Concourse

Concourse is an open-source continuous "thing-doer". It represents a general approach for automation which can be used for Continuous Integration and Continuous Delivery. Concourse CI follows a strict concept that is based on idempotency, immutability, declarative config, stateless workers, and reproducible builds. Pipelines are built on the mechanics of resources, tasks and jobs, which are all configured in one or multiple YAML files.

Concourse claims to be "simple" but has a steep learning curve in the beginning till it gets simple to use. Concourse is used in the Congstar Team to automate infrastructure deployments.