Gitlab run pipeline job only when previous job ran - gitlab-ci

Question

I'm trying to create a pipeline with a production and a development deployment. In both environments the application should be built with docker. But only when something changed in the according directory.
For example:
When something changed in the frontend directory the frontend should be build and deployed
When something changed in the backend directory the backend should be build and deployed
At first I didn't had the needs: keyword. The pipeline always executed the deploy_backend and deploy_frontend even when the build jobs were not executed.
Now I've added the needs: keyword, but Gitlab says yaml invalid when there was only a change in one directory. When there is a change in both directories the pipeline works fine. When there for exaple a change in the README.md outside the 2 directories the says yaml invalid as well.
Does anyone knows how I can create a pipeline that only runs when there is a change in a specified directory and only runs the according deploy job when the build job has ran?
gitlab-ci.yml:
stages:
- build
- deploy
build_frontend:
stage: build
only:
refs:
- master
- development
changes:
- frontend/*
script:
- cd frontend
- docker build -t frontend .
build_backend:
stage: build
only:
refs:
- master
- development
changes:
- backend/*
script:
- cd backend
- docker build -t backend .
deploy_frontend_dev:
stage: deploy
only:
refs:
- development
script:
- "echo deploy frontend"
needs: ["build_frontend"]
deploy_backend_dev:
stage: deploy
only:
refs:
- development
- pipeline
script:
- "echo deploy backend"
needs: ["build_backend"]

Answer 1

The problem here is that your deploy jobs require the previous build jobs to actually exist.
However, by using the only.changes-rule, they only exist if actually something changed within those directories.
So when only something in the frontend-folder changed, the build_backend-Job is not generated at all. But the deploy_backend_dev job still is and then misses it's dependency.
A quick fix would be to add the only.changes configuration also to the deployment-jobs like this:
deploy_frontend_dev:
stage: deploy
only:
refs:
- development
changes:
- frontend/*
script:
- "echo deploy frontend"
needs: ["build_frontend"]
deploy_backend_dev:
stage: deploy
only:
refs:
- development
- pipeline
changes:
- backend/*
script:
- "echo deploy backend"
needs: ["build_backend"]
This way, both jobs will only be created if the dependent build job is created as well and the yaml will not be invalid.