Deploying services in Swarm in Docker - Commands & Configuration

Introduction

When you want to run your app across multiple computers easily, Docker Swarm helps by managing many containers as one group. Deploying services in Swarm means starting your app so it runs reliably and can handle more users by spreading work across machines.

When you want your app to keep running even if one computer fails

When you need to run multiple copies of your app to handle more users

When you want to update your app without stopping it completely

When you want to manage many containers from one place

When you want to balance work evenly across several servers

Commands

This command starts Docker Swarm mode on your current machine, making it the manager that controls the cluster.

Terminal

docker swarm init

Expected Output

Swarm initialized: current node (abcdef123456) is now a manager.



To add a worker to this swarm, run the following command:

    docker swarm join --token SWMTKN-1-0abcdef1234567890abcdef1234567890abcdef1234567890abcdef123456 192.168.1.100:2377



To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.

This command deploys a service named 'my-web' running the nginx web server with 3 copies (replicas). It also maps port 8080 on your machine to port 80 in the containers so you can access the web server.

Terminal

docker service create --name my-web --replicas 3 -p 8080:80 nginx:1.23

Expected Output

q1w2e3r4t5y6z7x8y9a0b1c2d3e4f5g6h7i8j9k0l1m2n3o4p5q6r7s8t9u0v1w2

→

--name - Sets the name of the service for easy identification

→

--replicas - Specifies how many copies of the service to run

→

-p - Maps a port on the host to a port in the container

This command lists all running services in the swarm so you can check if your service is active and how many replicas are running.

Terminal

docker service ls

Expected Output

ID                  NAME                MODE                REPLICAS            IMAGE               PORTS

q1w2e3r4t5y6        my-web              replicated          3/3                 nginx:1.23          *:8080->80/tcp

This command shows detailed information about the tasks (containers) running for the 'my-web' service, including their current state and which node they run on.

Terminal

docker service ps my-web

Expected Output

ID                  NAME                IMAGE               NODE                DESIRED STATE       CURRENT STATE           ERROR               PORTS

x1y2z3a4b5c6        my-web.1            nginx:1.23          node-1              Running             Running 2 minutes ago                       

x7y8z9a0b1c2        my-web.2            nginx:1.23          node-2              Running             Running 2 minutes ago                       

x3y4z5a6b7c8        my-web.3            nginx:1.23          node-3              Running             Running 2 minutes ago                       

Key Concept

If you remember nothing else from this pattern, remember: Docker Swarm lets you run and manage many copies of your app easily across multiple machines with simple commands.

Common Mistakes

Trying to deploy a service before running 'docker swarm init'

The swarm mode is not active, so Docker cannot manage services across nodes.

Always run 'docker swarm init' first to enable swarm mode on your machine.

Not specifying the number of replicas when creating a service

Docker will create only one replica by default, which may not provide the desired availability or load distribution.

Use the '--replicas' flag to set how many copies of the service you want.

Forgetting to map ports with '-p' when deploying a web service

Without port mapping, you cannot access the service from outside the swarm nodes.

Use '-p host_port:container_port' to expose the service ports properly.

Summary

Initialize swarm mode with 'docker swarm init' to start managing services.

Deploy services using 'docker service create' with options for replicas and port mapping.

Check running services with 'docker service ls' and inspect tasks with 'docker service ps'.