01-Introduction
Concourse is a pipeline-based continuous thing-doer.
The word "pipeline" is all the rage in CI these days, so being more specific about this term is kind of important; Concourse's pipelines are significantly different from the rest.
Pipelines are built around Resources, which represent all external state, and Jobs, which interact with them. Concourse pipelines represent a dependency flow, kind of like distributed Makefiles. Pipelines are designed to be self-contained so as to minimize server-wide configuration. Maximizing portability also mitigates risk, making it easier for projects to recover from CI disasters.
Resources like the git resource and s3 resource are used to express source code, dependencies, deployments, and any other external state. This interface is also used to model more abstract things like scheduled or interval triggers, via the time resource.
Resource Types are defined as part of the pipeline itself, making the pipelines more self-contained and keeping Concourse itself small and generic without resorting to a complicated plugin system.
Jobs are sequences of get, put, and task steps to execute. These steps determine the job's inputs and outputs. Jobs are designed to be idempotent and loosely coupled, allowing the pipeline to grow with the project's needs without requiring engineers to keep too much in their head at a time.
Everything in Concourse runs in a container. Instead of modifying workers to install build tools, Tasks describe their own container image (typically using Docker images via the registry-image resource).
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