Docker Certified Associate Exam Course
Docker Swarm
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 |
Default Replicas
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.
Swarm Task vs. Container
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.
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