How to run admin commands on a non-leader node in Docker Swarm

Introduction

Docker Swarm is a powerful orchestration tool that allows you to manage a cluster of Docker hosts as a single system. In a Swarm, one node is designated as the leader, responsible for managing the cluster's state and making decisions. However, there may be times when you need to run administrative commands on non-leader nodes. This tutorial will guide you through the process of identifying the leader node and executing admin commands on non-leader nodes in a Docker Swarm.

Skills Graph

%%%%{init: {'theme':'neutral'}}%%%% flowchart RL docker(("`Docker`")) -.-> docker/SystemManagementGroup(["`System Management`"]) docker/SystemManagementGroup -.-> docker/info("`Display System-Wide Information`") docker/SystemManagementGroup -.-> docker/version("`Show Docker Version`") subgraph Lab Skills docker/info -.-> lab-411597{{"`How to run admin commands on a non-leader node in Docker Swarm`"}} docker/version -.-> lab-411597{{"`How to run admin commands on a non-leader node in Docker Swarm`"}} end

Understanding Docker Swarm Architecture

Docker Swarm is a container orchestration tool that allows you to manage a cluster of Docker hosts, known as a "swarm." In a Docker Swarm, you have two types of nodes: manager nodes and worker nodes.

Manager Nodes

Manager nodes are responsible for the overall management of the swarm. They handle tasks such as:

Scheduling and deploying containers
Maintaining the desired state of the swarm
Providing an API for interacting with the swarm

Manager nodes can also run containers, but their primary role is to manage the swarm.

Worker Nodes

Worker nodes are responsible for running the actual containers. They receive instructions from the manager nodes and execute them. Worker nodes can be added or removed from the swarm as needed to scale the cluster.

graph LR subgraph Docker Swarm Manager[Manager Node] --- Worker[Worker Node] Manager --- Worker end

The Docker Swarm architecture ensures high availability and fault tolerance by allowing multiple manager nodes to be part of the swarm. This way, if one manager node fails, the swarm can continue to operate with the remaining manager nodes.

Identifying the Leader Node in Docker Swarm

In a Docker Swarm, one of the manager nodes is elected as the leader. The leader node is responsible for making all the critical decisions and coordinating the actions of the other manager nodes.

Checking the Leader Node

To identify the leader node in your Docker Swarm, you can use the following command:

docker node ls

This command will list all the nodes in the swarm, and the leader node will be marked with a * symbol.

Alternatively, you can use the docker node inspect command to get more detailed information about a specific node:

docker node inspect < node-id > --pretty

This will display the node's role, status, and other relevant information, including whether it is the leader.

Understanding Leader Election

Docker Swarm uses the Raft consensus algorithm to elect a leader among the manager nodes. The leader is responsible for making decisions and coordinating the actions of the other manager nodes.

If the current leader node fails or becomes unavailable, a new leader will be automatically elected from the remaining manager nodes. This ensures that the swarm can continue to operate even if a manager node fails.

graph LR subgraph Docker Swarm Manager1[Manager Node 1] --- Manager2[Manager Node 2] Manager1 --- Manager3[Manager Node 3] Manager2 --- Manager3 Manager1 -- Leader --> Manager2 Manager2 -- Follower --> Manager3 end

Running Admin Commands on Non-Leader Nodes

In a Docker Swarm, the leader node is responsible for making all the critical decisions and coordinating the actions of the other manager nodes. However, there may be situations where you need to run administrative commands on the non-leader nodes, such as for troubleshooting or maintenance purposes.

Accessing Non-Leader Nodes

To access a non-leader node in your Docker Swarm, you can use the docker node ssh command. This command allows you to establish an SSH connection to a specific node in the swarm.

docker node ssh <node-id>

Replace <node-id> with the ID of the non-leader node you want to access.

Running Admin Commands

Once you have established an SSH connection to the non-leader node, you can run administrative commands as needed. Some common admin commands you might want to run include:

Checking the node's status and role:

docker node inspect < node-id > --pretty

Viewing the node's logs:
```
docker node logs <node-id>
```

Executing a command on the node:

docker exec -it <container-id> <command>

Keep in mind that while you can run admin commands on non-leader nodes, any changes or actions you take will not affect the overall state of the swarm. The leader node is still responsible for maintaining the desired state of the cluster.

Limitations

It's important to note that running admin commands on non-leader nodes has some limitations. For example, you cannot use the docker service or docker stack commands on non-leader nodes, as these commands require access to the swarm's state, which is managed by the leader node.

If you need to perform actions that require access to the swarm's state, you should do so on the leader node or use the docker node exec command to execute the command on the leader node from a non-leader node.

Summary

By the end of this tutorial, you will have a better understanding of the Docker Swarm architecture and be able to effectively run administrative commands on non-leader nodes. This knowledge will help you maintain high availability and efficiently manage your containerized applications in a Docker Swarm environment.