my-docs/Containerization-Orchestration/Docker/5-Docker-Swarm.md

📦 Docker Swarm Documentation

Comprehensive guide to managing a Docker Swarm cluster. This document includes core commands, workflows, and best practices for maintaining a healthy and operational environment.

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📚 Table of Contents

1. 🔧 Cluster Health & Manager Count
2. 🚀 Example Workflow: Setting Up Nginx
3. ⚙️ Cluster Initialization and Management
4. 🖥️ Node Management
5. 🛠️ Service Management
6. 🔑 Join Tokens & Node Configuration

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🔧 Cluster Health & Manager Count

A Docker Swarm cluster requires a majority of manager nodes to be functional for quorum.

Best Practice: Always maintain more than 50% manager nodes online. Losing quorum will render the cluster non-operational.

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🚀 Example Workflow: Setting Up Nginx

Docker Swarm handles service deployment through several internal components:

1. API – Receives service requests.
2. Allocator – Determines resource allocation.
3. Dispatcher – Assigns tasks to nodes.
4. Scheduler – Places tasks on optimal nodes.

This process ensures resilient and efficient service distribution.

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⚙️ Cluster Initialization and Management

🔹 Initialize Cluster

docker swarm init

🔹 Initialize with Specific Interface

docker swarm init --advertise-addr <ip-or-interface>

🔹 Join Existing Cluster

docker swarm join

🔹 Leave Cluster

docker swarm leave

🔹 Unlock a Manager Node

docker swarm unlock

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🖥️ Node Management

🔸 List Nodes

docker node ls

Example Output:

ID                            HOSTNAME   STATUS    AVAILABILITY   MANAGER STATUS   ENGINE VERSION
8yw8jrjeqczaci0qkuy060g09 *   docker-1   Ready     Active         Leader           24.0.5
v4gvf7xenw0izmxgvhr6hb2rj     docker-2   Ready     Active                          24.0.5
kd3ujmt1ey3pw6v9189fouxfa     docker-3   Ready     Active         Reachable        24.0.5
tm1msy58ztcltt36rs1lb76p7     docker-4   Down      Active                          24.0.5

🔸 Remove a Node

docker node rm <node-id>

🔸 Promote to Manager

docker node promote <hostname-or-id>

🔸 Inspect a Node

docker node inspect <nodename>

🔸 Change Node Role

docker node update --role manager <nodename>
docker node update --role worker <nodename>

🔸 Change Node Availability

docker node update --availability active <nodename>
docker node update --availability pause <nodename>
docker node update --availability drain <nodename>

🔸 Add or Remove Labels

Add:

docker node update --label-add env=development <nodename>
docker node update --label-add env=testing <nodename>

Remove:

docker node update --label-rm env= <nodename>
docker node update --label-rm env <nodename>

Use label constraints in service deployment:

deploy:
  placement:
    constraints:
      - node.labels.env == development

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🛠️ Service Management

🔹 Show Tasks on a Node

docker node ps

🔹 List All Services

docker service ls

🔹 Create a New Service

docker service create --name <service-name> <image-name>

🔹 Scale a Service

docker service scale <service-name>=<replica-count>

Example:

docker service scale nginx=5

🔹 Inspect a Service

docker service inspect <service-name>

🔹 Create Service with Replicas, Env Vars, and Port Mapping

docker service create \
  --name <service-name> \
  --replicas <count> \
  --env <ENV_VAR=value> \
  --publish <host-port>:<container-port> \
  <image-name>

Example:

docker service create \
  --name nginx \
  --replicas 3 \
  --env MY_ENV_VAR=value \
  --publish 8080:80 \
  nginx

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🔑 Join Tokens & Node Configuration

Securely add nodes to your Swarm using join tokens.

🔹 Get Worker Token

docker swarm join-token worker

🔹 Get Manager Token

docker swarm join-token manager