Caching Maven dependencies in Gitlab-CI correctly - maven

Question

I have configured and working following setup
gitlab-ci, which uses docker-machine runner and uploads cache to S3
maven build with configured caching
caching correctly loads and uploads on each job
But the problem is, that every time I run mvn install, something in the local maven repository changes (I assume it updates pom metadata) and gitlab runner keeps uploading new versions of the cache, on every single build.
It is still faster and more reliable to use this "busted" cache, than to download the deps from internet every time, but the upload can take a long time and I would like to shave off this extra time.
How can I modify my build to force maven, to generate cacheable local repository?
Simplified version of my .gitlab-ci.yml:
variables:
# we have a custom java+maven image, that uses this ENV variable,
# to auto-configure path where to put the local maven repository
MAVEN_LOCAL_REPOSITORY: $CI_PROJECT_DIR/.cache/maven
job-build:
stage: build
image: internal-gitlab/java/maven:3.6-jdk8-alpine
script:
- mvn -B clean package
cache:
key: backend-dependencies
paths:
- .cache/

Answer 1

You have a constant as a cache key. Maybe a more fine grained cache would help.
See the link here

Answer 2

In short - prepare your own maven image with required dependencies and use it instead of internal-gitlab/java/maven:3.6-jdk8-alpine.
Some details:
First of all, you need to create a maven docker image where all (or most of) required for your project dependencies are presented. Publish it to your registry (gitlab has one) and use it instead of internal-gitlab/java/maven:3.6-jdk8-alpine.
To create such an image I usually create an additional job in CI triggered manually. You need to trigger it at initial stage and when project dependencies are heavily modified.
Working sample can be found here:
https://gitlab.com/alexej.vlasov/syncer/blob/master/.gitlab-ci.yml
- this project is using the prepared image and also it has a job to prepare this image.
https://gitlab.com/alexej.vlasov/maven/blob/master/Dockerfile
- dockerfile to run maven and download dependencies once.
The pros:
don't need to download dependencies each time - they are inside a
docker image (and docker layers are cached on the runners)
don't need to upload artifacts when job is finished