Linux and Docker: How to migrate and synchronize containers across hosts?

Linux and Docker: How to implement cross-host migration and synchronization of containers?

Abstract: Docker is a popular containerization technology that provides a lightweight virtualization solution. In a multi-host environment, it is a very common requirement to migrate and synchronize containers across hosts. This article will introduce how to use Linux and Docker to implement cross-host migration and synchronization of containers, and provide some sample code for reference.

1. Introduction
   The rise of containerization technology makes application deployment and migration more flexible and efficient. In a multi-host environment, cross-host migration and synchronization of containers are essential functions that can help us achieve load balancing, high availability, and optimal utilization of resources. Linux provides some tools and functions to support cross-host migration and synchronization of containers, and Docker provides a more convenient way to manage and operate containers based on Linux.
2. Linux container migration and synchronization
   In Linux, container migration and synchronization mainly rely on two technologies: migration storage and network. Migrating storage refers to the process of migrating the data and file system of the container from the source host to the target host, and the network is the key to maintaining network connectivity during the container migration process.

2.1 Migration Storage
For container migration storage, there are several commonly used technologies to choose from, such as traditional replication and synchronization file systems, distributed file systems, and distributed block storage. Replicating and synchronizing file systems is the most common approach, and they are suitable for small-scale environments, but may cause performance bottlenecks in large-scale environments. Distributed file systems and distributed block storage can provide higher performance and scalability, but are relatively complex to configure and manage. Here, we will illustrate using copying and synchronizing file systems as an example.

Suppose we have two hosts, the source host and the target host. To migrate the container from the source host to the target host, we can perform the following steps:

Step 1: Stop the container on the source host running on.

$ docker stop container_id

Step 2: Export the container’s file system.

$ docker export container_id > container.tar

Step 3: Transfer the container’s file system to the target host.

$ scp container.tar user@target_host:/path/

Step 4: Import the container’s file system on the target host.

$ docker import /path/container.tar

Step 5: Start the container to run on the target host.

$ docker run -d --name container_name image_name

2.2 Network synchronization
During the container migration process, it is very important to maintain the stability of the network connection. Linux provides some tools and technologies to achieve network synchronization, such as iptables rules, network namespaces and macvlan. The exact implementation depends on the network architecture and requirements.

Assume we have two hosts, the source host and the target host. To maintain the network connection of the container during the migration process, we can perform the following steps:

Step 1: Create on the source host network namespace and move the container's network interface to that namespace.

$ ip link set dev eth0 netns container_ns

Step 2: Create a network namespace on the target host and move the container's network interface to the namespace.

$ ip link set dev eth0 netns container_ns

Step 3: Set iptables rules on the source host to redirect the container’s network traffic to the target host.

$ iptables -t nat -A PREROUTING -i eth0 -j DNAT --to-destination target_ip

Step 4: Start the container’s network interface on the target host.

$ ip link set dev eth0 up

Step 5: Set iptables rules on the target host to forward the container’s network traffic to the container’s network namespace.

$ iptables -t nat -A PREROUTING -i eth0 -j DNAT --to-destination container_ip

3. Docker container migration and synchronization
   In Docker, the cross-host migration and synchronization of containers mainly relies on Docker Swarm. Docker Swarm is a native Docker clustering and orchestration tool that can manage and schedule containers across multiple hosts.

Assuming that we have configured Docker Swarm, to implement cross-host migration and synchronization of containers in a multi-host environment, we can perform the following steps:

Step 1: Add the container to Docker Swarm cluster.

$ docker swarm join --token SWMTKN-abcdefg1234567890 manager_ip:2377

Step 2: Tag the container on the source host to indicate that it needs to be migrated across hosts.

$ docker service update --label-add com.docker.ucp.mesh.http.ports=80 container_name

Step 3: Start the service with the same name on the target host.

$ docker service create --name container_name image_name

Step 4: Docker Swarm will automatically migrate the containers on the source host to the target host.

4. Conclusion
   Through Linux and Docker, we can easily achieve cross-host migration and synchronization of containers. Whether we use replication and synchronization file systems directly in a Linux environment, or use Docker Swarm to manage and schedule containers across hosts, we can meet our needs in a multi-host environment. I hope this article has provided some help to readers in implementing cross-host migration and synchronization of containers.

Reference:

1. Docker documentation: https://docs.docker.com/
2. Linux documentation: https://www.kernel .org/doc/html/latest/

Appendix: Code Example

# 示例代码1：复制和同步文件系统
# 步骤1：停止容器在源主机上的运行。
$ docker stop container_id

# 步骤2：导出容器的文件系统。
$ docker export container_id > container.tar

# 步骤3：将容器的文件系统传输到目标主机。
$ scp container.tar user@target_host:/path/

# 步骤4：在目标主机上导入容器的文件系统。
$ docker import /path/container.tar

# 步骤5：启动容器在目标主机上运行。
$ docker run -d --name container_name image_name

# 示例代码2：网络同步
# 步骤1：在源主机上创建网络命名空间，并将容器的网络接口移动到该命名空间。
$ ip link set dev eth0 netns container_ns

# 步骤2：在目标主机上创建网络命名空间，并将容器的网络接口移动到该命名空间。
$ ip link set dev eth0 netns container_ns

# 步骤3：在源主机上设置iptables规则，将容器的网络流量重定向到目标主机。
$ iptables -t nat -A PREROUTING -i eth0 -j DNAT --to-destination target_ip

# 步骤4：在目标主机上启动容器的网络接口。
$ ip link set dev eth0 up

# 步骤5：在目标主机上设置iptables规则，将容器的网络流量转发到容器的网络命名空间。
$ iptables -t nat -A PREROUTING -i eth0 -j DNAT --to-destination container_ip

# 示例代码3：Docker Swarm的容器迁移和同步
# 步骤1：将容器加入到Docker Swarm集群。
$ docker swarm join --token SWMTKN-abcdefg1234567890 manager_ip:2377

# 步骤2：在源主机上标记容器，以指示它需要跨主机迁移。
$ docker service update --label-add com.docker.ucp.mesh.http.ports=80 container_name

# 步骤3：在目标主机上启动同名的服务。
$ docker service create --name container_name image_name

# 步骤4：Docker Swarm会自动将源主机上的容器迁移到目标主机上。

(Total word count: original text approximately 1306 words, sample code 607 words)

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