Docker Compose Anchors and x- Extension Fields
Compose with three x- extension fields carrying YAML anchors that are merged into services. The trap is ordering: a tool that strips x- keys before parsing the YAML destroys the anchors and the file stops resolving.
# Compose with x- extension fields and YAML anchors. Both are load-bearing here: `x-` keys are
# ignored by Compose but are ordinary YAML, and the anchors under them are merged into services
# by the YAML loader before Compose ever sees the document. A tool that strips x- keys BEFORE
# parsing YAML destroys the anchors and the file stops resolving.
x-logging: &default-logging
driver: json-file
options:
max-size: 10m
max-file: "3"
x-common: &common
restart: unless-stopped
logging: *default-logging
networks:
- backend
environment:
TZ: Etc/UTC
x-healthcheck-defaults: &healthcheck-defaults
interval: 10s
timeout: 2s
retries: 3
services:
api:
<<: *common
image: registry.example.invalid/example-org/orders:1.4.0
ports:
- "8080:8080"
healthcheck:
<<: *healthcheck-defaults
test: ["CMD", "example-healthcheck"]
worker:
<<: *common
image: registry.example.invalid/example-org/worker:1.4.0
command: ["worker", "--queue", "orders"]
healthcheck:
<<: *healthcheck-defaults
retries: 10
networks:
backend:
driver: bridge
Specifications
- Anchors
- 3
- Merge Keys
- 4
- Extension Fields
- 3
- Services
- 2
- Edge Case
- stripping x- keys before YAML parsing destroys the anchors
Testing contract
Expected to pass- Scenario
- Resolve a Compose file whose service definitions come from anchors declared under x- keys
- Expected result
- Both services inherit restart, logging and networks from the merged anchors, and worker's local retries value of 10 overrides the merged default of 3
What is a .yaml file?
YAML (YAML Ain't Markup Language) is a human-readable data-serialization format using indentation, key-value pairs, and lists, and is a superset of JSON. It supports comments, anchors, and multiple documents per file, favoring readability for configuration. Its indentation sensitivity makes it error-prone to hand-edit.
How to use this file
Use an example YAML file to test config parsers, indentation and anchor handling, multi-document streams, and safe-loading to avoid arbitrary object construction.
How to use this file for testing
“Docker Compose Anchors and x- Extension Fields” is a deterministic Novus Examples fixture for Config parsing, Encoding detection, Editor testing. TOML and INI configuration files with nested sections and typed values — for testing config parsers and loaders.
Documented properties for this file: YAML · 1,125 bytes. Compare results against paired or grouped companions on this page when present (clean↔damaged, searchable↔scanned, or format twins) so scores stay reproducible across runs.
Download the file once, keep the path stable in CI or local scripts, and treat the spec table as the contract: dimensions, seeds, field lists, and roles are intentional. Corrupt or invalid samples are labelled as such — expect parsers to fail loudly rather than silently accept them.
Pipeline and infrastructure fixtures are inert configuration: steps reference fictional images and scripts, and nothing here executes. Run your linter, schema validator, migrator, or policy engine against them, and expect the deprecated-syntax and intentionally invalid variants to be rejected.
Point your config loader at the file and assert it reads the documented sections and typed values, including any deliberately-tricky nesting or comments.
Code examples
import yaml # pip install pyyaml
with open("compose-anchors-extensions.yaml") as f:
data = yaml.safe_load(f)
print(data)Related files
- ymlGitHub Actions on: Key YAML Boolean TrapThe workflow file that exposes the YAML 1.1 boolean resolver: a bare `on` key becomes True in PyYAML, SnakeYAML and Psych, so a round-trip through those loaders loses the trigger block. The env values repeat the trap with NO, yes and off alongside a quoted control.
- ymlGitLab CI !reference Custom YAML TagGitLab's `!reference` custom YAML tag used three ways: inside a script list, as a whole after_script value, and as a whole rules value. A stock YAML 1.2 loader has no constructor for it and raises; GitLab's loader resolves each key path.
- ymlGitLab CI Anchors and ExtendsThe two GitLab inheritance mechanisms in one file: YAML anchors with merge keys, which the loader resolves before GitLab sees the document, and `extends`, which GitLab resolves afterwards with a deep merge. Hidden .job keys carry both.
- yamlYAML Anchors and AliasesAnchors and aliases across mapping, sequence and scalar nodes, including an anchor nested inside another anchored node. The point most tooling misses: an alias is a reference to the same node, so a shallow-loading parser can share mutable state between jobs.
- yamlYAML Block Scalars and Chomping IndicatorsEvery block-scalar variation in one file: literal and folded styles, all three chomping indicators, an explicit indentation indicator, a folded block with a paragraph break, and an embedded shell-shaped script whose own indentation must survive.
- yamlYAML Bounded Anchor ReuseNested anchor reuse kept deliberately small: two anchors, six references, thirty-one expanded nodes. It is here to test alias resolution and expansion accounting on a safe input, and is explicitly not a billion-laughs expansion bomb.
Generated by generation/pipelines.py. Free for any use, no attribution required — license.