06. Docker pause & stop

docker linux signals

Core Idea

Pause suspends a container’s processes with the kernel’s cgroup freezer; stop signals it and shuts it down, and the SIGTERM versus SIGKILL story carries into Kubernetes.

• Docker pause: freeze all processes in the container without killing it.
• Docker stop: graceful shutdown of the container.
• The note connects both to SIGTERM and SIGKILL behavior in containerized and Kubernetes environments.

Docker Pause

Suspends all processes in the container using the cgroup freezer feature of the Linux kernel.

• Effect:

  • The container remains **running** but is in a **paused** state, meaning all processes are frozen and cannot execute.
  • Resources (e.g., memory, CPU) remain allocated.

• Use case: Temporarily halting a container’s activity without restarting or affecting its state, e.g., during maintenance or debugging.

Docker Stop

Gracefully shuts down the container by sending a SIGTERM signal to its main process, followed by a SIGKILL if the process doesn’t terminate within a timeout.

• Effect:

  • The container stops executing and is moved to the stopped state.
  • Resources are released, but the container can be restarted later.

• Use case: To halt a container completely, saving resources or preparing for updates.

SIGTERM and SIGKILL in Linux

👉 What is SIGTERM What’s the difference between SIGKILL & SIGTERM

Docker and Kubernetes: SIGTERM and SIGKILL in Containerized Environments

Info

Understanding how SIGTERM and SIGKILL work becomes even more critical in containerized environments like Docker and Kubernetes

Docker SIGKILL and SIGTERM Handling

Docker uses a combination of SIGTERM and SIGKILL for graceful container shutdown:

1. When you run docker stop, Docker sends a SIGTERM to the main process in the container.
2. Docker then waits for a grace period (default 10 seconds) for the process to exit.
3. If the process doesn’t exit within the grace period, Docker sends a SIGKILL to forcefully terminate it.

You can adjust the grace period when stopping a container:

docker stop --time 20 my_container # Wait for 20 seconds before sending SIGKILL

It’s crucial to design your containerized applications to handle SIGTERM properly to ensure graceful shutdowns.

Kubernetes SIGTERM SIGKILL Process

Kubernetes follows a similar but more complex pattern when terminating pods:

1. The Pod’s status is updated to “Terminating”.
2. If the Pod has a preStop hook defined, it is executed.
3. Kubernetes sends SIGTERM to the main process in each container.
4. Kubernetes waits for a grace period (30 seconds by default, but configurable).
5. If containers haven’t terminated after the grace period, Kubernetes sends SIGKILL.

Here’s an example of how to set a custom termination grace period in a Kubernetes Pod specification:

apiVersion: v1
kind: Pod
metadata:
    name: my-pod
spec:
    terminationGracePeriodSeconds: 60
    containers:
    - name: my-container
    image: my-image

This configuration gives the Pod 60 seconds to shut down gracefully before being forcefully terminated.

👉 SIGKILL vs SIGTERM: A Developer’s Guide to Process Termination | StackState