GitLab CI comparison in Jenkins - Step-by-Step Execution

Process Flow - GitLab CI comparison

Write Jenkinsfile

↓

Commit to Repo

↓

Jenkins Polls or Webhook Trigger

↓

Jenkins Master Schedules Job

↓

Agent Executes Pipeline Steps

↓

Job Status Reported

↓

Developer Sees Results

↓

Write .gitlab-ci.yml

↓

Commit to Repo

↓

GitLab Detects Commit

↓

GitLab Runner Executes Jobs

↓

Job Status Reported

↓

Developer Sees Results

Shows the step-by-step flow of how Jenkins and GitLab CI pipelines are triggered and executed after code commits.

Execution Sample

Jenkins

pipeline {
  agent any
  stages {
    stage('Build') {
      steps { echo 'Building...' }
    }
  }
}

A simple Jenkins pipeline that runs a build stage printing 'Building...'

Process Table

Step	Action	Trigger	Result	Notes
1	Write Jenkinsfile	Manual	Jenkinsfile created in repo	Defines pipeline steps
2	Commit Jenkinsfile	Developer commit	Code pushed to repo	Triggers Jenkins polling or webhook
3	Jenkins detects commit	Polling/Webhook	Schedules pipeline job	Master assigns job to agent
4	Agent runs 'Build' stage	Job start	Prints 'Building...'	Executes pipeline steps
5	Job completes	Job end	Status reported to Jenkins UI	Success or failure shown
6	Write .gitlab-ci.yml	Manual	.gitlab-ci.yml created	Defines GitLab CI jobs
7	Commit .gitlab-ci.yml	Developer commit	Code pushed to GitLab repo	Triggers GitLab pipeline
8	GitLab detects commit	Push event	Pipeline scheduled	GitLab Runner picks job
9	Runner executes job	Job start	Job runs defined steps	Outputs logs in GitLab UI
10	Job completes	Job end	Status shown in GitLab UI	Success or failure visible
11	Exit	No more steps	Pipeline finished	End of process

💡 Pipeline finishes after all stages/jobs complete and status is reported

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	After Step 7	After Step 9	Final
Jenkinsfile	None	Created	Exists	Exists	Exists	Exists	Exists
.gitlab-ci.yml	None	None	None	None	Created	Exists	Exists
Pipeline Status	None	Pending	Running	Success	Pending	Running	Success

Key Moments - 3 Insights

Why does Jenkins need a separate agent to run jobs?

Jenkins master schedules jobs but agents do the actual work to keep master free for coordination, as shown in execution_table step 3 and 4.

How does GitLab CI detect when to start a pipeline?

GitLab CI automatically detects commits via push events to the repo, triggering pipelines without extra setup, as seen in execution_table step 8.

Can Jenkins and GitLab CI pipelines run on the same repository?

Yes, both can run independently if configured, but they use different config files and runners/agents, shown by separate steps for Jenkinsfile and .gitlab-ci.yml.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Jenkins agent start running the build stage?

A Step 4

B Step 5

C Step 3

D Step 2

💡 Hint

Check the 'Action' and 'Result' columns for when the build stage runs.

According to the variable tracker, what is the pipeline status after step 5?

A Pending

B Running

C Success

D None

💡 Hint

Look at the 'Pipeline Status' row under 'After Step 5' column.

If the .gitlab-ci.yml file is not committed, what happens to the GitLab pipeline?

A Pipeline runs with default settings

B Pipeline is not triggered

C Pipeline runs but fails immediately

D Pipeline triggers Jenkins instead

💡 Hint

Refer to execution_table steps 6 and 7 about .gitlab-ci.yml creation and commit.

Concept Snapshot

GitLab CI and Jenkins both automate code testing and deployment.
Jenkins uses a Jenkinsfile and agents to run pipelines.
GitLab CI uses .gitlab-ci.yml and runners triggered by repo pushes.
Both report job status to developers.
Jenkins requires separate agents; GitLab CI integrates runners.
Choose based on your project needs and environment.

Full Transcript

This visual execution compares Jenkins and GitLab CI pipelines. Jenkins requires writing a Jenkinsfile, committing it, then Jenkins detects the commit via polling or webhook. The master schedules the job, and an agent runs the pipeline steps like 'Build'. The job status is reported back to Jenkins UI. GitLab CI uses a .gitlab-ci.yml file committed to the repo. GitLab detects the commit automatically and schedules the pipeline. GitLab runners execute the jobs and show status in GitLab UI. Variables like pipeline status change from pending to running to success as jobs progress. Key points include Jenkins needing agents to run jobs, GitLab CI triggering pipelines automatically on push, and both systems being able to run on the same repo independently. The quiz checks understanding of when Jenkins agents run, pipeline status changes, and GitLab pipeline triggers. This helps beginners see the step-by-step flow and differences between Jenkins and GitLab CI.