What is a Parallel Job in CI? What is the sequential execution called?

Parallel Job in CI is the execution of multiple tasks or stages simultaneously, which speeds up the overall build or testing process. Sequential Job, on the other hand, refers to tasks running one after another.

For example, in GitLab CI, you can define multiple jobs that will run in parallel if they do not depend on each other:

job1:
  script: echo "Job 1"

job2:
  script: echo "Job 2"

# job1 and job2 can run in parallel

job3:
  script: echo "Job 3"
  needs: [job1, job2]  # job3 will run after job1 and job2

Here, job1 and job2 are executed in parallel, and job3 runs sequentially after them.