Concourse is a CI system composed of simple tools and ideas. It can express entire pipelines, integrating with arbitrary resources, or it can be used to execute one-off tasks, either locally or in another CI system. So far, it has largely been developed by Cloud Foundry.
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Pro Local iteration
Debugging on remote build agents is a nightmare (especially without isolated builds). Concourse CI can be run locally. When there are problems with the pipeline definition, it can be run and debugged locally. That means it takes less time to find and fix problems.
Resources are to Concourse as plugins are to Jenkins. In other words, resources allow Concourse CI to do just about any work necessary in a build. But resources follow a "service provider interface" that makes them easy to build in any language (not just JVM languages) and have a clearly defined computing model, built for composition. Resources don't clutter UI or tax performance.
Pro Scalable, reproducible deployment
BOSH is an open source tool for release engineering, deployment, lifecycle management, and monitoring of distributed systems. Since Concourse CI is built on top of BOSH, Concourse can scale across many servers or be run in the Cloud.
Pro Isolated builds
Build isolation keeps workers "clean". There's no configuration drift of agents. Or flaky interactions between build jobs.
Visual pipeline view makes it clear what the automation does. Simple navigation to logs makes it easy to understand what happened in a build.
Concourse defines three primitives that, together, can express arbitrary features and pipelines.
Con Limited infrastructure options
The downside of building on BOSH is that a full, scalable deployment of Concourse CI requires AWS, vSphere, or OpenStack. If you don't already have these, any one of them can be a big effort to set up, just to get a build server running. Might not be a good fit for smaller teams.