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DevOps / GitLab CI Basics Interview Questions

What is a merge request pipeline in GitLab CI and how does it differ from a branch pipeline?

GitLab supports two types of pipelines related to merge requests: branch pipelines (triggered by a push to a branch) and merge request pipelines (triggered specifically by merge request events). Understanding the difference is essential for designing efficient review workflows.

Branch pipeline vs MR pipeline
AspectBranch PipelineMerge Request Pipeline
TriggerPush to a branchMR created, updated, or rebased
Source variablepushmerge_request_event
Runs onThe feature branch's HEADMerge result (branch + target merged)
MR status checksNot shown in MR (unless configured)Shown in MR as status checks
MR variables$CI_MERGE_REQUEST_* not set$CI_MERGE_REQUEST_* fully populated
Merged resultTests the branch as-isTests what the code will look like after merge
# Jobs that run only in MR pipelines:
code-review:
  script:
    - run-code-quality.sh
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"

# Useful MR-specific variables:
check-mr:
  script:
    - echo "MR ID: $CI_MERGE_REQUEST_ID"
    - echo "Target branch: $CI_MERGE_REQUEST_TARGET_BRANCH_NAME"
    - echo "Source branch: $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME"
    - echo "MR title: $CI_MERGE_REQUEST_TITLE"
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"

# Merged results pipeline (tests the post-merge state):
# Enable in Settings > Merge requests > Merged results pipelines
# Runs on the merge commit that would result from accepting the MR

Merged results pipelines (enabled in project settings) run the CI on the simulated merge result - catching conflicts between the MR branch and the target branch that would only appear after merging. This is safer than testing just the source branch.

What predefined variable is populated in a merge request pipeline but NOT in a branch pipeline?
What advantage does a 'merged results pipeline' have over a regular branch pipeline?

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