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We have collected several resources that you may find useful before starting to migrate.
The [Quick Start Guide](../quick_start/README.md) is a good overview of how GitLab CI/CD works. You may also be interested in [Auto DevOps](../../topics/autodevops/index.md) which can be used to build, test, and deploy your applications with little to no configuration needed at all.
For advanced CI/CD teams, [custom project templates](../../user/admin_area/custom_project_templates.md) can enable the reuse of pipeline configurations.
If you have questions that are not answered here, the [GitLab community forum](https://forum.gitlab.com/) can be a great resource.
## `config.yml` vs `gitlab-ci.yml`
CircleCI's `config.yml` configuration file defines scripts, jobs, and workflows (known as "stages" in GitLab). In GitLab, a similar approach is used with a `.gitlab-ci.yml` file in the root directory of your repository.
In CircleCI, jobs are a collection of steps to perform a specific task. In GitLab, [jobs](../pipelines/index.md#about-jobs) are also a fundamental element in the configuration file. The `checkout` keyword is not necessary in GitLab CI/CD as the repository is automatically fetched.
Example of the same job definition in GitLab CI/CD:
``` yaml
job1:
script: "execute-script-for-job1"
```
### Docker image definition
CircleCI defines images at the job level, which is also supported by GitLab CI/CD. Additionally, GitLab CI/CD supports setting this globally to be used by all jobs that don't have `image` defined.
CircleCI example image definition:
```yaml
jobs:
job1:
docker:
- image: ruby:2.6
```
Example of the same image definition in GitLab CI/CD:
```yaml
job1:
image: ruby:2.6
```
### Workflows
CircleCI determines the run order for jobs with `workflows`. This is also used to determine concurrent, sequential, scheduled, or manual runs. The equivalent function in GitLab CI/CD is called [stages](../yaml/README.md#stages). Jobs on the same stage run in parallel, and only run after previous stages complete. Execution of the next stage is skipped when a job fails by default, but this can be allowed to continue even [after a failed job](../yaml/README.md#allow_failure).
See [the Pipeline Architecture Overview](../pipelines/pipeline_architectures.md) for guidance on different types of pipelines that you can use. Pipelines can be tailored to meet your needs, such as for a large complex project or a monorepo with independent defined components.
#### Parallel and sequential job execution
The following examples show how jobs can run in parallel, or sequentially:
1.`job1` and `job2` run in parallel (in the `build` stage for GitLab CI/CD).
1.`job3` runs only after `job1` and `job2` complete successfully (in the `test` stage).
1.`job4` runs only after `job3` completes successfully (in the `deploy` stage).
CircleCI example with `workflows`:
```yaml
version: 2
jobs:
job1:
steps:
- checkout
- run: make build dependencies
job2:
steps:
- run: make build artifacts
job3:
steps:
- run: make test
job4:
steps:
- run: make deploy
workflows:
version: 2
jobs:
- job1
- job2
- job3:
requires:
- job1
- job2
- job4:
requires:
- job3
```
Example of the same workflow as `stages` in GitLab CI/CD:
GitLab CI/CD has an easy to use UI to [schedule pipelines](../pipelines/schedules.md). Also, [rules](../yaml/README.md#rules) can be used to determine if jobs should be included or excluded from a scheduled pipeline.
CircleCI example of a scheduled workflow:
```yaml
commit-workflow:
jobs:
- build
scheduled-workflow:
triggers:
- schedule:
cron: "0 1 * * *"
filters:
branches:
only: try-schedule-workflow
jobs:
- build
```
Example of the same scheduled pipeline using [`rules`](../yaml/README.md#rules) in GitLab CI/CD:
After the pipeline configuration is saved, you configure the cron schedule in the [GitLab UI](../pipelines/schedules.md#configuring-pipeline-schedules), and can enable or disable schedules in the UI as well.
#### Manual run
CircleCI example of a manual workflow:
```yaml
release-branch-workflow:
jobs:
- build
- testing:
requires:
- build
- deploy:
type: approval
requires:
- testing
```
Example of the same workflow using [`when: manual`](../yaml/README.md#whenmanual) in GitLab CI/CD:
```yaml
deploy_prod:
stage: deploy
script:
- echo "Deploy to production server"
when: manual
```
### Filter job by branch
[Rules](../yaml/README.md#rules) are a mechanism to determine if the job will or will not run for a specific branch.
CircleCI example of a job filtered by branch:
```yaml
jobs:
deploy:
branches:
only:
- master
- /rc-.*/
```
Example of the same workflow using `rules` in GitLab CI/CD:
```yaml
deploy_prod:
stage: deploy
script:
- echo "Deploy to production server"
rules:
- if: '$CI_COMMIT_BRANCH == "master"'
```
### Caching
GitLab provides a caching mechanism to speed up build times for your jobs by reusing previously downloaded dependencies. It's important to know the different between [cache and artifacts](../caching/index.md#cache-vs-artifacts) to make the best use of these features.
CircleCI example of a job using a cache:
```yaml
jobs:
job1:
steps:
- restore_cache:
key: source-v1-<.Revision>
- checkout
- run: npm install
- save_cache:
key: source-v1-<.Revision>
paths:
- "node_modules"
```
Example of the same pipeline using `cache` in GitLab CI/CD:
CircleCI provides [Contexts](https://circleci.com/docs/2.0/contexts/) to securely pass environment variables across project pipelines. In GitLab, a [Group](../../user/group/index.md) can be created to assemble related projects together. At the group level, [variables](../variables/README.md#group-level-environment-variables) can be stored outside the individual projects, and securely passed into pipelines across multiple projects.
## Orbs
There are two GitLab issues open addressing CircleCI Orbs and how GitLab can achieve similar functionality.
