Swarm Services

Docker services enable you to deploy, scale, and manage containers across a Swarm cluster. With built-in load balancing, health checks, and automatic rescheduling, services take container orchestration to the next level.

From `docker run` to Swarm Services

On a single Docker host, you might launch a web server like this:

docker run --name webserver httpd

But scaling out across multiple nodes manually requires SSHing into each host:

docker run httpd

Challenges of this approach:

You must configure external load balancing.
Health monitoring and restarts are manual.
Deployment becomes error-prone as the cluster grows.

Swarm services automate all of these tasks:

Unified load balancing across nodes
Built-in health monitoring and self-healing
Declarative desired state and scaling

Creating a Docker Service

A Swarm service represents one or more replicas of the same container. On a manager node, run:

docker service create \
  --name web \
  --replicas 3 \
  --publish 80:80 \
  httpd:alpine

Flag	Description	Example
`--name`	Assigns a name to the service	`web`
`--replicas`	Number of container replicas to run	`3`
`--publish`	Maps a host port to container port	`80:80`
Image	Container image (with optional tag)	`httpd:alpine`

If you omit --replicas, Docker defaults to a single replica (1).

How Swarm Orchestration Works

When you issue docker service create, the Swarm manager components collaborate:

API Server
Accepts the CLI/API request.
Orchestrator
Calculates desired tasks (one per replica).
Allocator
Assigns virtual IPs and ports to each task.
Dispatcher
Sends tasks to available worker nodes.
Worker Node
Launches containers and reports status back to the manager.
Rescheduler
Detects failures and automatically replaces failed tasks.

A task is the Swarm abstraction for running a container and maintaining its desired state.

Managing Services

Use the following commands to inspect and troubleshoot services:

Command	Description
`docker service ls`	List all services in the Swarm cluster
`docker service ps <service>`	View tasks (containers) for a specific service
`docker service inspect <service> --pretty`	Show detailed service configuration in human-readable form
`docker service logs <service>`	Retrieve aggregated logs from all service replicas
`docker service scale <service>=<replicas>`	Adjust the number of replicas on the fly

Example:

docker service ls

ID            NAME  MODE        REPLICAS  IMAGE         PORTS
3zhe91mns5vz  web   replicated  3/3       httpd:alpine  *:80->80/tcp

Sample logs:

web.1.xxxxxx@worker1 | [mpm_event:notice] AH00489: Apache/2.4.43 configured
web.2.yyyyyy@worker2 | [core:notice] AH00994: Command line: 'httpd -D FOREGROUND'
web.3.zzzzzz@worker3 | 10.0.0.4 - - [24/Apr/2020] "POST /cgi-bin/main.cgi" 400 226

Removing a Service

To tear down a service and its tasks:

docker service rm web

All associated containers will be stopped and removed.

Links and References

Happy containerizing! See you in the next guide.

Introduction

Docker Engine

Docker Image Management

Docker Engine - Security

Docker Engine - Networking

Docker Engine - Storage

Docker Compose

Docker Swarm

Kubernetes

Docker Engine Enterprise

Docker Trusted Registry

Disaster Recovery

From `docker run` to Swarm Services

Creating a Docker Service

How Swarm Orchestration Works

Managing Services

Removing a Service

Links and References

Watch Video

​From docker run to Swarm Services

​Creating a Docker Service

​How Swarm Orchestration Works

​Managing Services

​Removing a Service

​Links and References

Watch Video

From `docker run` to Swarm Services

Creating a Docker Service

How Swarm Orchestration Works

Managing Services

Removing a Service

Links and References