Liveness Probe

Kubernetes provides three types of probes to monitor and manage the health and readiness of containers in a pod:

1. Liveness Probe

• Purpose: Checks if a container is alive (running properly).
• What happens on failure: If this probe fails, Kubernetes restarts the container, helping recover from deadlocks or crashes.
• Use case: Useful for applications that might get stuck or hang.
• How to use: Define a check endpoint, command, or TCP socket in the pod spec under livenessProbe. For example, an HTTP GET to /health endpoint or a command that returns a success status.

2. Readiness Probe

• Purpose: Checks if a container is ready to receive traffic.
• What happens on failure: If it fails, the pod is temporarily removed from service endpoints, so no traffic is sent to it until it’s ready again.
• Use case: Ideal for applications that need startup time, load external resources, or recover from overload.
• How to use: Configured similarly to Liveness probes using HTTP, TCP, or command checks, but under readinessProbe in pod spec.

3. Startup Probe

• Purpose: Checks if the application inside the container has started successfully.
• What happens on failure: Until the startup probe succeeds, readiness and liveness probes are disabled to avoid premature restarts.
• Use case: Designed for slow-starting containers where startup time might exceed liveness/readiness probe timeouts.
• How to use: Defined under startupProbe in pod spec and run only during container startup.

Probe Types (Applicable to all probes)

Each probe can be configured using one of three methods:

• HTTP GET: Kubernetes sends an HTTP GET request to a specified path and port inside the container. Success means response codes 200–399.
• TCP Socket: Kubernetes attempts to open a TCP connection on a specified port. Success if connection established.
• Exec Command: Kubernetes runs a command inside the container. Success if command exits with 0 status.

Example of Configuring a Liveness Probe (HTTP)

livenessProbe:
  httpGet:
    path: /healthz
    port: 8080
  initialDelaySeconds: 10
  periodSeconds: 5
  timeoutSeconds: 3
  failureThreshold: 3

This will start checking the health of the container after 10 seconds delay, every 5 seconds, and consider it failed if no response in 3 seconds for 3 consecutive times.

Summary Use Cases for Each Probe

• Liveness Probe

  • Purpose: Check if container needs restart
  • When To Use: To catch crashes or deadlocks

• Readiness Probe

  • Purpose: Check if container is ready for traffic
  • When To Use: For startup delays or overloaded states

• Startup Probe

  • Purpose: Check if container has started
  • When To Use: For slow-starting applications