Alias and anchors
Anchors and aliases let you identify an item with an anchor in a YAML document, and then refer to
that item with an alias later in the same document. Anchors are identified by an & character, and
aliases by an * character.
The following example will start up 3 nginx servers with the same volumes in ports 80, 81 and 82.
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version: '3'
services:
web1: &web
image: nginx
volumes:
- ./nginx.conf:/etc/nginx/nginx.conf:ro
- ./templates:/etc/nginx/templates
port:
- 80:80
web2:
<<: *web
port:
- 81:80
web3:
<<: *web
port:
- 82:80
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We can also use the prefix x- to set generic definition outside of the services definition. It
allows us to do something like this:
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version: '3'
# basic backend definition
x-backend: &function
labels:
backend: 'true'
depends_on:
- db
networks:
- backend
# deploy mode replicated
x-replicated:
deploy:
mode: replicated
replicas: 3
# deploy mode global
x-global:
deploy:
mode: global
services:
app1:
<<: *x-backend
<<: *x-replicated
image: app1
app2:
<<: *x-backend
<<: *x-global
image: app2
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Scaling and port mapping
We can scale applications with the following commands:
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docker-compose up --scale app1=3 --scale app2=4
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However, if we want to bind ports to local ports, we need to set a range of ports instead in the
docker-compose.yml file:
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version: '3'
services:
app1:
image: app1
ports:
# app1 will start in a port from 8180 to 8199
- "8180:8199:8080"
app2:
image: app2
ports:
# app2 will start in a port from 8280 to 8299
- "8280:8299:8080"
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