How do you perform a live migration of Docker containers between hosts?

When performing real-time migration of Docker containers, the primary goal is to migrate a running container from one physical or virtual machine to another without interrupting service. This process involves several key steps and technical choices, which I will explain in detail in order.

1. Selecting the Right Tools and Technologies:

   • CRIU (Checkpoint/Restore In Userspace): This is a Linux software tool that can freeze a running application and save its state to disk, which can then be restored on another machine. CRIU is one of the key technologies for achieving real-time container migration.
   • Docker's Built-in Migration Tools: Although Docker does not natively support real-time migration, it can be achieved by integrating tools like CRIU.

2. Preparing the Migration Environment:

   • Ensure that the source and target hosts have compatible environment configurations, including matching operating system versions, Docker versions, and network settings.
   • The two hosts must be able to communicate with each other, preferably within the same local area network.

3. Creating and Using Checkpoints:

   • On the source host, use CRIU to create a checkpoint of the container. This step involves saving the container's memory state, network configuration, and all dependent filesystem states.

   bash
   docker checkpoint create <container-name> <checkpoint-name>

   • The checkpoint data must be transferred to the target host, typically via network transmission using tools like rsync, scp, or other file transfer protocols.

4. Restoring the Container on the Target Host:

   • Use the previously transferred checkpoint data to restore the container on the target host.

   bash
   docker start --checkpoint <checkpoint-name> <container-name>

5. Verifying the Migration Results:

   • Confirm that the container is running normally on the target host with no service interruption.
   • Check network connectivity, application logs, and performance metrics to ensure everything is functioning correctly.

Example Application Scenario

Suppose I work at a company providing online gaming services. We need to migrate some Docker containers of game servers to other machines during maintenance to avoid interrupting users' gaming experience. By leveraging CRIU and Docker's migration capabilities, we can smoothly complete server maintenance and software upgrades without affecting online users.

Through this approach, we successfully migrated containers from one host to another, achieving nearly zero downtime service.