

CREATE USER '<username>'@'<ip_of_haproxy|network_of_haproxy/netmask>'
/*!50701 WITH MAX_QUERIES_PER_HOUR 1 MAX_UPDATES_PER_HOUR 0 */
/*M!100201 MAX_STATEMENT_TIME 0.0001 */;
Important

This health check method requires MySQL >=3.22. For older versions, we recommend using a TCP health check.

PGSQL

For PGSQL health checks, you are prompted to select the database username. The check sends a PostgreSQL StartupMessage and waits for either an Authentication request or ErrorResponse message. This basic but useful test does not produce errors or aborted connections on the server. This check is identical to the mysql-check.

Important

There is a known bug with PGSQL health checks where the PGSQL server version is higher than 14. A fix is currently underway. In the meantime, we recommend using a TCP health check for PGSQL servers of version 14 or higher.

LDAP

No additional settings are required for LDAP health checks. LDAP health checks test connection to the LDAP server by sending an a LDAPv3 anonymous simple bind message. This checks the LDAP service is actually up, and not just that a TCP session can be established.

Redis

No additional settings are required for REDIS health checks. A PING redis command is sent to the server, and the response is analyzed for the +PONG response message. This checks that the server is correctly communicating with the REDIS protocol, rather than just accepting the TCP connection.

TCP

No additional settings are required for TCP health checks. This check consists of a connection attempt that either fails or succeeds.

TLS

You can choose to activate or deactivate TLS for health checks. Activating TLS encrypts connections between the Load Balancer and the backend server(s) during health checks. Note that the backend server should have its own SSL/TLS certificate

The activation setting is inherited from your backend configuration, however, you can choose to override this for health checks if you wish.

Verify certificate

If you activated TLS encryption for your health check, the Verify Certificate setting displays. This determines whether the backend server’s certificate is verified during health checks.

This setting is inherited from the backend configuration and cannot be overridden:

  • If you activated TLS on your backend, the Verify Certificate setting you chose there is inherited for health checks.
  • If you did not activate TLS on your backend configuration, the backend server’s certificate will not be verified during health checks and you do not have the option to activate the setting here.

The above also applies if you edit your backend settings after creating the backend: if you change your Verify Certificate setting in the backend, configuration the new setting will also be applied to the health check.

Proxy Protocol

Proxy Protocol enables the original client IP address to be passed to the backend server in a standardized way during health checks. Note that the backend server must support the selected Proxy Protocol.

The following versions of Proxy Protocol are available:

  • Proxy Protocol v1: Version one (text format).
  • Proxy Protocol v2: Version two (binary format).
  • Proxy Protocol v2-ssl: Version two with SSL information extension added, which provides information on SSL connection settings originally sent by the client.
  • Proxy Protocol v2-ssl-cn: Version two with SSL information extension added as above, as well as the common name from the subject of the client certificate (if any).

If you activated Proxy Protocol on your backend configuration, it will also be activated by default for your health checks. Note that:

  • You can choose to override the inherited setting and deactivate Proxy Protocol for health checks if you wish.
  • If you keep Proxy Protocol activated for health checks, the Proxy Protocol version is inherited from the backend configuration and cannot be overridden.

If you deactivated Proxy Protocol on your backend, it will also be deactivated for health checks, and you cannot override this setting. That is to say, you cannot have Proxy Protocol deactivated on the general backend configuration, but activated for health checks.

Advanced health check settings

Recommended settings for all the parameters below are selected by default, however you can modify them if you wish. These settings allow you to define more precisely the parameters that the Load Balancer should use when carrying out health checks. The following parameters are available:

  • Interval: The interval between consecutive health checks (in milliseconds). The Load Balancer will carry out health checks every {interval} ms. The default value is 3 000, the minimum value is 1 000 and the maximum value is 27 670 116 110 564.
  • Timeout: The maximum amount of time (in milliseconds) a backend server has to respond to a health check before the health check is considered as failed. The default value is 1 000, the minimum value is 1 000 and the maximum value is 27 670 116 110 564.
  • Unhealthy threshold: The number of consecutive failed health checks after which a backend server is diagnosed as unhealthy, and its status is changed to Down. Requests/connections will no longer be forwarded to servers with a down status.The default value is 3, the minimum value is 1 and the maximum value is 9 999.
  • Interval (transient state): The interval between two consecutive health checks when a server is in a transient state. The default value is 0.5, the minimum value is 0.1 and the maximum value is 2 147 483. Note that a backend server is in a transient state when:
    • It is considered up, but then fails a health check. It stays in this transient state until the number of consecutive unsuccessful health checks goes above the unhealthy threshold, at which point it switches to down.
    • It is considered down, but then passes a health check. It stays in this transient state until the number of consecutive successful health checks goes above two, at which point it switches to up.
  • Shut down sessions if unhealthy: Activate this option if you want the Load Balancer to shut down any open sessions to a given backend server as soon as that server is diagnosed as unhealthy.
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