gitlab-org--gitlab-foss/doc/ci/yaml/yaml_optimization.md

---
stage: Verify
group: Pipeline Authoring
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type: reference
---

# Optimize GitLab CI/CD configuration files **(FREE)**

You can reduce complexity and duplicated configuration in your GitLab CI/CD configuration
files by using:

- YAML-specific features like [anchors (`&`)](#anchors), aliases (`*`), and map merging (`<<`).
  Read more about the various [YAML features](https://learnxinyminutes.com/docs/yaml/).
- The [`extends` keyword](#use-extends-to-reuse-configuration-sections),
  which is more flexible and readable. We recommend you use `extends` where possible.

## Anchors

YAML has a feature called 'anchors' that you can use to duplicate
content across your document.

Use anchors to duplicate or inherit properties. Use anchors with [hidden jobs](../jobs/index.md#hide-jobs)
to provide templates for your jobs. When there are duplicate keys, GitLab
performs a reverse deep merge based on the keys.

You can use YAML anchors to merge YAML arrays.

You can't use YAML anchors across multiple files when using the [`include`](index.md#include)
keyword. Anchors are only valid in the file they were defined in. To reuse configuration
from different YAML files, use [`!reference` tags](#reference-tags) or the
[`extends` keyword](#use-extends-to-reuse-configuration-sections).

The following example uses anchors and map merging. It creates two jobs,
`test1` and `test2`, that inherit the `.job_template` configuration, each
with their own custom `script` defined:

```yaml
.job_template: &job_configuration  # Hidden yaml configuration that defines an anchor named 'job_configuration'
  image: ruby:2.6
  services:
    - postgres
    - redis

test1:
  <<: *job_configuration           # Merge the contents of the 'job_configuration' alias
  script:
    - test1 project

test2:
  <<: *job_configuration           # Merge the contents of the 'job_configuration' alias
  script:
    - test2 project
```

`&` sets up the name of the anchor (`job_configuration`), `<<` means "merge the
given hash into the current one," and `*` includes the named anchor
(`job_configuration` again). The expanded version of this example is:

```yaml
.job_template:
  image: ruby:2.6
  services:
    - postgres
    - redis

test1:
  image: ruby:2.6
  services:
    - postgres
    - redis
  script:
    - test1 project

test2:
  image: ruby:2.6
  services:
    - postgres
    - redis
  script:
    - test2 project
```

You can use anchors to define two sets of services. For example, `test:postgres`
and `test:mysql` share the `script` defined in `.job_template`, but use different
`services`, defined in `.postgres_services` and `.mysql_services`:

```yaml
.job_template: &job_configuration
  script:
    - test project
  tags:
    - dev

.postgres_services:
  services: &postgres_configuration
    - postgres
    - ruby

.mysql_services:
  services: &mysql_configuration
    - mysql
    - ruby

test:postgres:
  <<: *job_configuration
  services: *postgres_configuration
  tags:
    - postgres

test:mysql:
  <<: *job_configuration
  services: *mysql_configuration
```

The expanded version is:

```yaml
.job_template:
  script:
    - test project
  tags:
    - dev

.postgres_services:
  services:
    - postgres
    - ruby

.mysql_services:
  services:
    - mysql
    - ruby

test:postgres:
  script:
    - test project
  services:
    - postgres
    - ruby
  tags:
    - postgres

test:mysql:
  script:
    - test project
  services:
    - mysql
    - ruby
  tags:
    - dev
```

You can see that the hidden jobs are conveniently used as templates, and
`tags: [postgres]` overwrites `tags: [dev]`.

### YAML anchors for scripts

> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/23005) in GitLab 12.5.

You can use [YAML anchors](#anchors) with [script](index.md#script), [`before_script`](index.md#before_script),
and [`after_script`](index.md#after_script) to use predefined commands in multiple jobs:

```yaml
.some-script-before: &some-script-before
  - echo "Execute this script first"

.some-script: &some-script
  - echo "Execute this script second"
  - echo "Execute this script too"

.some-script-after: &some-script-after
  - echo "Execute this script last"

job1:
  before_script:
    - *some-script-before
  script:
    - *some-script
    - echo "Execute something, for this job only"
  after_script:
    - *some-script-after

job2:
  script:
    - *some-script-before
    - *some-script
    - echo "Execute something else, for this job only"
    - *some-script-after
```

### YAML anchors for variables

Use [YAML anchors](#anchors) with `variables` to repeat assignment
of variables across multiple jobs. You can also use YAML anchors when a job
requires a specific `variables` block that would otherwise override the global variables.

The following example shows how override the `GIT_STRATEGY` variable without affecting
the use of the `SAMPLE_VARIABLE` variable:

```yaml
# global variables
variables: &global-variables
  SAMPLE_VARIABLE: sample_variable_value
  ANOTHER_SAMPLE_VARIABLE: another_sample_variable_value

# a job that must set the GIT_STRATEGY variable, yet depend on global variables
job_no_git_strategy:
  stage: cleanup
  variables:
    <<: *global-variables
    GIT_STRATEGY: none
  script: echo $SAMPLE_VARIABLE
```

## Use `extends` to reuse configuration sections

You can use the [`extends` keyword](index.md#extends) to reuse configuration in
multiple jobs. It is similar to [YAML anchors](#anchors), but simpler and you can
[use `extends` with `includes`](#use-extends-and-include-together).

`extends` supports multi-level inheritance. You should avoid using more than three levels,
but you can use as many as eleven. The following example has two levels of inheritance:

```yaml
.tests:
  rules:
    - if: $CI_PIPELINE_SOURCE == "push"

.rspec:
  extends: .tests
  script: rake rspec

rspec 1:
  variables:
    RSPEC_SUITE: '1'
  extends: .rspec

rspec 2:
  variables:
    RSPEC_SUITE: '2'
  extends: .rspec

spinach:
  extends: .tests
  script: rake spinach
```

### Exclude a key from `extends`

To exclude a key from the extended content, you must assign it to `null`, for example:

```yaml
.base:
  script: test
  variables:
    VAR1: base var 1

test1:
  extends: .base
  variables:
    VAR1: test1 var 1
    VAR2: test2 var 2

test2:
  extends: .base
  variables:
    VAR2: test2 var 2

test3:
  extends: .base
  variables: {}

test4:
  extends: .base
  variables: null
```

Merged configuration:

```yaml
test1:
  script: test
  variables:
    VAR1: test1 var 1
    VAR2: test2 var 2

test2:
  script: test
  variables:
    VAR1: base var 1
    VAR2: test2 var 2

test3:
  script: test
  variables:
    VAR1: base var 1

test4:
  script: test
  variables: null
```

### Use `extends` and `include` together

To reuse configuration from different configuration files,
combine `extends` and [`include`](index.md#include).

In the following example, a `script` is defined in the `included.yml` file.
Then, in the `.gitlab-ci.yml` file, `extends` refers
to the contents of the `script`:

- `included.yml`:

  ```yaml
  .template:
    script:
      - echo Hello!
  ```

- `.gitlab-ci.yml`:

  ```yaml
  include: included.yml

  useTemplate:
    image: alpine
    extends: .template
  ```

### Merge details

You can use `extends` to merge hashes but not arrays.
The algorithm used for merge is "closest scope wins," so
keys from the last member always override anything defined on other
levels. For example:

```yaml
.only-important:
  variables:
    URL: "http://my-url.internal"
    IMPORTANT_VAR: "the details"
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
    - if: $CI_COMMIT_BRANCH == "stable"
  tags:
    - production
  script:
    - echo "Hello world!"

.in-docker:
  variables:
    URL: "http://docker-url.internal"
  tags:
    - docker
  image: alpine

rspec:
  variables:
    GITLAB: "is-awesome"
  extends:
    - .only-important
    - .in-docker
  script:
    - rake rspec
```

The result is this `rspec` job:

```yaml
rspec:
  variables:
    URL: "http://docker-url.internal"
    IMPORTANT_VAR: "the details"
    GITLAB: "is-awesome"
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
    - if: $CI_COMMIT_BRANCH == "stable"
  tags:
    - docker
  image: alpine
  script:
    - rake rspec
```

In this example:

- The `variables` sections merge, but `URL: "http://docker-url.internal"` overwrites `URL: "http://my-url.internal"`.
- `tags: ['docker']` overwrites `tags: ['production']`.
- `script` does not merge, but `script: ['rake rspec']` overwrites
  `script: ['echo "Hello world!"']`. You can use [YAML anchors](yaml_optimization.md#anchors) to merge arrays.

## `!reference` tags

> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/266173) in GitLab 13.9.
> - `rules` keyword support [introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/322992) in GitLab 14.3.

Use the `!reference` custom YAML tag to select keyword configuration from other job
sections and reuse it in the current section. Unlike [YAML anchors](#anchors), you can
use `!reference` tags to reuse configuration from [included](index.md#include) configuration
files as well.

In the following example, a `script` and an `after_script` from two different locations are
reused in the `test` job:

- `setup.yml`:

  ```yaml
  .setup:
    script:
      - echo creating environment
  ```

- `.gitlab-ci.yml`:

  ```yaml
  include:
    - local: setup.yml

  .teardown:
    after_script:
      - echo deleting environment

  test:
    script:
      - !reference [.setup, script]
      - echo running my own command
    after_script:
      - !reference [.teardown, after_script]
  ```

In the following example, `test-vars-1` reuses all the variables in `.vars`, while `test-vars-2`
selects a specific variable and reuses it as a new `MY_VAR` variable.

```yaml
.vars:
  variables:
    URL: "http://my-url.internal"
    IMPORTANT_VAR: "the details"

test-vars-1:
  variables: !reference [.vars, variables]
  script:
    - printenv

test-vars-2:
  variables:
    MY_VAR: !reference [.vars, variables, IMPORTANT_VAR]
  script:
    - printenv
```

You can't reuse a section that already includes a `!reference` tag. Only one level
of nesting is supported.