GitLab CI basics - Time & Space Complexity

Time Complexity: GitLab CI basics

O(n)

Understanding Time Complexity

We want to understand how the time GitLab CI takes to run grows as the number of jobs or stages increases.

How does adding more jobs affect the total time GitLab CI spends executing pipelines?

Scenario Under Consideration

Analyze the time complexity of this simple GitLab CI pipeline configuration.

stages:
  - build
  - test

build_job:
  stage: build
  script:
    - echo "Building..."

test_job_1:
  stage: test
  script:
    - echo "Testing part 1..."

test_job_2:
  stage: test
  script:
    - echo "Testing part 2..."

This pipeline has two stages: build and test. The build stage has one job, and the test stage has two jobs running in parallel.

Identify Repeating Operations

Look for repeated actions that affect total execution time.

• Primary operation: Running each job's script commands.
• How many times: Once per job, so total jobs count.

How Execution Grows With Input

As you add more jobs, the total commands to run increase roughly by the number of jobs.

Input Size (jobs)	Approx. Operations (job scripts run)
10	10
100	100
1000	1000

Pattern observation: The total work grows directly with the number of jobs.

Final Time Complexity

Time Complexity: O(n)

This means the total time to run the pipeline grows linearly with the number of jobs if jobs run sequentially.

Common Mistake

[X] Wrong: "Adding more jobs always makes the pipeline take that many times longer."

[OK] Correct: Jobs in the same stage run in parallel, so adding jobs in one stage may not increase total time as much as expected.

Interview Connect

Understanding how pipeline time grows helps you design efficient CI workflows and explain your choices clearly in discussions.

Self-Check

"What if all jobs were in a single stage running sequentially? How would the time complexity change?"

Practice

1. What is the main purpose of the .gitlab-ci.yml file in GitLab CI?

easy

A. To create branches automatically

B. To store the source code of the project

C. To configure user permissions in GitLab

D. To define the stages and jobs for automated tasks like build and test

Solution

1. Step 1: Understand the role of .gitlab-ci.yml

   This file is used to tell GitLab CI what tasks to run automatically, such as building or testing code.

2. Step 2: Identify what the file does not do

   It does not store code, manage permissions, or create branches; those are handled elsewhere.

3. Final Answer:

   To define the stages and jobs for automated tasks like build and test -> Option D

4. Quick Check:

   .gitlab-ci.yml defines jobs and stages [OK]

Hint: Remember: .gitlab-ci.yml controls automation steps [OK]

Common Mistakes:

• Confusing .gitlab-ci.yml with source code files
• Thinking it manages user permissions
• Assuming it creates branches automatically

2. Which of the following is the correct way to define a job named test in .gitlab-ci.yml?

easy

A. job test {
run: echo "Running tests"
}

B. test:
script:
- echo "Running tests"

C. job:
name: test
command: echo "Running tests"

D. task test:
commands:
- echo "Running tests"

Solution

1. Step 1: Recall GitLab CI job syntax

   A job is defined by its name followed by a script section listing commands to run.

2. Step 2: Compare options to correct YAML format

   test: script: - echo "Running tests" uses correct YAML with job name and script list. Others use incorrect keywords or syntax.

3. Final Answer:

   test: script: - echo "Running tests" -> Option B

4. Quick Check:

   Job name + script list = correct job definition [OK]

Hint: Job name + script list is the YAML way to define jobs [OK]

Common Mistakes:

• Using curly braces or incorrect keywords like 'job' or 'task'
• Not using a list for script commands
• Incorrect indentation or missing colons

3. Given this .gitlab-ci.yml snippet:

stages:
  - build
  - test

build_job:
  stage: build
  script:
    - echo "Building"
test_job:
  stage: test
  script:
    - echo "Testing"

What will be the order of job execution?

medium

A. Both jobs run at the same time

B. test_job runs first, then build_job

C. build_job runs first, then test_job

D. Only build_job runs

Solution

1. Step 1: Understand stages order

   Stages run in the order listed: first 'build', then 'test'.

2. Step 2: Match jobs to stages

   'build_job' is in 'build' stage, so it runs before 'test_job' in 'test' stage.

3. Final Answer:

   build_job runs first, then test_job -> Option C

4. Quick Check:

   Stages run in order listed [OK]

Hint: Jobs run by stage order top to bottom [OK]

Common Mistakes:

• Assuming jobs run in alphabetical order
• Thinking jobs run in parallel by default
• Ignoring the stages list order

4. You wrote this job in .gitlab-ci.yml but it never runs:

deploy_job:
  script:
    - echo "Deploying"
  only:
    - main

What is the likely reason?

medium

A. The job is missing a stage definition

B. The branch name is incorrect; it should be 'master' instead of 'main'

C. The 'only' keyword is deprecated; use 'rules' instead

D. The script command is invalid

Solution

1. Step 1: Check the stage assignment

   The job does not specify a 'stage' key, so it defaults to the 'test' stage.

2. Step 2: Impact of missing stage in global stages

   If the root-level 'stages' does not include 'test', GitLab fails the job because the stage is undefined in the global stages list.

3. Final Answer:

   The job is missing a stage definition -> Option A

4. Quick Check:

   Default stage 'test' must be in 'stages' list [OK]

Hint: Default stage is 'test'; must be defined in 'stages' if used [OK]

Common Mistakes:

• Assuming branch name is wrong without checking
• Forgetting to define a stage (defaults to 'test' if missing)
• Thinking script command syntax is wrong

5. You want a job to run only on the develop branch and after the build stage completes successfully. Which .gitlab-ci.yml snippet correctly achieves this?

hard

A. deploy_job:
stage: deploy
script:
- echo "Deploying"
rules:
- if: '$CI_COMMIT_BRANCH == "develop"'
when: on_success
needs:
- build_job

B. deploy_job:
stage: deploy
script:
- echo "Deploying"
only:
- develop
after_script:
- build_job

C. deploy_job:
stage: deploy
script:
- echo "Deploying"
rules:
- branch: develop
when: always
dependencies:
- build_job

D. deploy_job:
stage: deploy
script:
- echo "Deploying"
except:
- main
needs:
- build

Solution

1. Step 1: Use 'rules' to run job only on 'develop' branch

   The 'rules' keyword with condition '$CI_COMMIT_BRANCH == "develop"' ensures the job runs only on that branch.

2. Step 2: Use 'needs' to wait for 'build_job'

   The 'needs' keyword makes this job wait for 'build_job' to finish successfully before running.

3. Final Answer:

   deploy_job: stage: deploy script: - echo "Deploying" rules: - if: '$CI_COMMIT_BRANCH == "develop"' when: on_success needs: - build_job -> Option A

4. Quick Check:

   'rules' + 'needs' control branch and stage order [OK]

Hint: Use 'rules' for branch and 'needs' for stage dependency [OK]

Common Mistakes:

• Using deprecated 'only' or 'except' keywords
• Confusing 'after_script' with job dependencies
• Incorrect 'needs' or 'dependencies' usage