Swarm Service Types

In Docker Swarm, services define how containers are deployed and managed across a cluster. Swarm supports two primary service modes: replicated and global. This guide explains each mode, shows how to deploy them, and compares their use cases.

Replicated Service

A replicated service launches a specified number of identical tasks (containers). This is the default mode in Swarm.

To deploy five replicas of a web service:

docker service create \
  --name web \
  --replicas 5 \
  nginx:latest

Verify the service mode and replica count:

docker service inspect web \
  --format '{{json .Spec.Mode}}'

Sample output:

{"Replicated":{"Replicas":5}}

Key points for replicated services:

• Tasks are spread evenly across available nodes.
• Scale up or down by updating --replicas.
• Ideal for stateless applications like web servers or APIs.

Global Service

A global service ensures exactly one task runs on every node in the Swarm.

To deploy a monitoring agent on all nodes:

docker service create \
  --name agent \
  --mode global \
  my-monitoring-agent:1.0

Global mode behavior:

• New nodes automatically receive one task.
• When a node leaves, its task is removed and not rescheduled.
• Perfect for logging, monitoring, and security daemons.

Comparison Table

References

• Docker Swarm Overview
• docker service create
• docker service inspect
• Docker Blog: Introduction to Swarm Mode