Configuring a Health Check

Scenarios

You can modify the health check configuration after creating a backend server group. After the health check protocol is changed, the load balancer uses the new protocol to check the backend servers. If the backend servers are detected healthy, the load balancer continues routing traffic to them.

During the protocol-change window, if the backend servers cannot be connected using the new health check protocol, the backend servers will be considered unhealthy and removed from the forwarding list. If no backend server is available, the load balancer returns error code 503, causing temporary request failures. Before changing the health check protocol, ensure that the backend servers support the new health check protocol. Alternatively, change the protocol during off-peak hours to minimize the impact of the change.

Constraints

• The health check protocol does not have to be the same as the backend protocol of the backend server group. However, in some special scenarios (for example, incompatible protocol message formats), service faults may occur. Therefore, using the same protocol is recommended.
• To reduce the vCPU usage of the backend servers, it is recommended that you use TCP for health checks. If you want to use HTTP for health checks, you can use static files to return the health check results.
• If health check is enabled, security group rules must allow traffic from the health check port to the backend servers over the health check protocol. For details, see Security Group and Network ACL Rules.

After you enable health check, the load balancer immediately checks the health of backend servers.

• If a backend server is detected healthy, the load balancer will start routing requests to it over new connections based on the configured load balancing algorithms and weights.
• If a backend server is detected unhealthy, the load balancer will stop routing traffic to it.

Enabling Health Check

1. Go to the backend server group list page.
2. On the Backend Server Groups page, locate the backend server group and click its name.
3. On the Summary page, click Health Check on the right.
4. In the Configure Health Check dialog box, enable health check and configure the parameters based on Table 1.

   Table 1 Parameters required for configuring health check

   Parameter	Description	Example Value
   Health Check	Specifies whether to enable the health check option. NOTE: When the health check is enabled or disabled, the number of healthy or unhealthy backend servers may temporarily fluctuate but will stabilize after a monitoring period.	Enable it.
   Health Check Protocol	Specifies the protocol that will be used by the load balancer to check the health of backend servers. If the protocol of the backend server group is UDP, the health check protocol is UDP by default. Dedicated load balancers support TCP, HTTP, TLS, gRPC, and HTTPS.	HTTP
   HTTP Method	Specifies the request method for the health check. This parameter is mandatory for dedicated load balancers and the health check protocol is HTTP, gRPC, or HTTPS. GET: The backend server returns all the information. HEAD: The backend server returns only HTTP headers, improving request efficiency. Ensure that your backend servers support HEAD requests. Otherwise, the health check may fail. In this case, you can use GET to perform the health check. POST: Ensure that your backend servers support POST requests. Otherwise, the health check may fail. In this case, you can use GET to perform the health check. NOTE: If the health check protocol is HTTP or HTTPS, the GET and HEAD methods are supported. If the health check protocol is gRPC, the GET and POST methods are supported. This feature will be available in more regions. See details on the management console.	GET
   Domain Name	Specifies the domain name that will be used for the health check. This parameter is mandatory if the health check protocol is HTTP, gRPC, or HTTPS. You can use the private IP address of the backend server as the domain name. You can also specify a domain name that consists of at least two labels separated by periods (.). Use only letters, digits, and hyphens (-). Do not start or end strings with a hyphen. Max total: 100 characters. Max label length: 63 characters.	www.elb.com
   Health Check Port	Specifies the port that will be used by the load balancer to check the health of backend servers. The port number ranges from 1 to 65535. By default, the service port on each backend server is used. You can specify a port for a health check.	80
   Path	Specifies the health check path, which is the destination on backend servers for health checks. This parameter is mandatory if the health check protocol is HTTP, gRPC, or HTTPS. The path can contain 1 to 80 characters and must start with a slash (/). If the backend server group is associated with a dedicated load balancer, the check path can contain letters, digits, hyphens (-), slashes (/), periods (.), question marks (?), number signs (#), percent signs (%), ampersands (&), and extended character sets _;~!. () *[]@$^:',+	/index.html
   Interval (s)	Specifies the maximum time between two consecutive health checks, in seconds. The interval ranges from 1 to 50.	5
   Timeout (s)	Specifies the maximum time required for waiting for a response from the health check, in seconds. The value ranges from 1 to 50.	3
   Healthy Threshold	Specifies the number of consecutive successful health checks required for declaring a backend server healthy. The value ranges from 1 to 10.	3
   Unhealthy Threshold	Specifies the number of consecutive failed health checks required for declaring a backend server unhealthy. The value ranges from 1 to 10.	3
   Status Code	Specifies the status codes that will be returned to the load balancer to indicate the health of backend servers. This parameter is available only when you set the health check protocol to HTTP, gRPC, or HTTPS. You can enter a unique number or a positive number range within the status code range, for example, 0-10 and 200-300. A maximum of five HTTP status codes and number ranges are supported. If there is more than one status code or number range, press Enter to separate them. If the check protocol is HTTP or HTTPS, the status code ranges from 200 to 599. When the gRPC protocol is used, the status code ranges from 0 to 99. NOTE: This feature will be available in more regions. See details on the management console.	200

5. Click OK.

Disabling Health Check

1. Go to the backend server group list page.
2. On the Backend Server Groups page, click the name of the target backend server group.
3. On the Summary page, click Health Check on the right.
4. In the Configure Health Check dialog box, disable health check.
5. Click OK.

Helpful Links

• Troubleshooting an Unhealthy Backend Server
• Other Issues
• How Do I Troubleshoot an Unhealthy Backend Server of a Dedicated Load Balancer?
• APIs:
  • Configuring a Health Check
  • Querying Health Check Details
  • Updating a Health Check
  • Deleting a Health Check