Health Check

What is Health Check?

Health Check refers to a mechanism used by load balancers to monitor the operational status of servers. This functionality is commonly known as the "health check feature" or "health monitor feature."

Load balancers receive client requests from external networks and distribute them across multiple servers within an internal network. If requests are sent to servers that are down or unresponsive, proper processing cannot be performed. To prevent this, load balancers continually verify the status of servers to ensure they are operational. This verification process is called a health check.

There are two main types of health checks: active and passive.

• Active Health Check: The load balancer sends packets to the server specifically for health verification and confirms server responses. Depending on the protocol layer used, there are three types:
  • L3 Check: Sends ICMP echo requests and verifies if an echo reply is received.
  • L4 Check: Performs a TCP handshake to confirm the server's status.
  • L7 Check: Simulates an HTTP or application-layer request and examines the server's response.
• Passive Health Check: Instead of sending specific packets, the load balancer monitors responses to actual client requests. If errors occur—such as an invalid response—the load balancer redirects incoming requests to alternative servers.

F5’s BIG-IP solution supports all types of health checks, offering robust flexibility. It also allows the combination of both active and passive health check methods for enhanced performance and reliability.