10 -- Discovery: Docker

Prerequisite: 01 -- HTTP Gateway You will need: Running Hangar, Docker or Podman Time: 10 minutes Adds: Auto-discover MCP servers from Docker container labels

The Problem

You have MCP servers running as Docker containers. You don't want to manually update config.yaml every time a container starts or stops. You want Hangar to detect them automatically.

The Config

# config.yaml -- Recipe 10: Docker Discovery
discovery:                               # NEW: discovery configuration
  enabled: true                          # NEW: enable auto-discovery
  refresh_interval_s: 30                 # NEW: scan every 30 seconds
  auto_register: false                   # NEW: require manual approval

  sources:                               # NEW: discovery sources
    - type: docker                       # NEW: Docker source
      mode: additive                     # NEW: only add, never remove

Try It

Start an MCP server container with labels:

docker run -d --name my-mcp-server \
  -l mcp.hangar.enabled=true \
  -l mcp.hangar.name=docker-math \
  -l mcp.hangar.mode=http \
  -l mcp.hangar.port=8080 \
  my-mcp-server:latest

Start Hangar:
```
mcp-hangar serve --http --port 8000
```

Trigger a discovery scan:

curl -X POST http://localhost:8000/api/discovery/sources/docker/scan

Check pending MCP servers:

curl http://localhost:8000/api/discovery/pending

{"pending": [{"name": "docker-math", "source": "docker", "mode": "remote"}]}

Approve the MCP server:

curl -X POST http://localhost:8000/api/discovery/approve/docker-math

Verify it's registered:

mcp-hangar status

docker-math    remote    cold    source=docker:auto-discovery

What Just Happened

The Docker discovery source connects to the Docker socket and lists containers with mcp.hangar.enabled=true labels. In additive mode, it only adds new MCP servers -- never removes existing ones. With auto_register: false, discovered MCP servers go to a pending queue for manual approval.

Set auto_register: true if you trust all labeled containers and want zero-touch registration.

Key Config Reference

Key	Type	Default	Description
`discovery.enabled`	bool	`false`	Enable auto-discovery
`discovery.refresh_interval_s`	int	`30`	Seconds between scans
`discovery.auto_register`	bool	`false`	Register without approval
`discovery.sources[].type`	string	--	`docker`, `filesystem`, `kubernetes`, `entrypoint`
`discovery.sources[].mode`	string	--	`additive` (add only) or `authoritative` (add and remove)

Docker Labels

Label	Required	Default	Description
`mcp.hangar.enabled`	Yes	--	Must be `"true"`
`mcp.hangar.name`	No	Container name	MCP Server name
`mcp.hangar.mode`	No	`http`	MCP Server mode
`mcp.hangar.port`	No	`8080`	MCP Server port
`mcp.hangar.group`	No	--	Auto-add to group

What's Next

Docker discovery works for local and CI environments. For Kubernetes, you need annotation-based discovery.

--> 11 -- Discovery: Kubernetes