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pod lifecycles [e0b55c49]

Tags: kubernetes (k8s)

1. Three states

  • waiting
  • running

  • terminated

    • use kubectl describe

2. Terminating Gracefully

  • Cleanups?
  • Resources?
  • Forced termination?
  • SIGTERM/SIGKILL

  • Lifecycle

    1. Set the grace period -> terminating state -> traffic
    2. preStop hook -> grace of 2 seconds
    3. send SIGTERM to pid 1
    4. grace period sends SIGKILL
    5. API server deletes the pod's API object

3. Health Probes

  • Kubelet brings up the containers and keeps them running, also responsible for restarts
  • uses some Process Health Checks to check the status
  • Stalls?

  • Types of probes

    • Exec

      • Runs every so often, checks for the exit code
    • TCP socket
    • HTTP get

  • Liveness Probes

    • Check if the container is alive or dead
    • Always define a liveness probe for prod apps
    • Have the application expose a health-check API that doesn't require auth
    • Keep probes light as CPU resources
  • Readniess Probes

  • Startup probes

    • Indicates whether the container has started
    • Disables other probes until this is done

4. Hooks

4.1. postStart hook

  • Runs after container is created, async
  • No guarantees of running
  • postStart is a blocking call
  • container remains in Waiting until the handler completes

4.2. preStop

  • Call sent to container before terminated
  • use when reacting to SIGTERM
  • always ends before SIGKILL is sent
  • used for graceful termination

4.3. initContainers

  • runs various preconditions before the container startup

  • exaples

    • DB, create user accounts, DB migrations

4.4. Scheduling and resources

  • Effective request/limit is the max(sum request/limit for all the application containers, effective request limit)

5. Init vs postStart hook vs Startup Probe

  • Probes and hooks run in the same container, init containers run on the whole pod
  • hooks have no guarantees

  • usages:

    • postStart

      • signal to external listeners

    • init

      • initalization

    • probes

      • slow starting containers