Pipeline Runners
Pipelines are just a piece of code waiting to be executed. Pipeline runners (lambdacd.runners) provide the logic when to start a new run of the pipeline.
They run in the background to make sure you always have an instance ready to go when you need it.
As the name suggests, this runner only allows one pipeline to run at a time, starting the next run only after a pipeline completed.
Use this if you are concerned about more than one pipeline running at the same time, maybe because they might conflict with each other or simply to control resource utilization.
This runner assumes that the first step of your pipeline is a trigger-step that waits for some event that will trigger the pipeline. It'll make sure you always have one such trigger step waiting by starting a new pipeline-run when the first step of your pipeline (the trigger) completed (i.e. stopped waiting because of an event).
Use this if you want to start new builds immediately upon an event and you can allow pipelines to run in parallel.