Docker Swarm
Docker Swarm is Docker’s native clustering and orchestration tool that allows you to manage a group of Docker engines as a single virtual system. It simplifies the deployment and scaling of containerized applications by providing built-in orchestration capabilities.
Key Concepts
1. **Swarm**: A cluster of Docker nodes, consisting of manager nodes and worker nodes.
2. **Node**: An individual Docker engine participating in the swarm, which can be either a manager or a worker.
3. **Service**: The definition of a task, including the Docker image to use, the desired state, and how to scale.
4. **Task**: A single instance of a container managed by the swarm.
5. **Manager Node**: Responsible for managing the swarm and distributing tasks to worker nodes.
6. **Worker Node**: Executes tasks assigned by manager nodes.
## Setting Up Docker Swarm
### Prerequisites
– Docker installed on all nodes (manager and workers).
– Basic understanding of Docker commands.
### Installation Steps
1. **Install Docker**:
Ensure Docker is installed on all nodes. Follow the installation steps for your operating system:
– [Docker installation guide](https://docs.docker.com/get-docker/)
2. **Initialize the Swarm**:
On the manager node, initialize the swarm:
“`sh
docker swarm init –advertise-addr <MANAGER-IP>
“`
This command will output a join token for worker nodes.
3. **Join Worker Nodes to the Swarm**:
On each worker node, run the join command provided by the `docker swarm init` output:
“`sh
docker swarm join –token <WORKER-TOKEN> <MANAGER-IP>:2377
“`
4. **Verify Nodes in the Swarm**:
On the manager node, list the nodes in the swarm:
“`sh
docker node ls
“`
### Deploying Services
1. **Create a Service**:
Deploy a service to the swarm:
“`sh
docker service create –name my-web-service -p 80:80 –replicas 3 nginx
“`
2. **List Services**:
Check the running services:
“`sh
docker service ls
“`
3. **Inspect a Service**:
Get details about a specific service:
“`sh
docker service inspect –pretty my-web-service
“`
4. **Scale a Service**:
Scale the number of replicas for a service:
“`sh
docker service scale my-web-service=5
“`
5. **Update a Service**:
Update the image or configuration of a service:
“`sh
docker service update –image nginx:latest my-web-service
“`
### Managing the Swarm
1. **Promote a Worker to Manager**:
Promote a worker node to a manager node:
“`sh
docker node promote <NODE-ID>
“`
2. **Demote a Manager to Worker**:
Demote a manager node to a worker node:
“`sh
docker node demote <NODE-ID>
“`
3. **Drain a Node**:
Prevent a node from receiving new tasks and move its current tasks to other nodes:
“`sh
docker node update –availability drain <NODE-ID>
“`
4. **Remove a Node**:
Remove a node from the swarm:
“`sh
docker node rm <NODE-ID>
“`
### Networking in Docker Swarm
1. **Overlay Network**:
Create an overlay network for multi-host communication:
“`sh
docker network create –driver overlay my-overlay-network
“`
2. **Attach a Service to an Overlay Network**:
Deploy a service attached to an overlay network:
“`sh
docker service create –name my-overlay-service –network my-overlay-network nginx
“`
3. **List Networks**:
View all networks:
“`sh
docker network ls
“`
4. **Inspect a Network**:
Get details about a specific network:
“`sh
docker network inspect my-overlay-network
“`
Conclusion
Docker Swarm provides a simple yet powerful way to manage containerized applications in a clustered environment. With its built-in orchestration capabilities, you can easily deploy, scale, and manage services across multiple Docker nodes. Understanding these basic concepts and commands will help you leverage Docker Swarm effectively for your container orchestration needs.