Fragments
Anchors are created using the &
sign. The sign is followed by an alias name. You can use this alias with the *
sign later to reference the value following the anchor. Make sure there is no space between the &
and the *
characters and the following alias name.
You can use more than one anchor and alias in a single Compose file.
Example 1
volumes:
db-data: &default-volume
driver: default
metrics: *default-volume
In the example above, a default-volume
anchor is created based on the db-data
volume. It is later reused by the alias *default-volume
to define the metrics
volume.
Anchor resolution takes place before variables interpolation, so variables can't be used to set anchors or aliases.
Example 2
services:
first:
image: my-image:latest
environment: &env
- CONFIG_KEY
- EXAMPLE_KEY
- DEMO_VAR
second:
image: another-image:latest
environment: *env
If you have an anchor that you want to use in more than one service, use it in conjunction with an extension to make your Compose file easier to maintain.
Example 3
You may want to partially override values. Compose follows the rule outlined by YAML merge type.
In the following example, metrics
volume specification uses alias
to avoid repetition but overrides name
attribute:
services:
backend:
image: example/database
volumes:
- db-data
- metrics
volumes:
db-data: &default-volume
driver: default
name: "data"
metrics:
<<: *default-volume
name: "metrics"
Example 4
You can also extend the anchor to add additional values.
services:
first:
image: my-image:latest
environment: &env
FOO: BAR
ZOT: QUIX
second:
image: another-image:latest
environment:
<<: *env
YET_ANOTHER: VARIABLE
Note
YAML merge only applies to mappings, and can't be used with sequences.
In example above, the environment variables must be declared using the FOO: BAR
mapping syntax, while the sequence syntax - FOO=BAR
is only valid when no fragments are involved.