Metahelm is a CLI and library for installing dependency graphs of Helm charts.
Helm dependency handling doesn't handle complex trees of dependencies well. It creates all Kubernetes objects at once and waits for them to settle into a consistent state.
With complex sets of dependency relationships, this means one or more applications go into CrashLoopBackoff, greatly lengthening the time for the entire tree to come up. Sometimes they will completely fail when one or more applications gives up entirely.
What does Metahelm do?
Metahelm takes a list of charts (as YAML) with dependency relationships specified, and optionally a way for Metahelm to tell when a chart is "up" or successful.
Metahelm will build a dependency graph (DAG) of the charts and install them in order such that all dependencies are satisfied prior to each chart install.
Say you have a primary application ("YOLO") that depends upon several microservices to be running for the primary app to start. Those microservices are called "Alpha", "Bravo" and "Charlie". "Alpha" depends upon a MySQL database, "Bravo" needs PostgreSQL and all three need Redis.
An example YAML input might look like the following:
- name: YOLO path: /home/charts/yolo values_path: /home/releases/yolo/values.yml primary_deployment: yolo dependencies: - alpha - bravo - charlie - name: alpha path: /home/charts/alpha values_path: /home/releases/alpha/values.yml primary_deployment: alpha wait_for_all_pods: true dependencies: - mysql - redis - name: bravo path: /home/charts/bravo values_path: /home/releases/bravo/values.yml primary_deployment: bravo wait_for_all_pods: true dependencies: - postgres - redis - name: charlie path: /home/charts/charlie values_path: /home/releases/charlie/values.yml primary_deployment: charlie wait_for_all_pods: true dependencies: - redis - name: mysql path: /home/charts/mysql values_path: /home/releases/mysql/values.yml primary_deployment: mysql - name: postgres path: /home/charts/postgres values_path: /home/releases/postgres/values.yml primary_deployment: postgres - name: redis path: /home/charts/redis values_path: /home/releases/redis/values.yml primary_deployment: redis
...and an execution plan:
Graph Root: YOLO Phase 1: ["mysql" "postgres" "redis"] Phase 2: ["charlie" "alpha" "bravo"] Phase 3: ["YOLO"]
Phase 1 is installed first. The Mysql, postgres and redis charts would be installed in parallel. When they are all determined to be healthy, Phase 2 ("Charlie", "Alpha", "Bravo") would be installed in a similar fashion. Finally, Phase 3 ("YOLO") would be installed.