As the development and testing of builds continues to grow not only in complexity but also in scale, the need for improved, efficient management of approvals and merges has evolved as well. This is where the concepts and practices of CI/CD tools have shown their value, and, as a result, have given way to the development of CI tools such as Zuul.
Zuul was originally developed back in 2012 for CI testing, enabling users and contributors, spanning numerous diverse organizations, to efficiently and effectively develop together across multiple projects. Essentially, the problem that Zuul was designed to solve is that of providing developers with a tool to test serialized future states faster than was possible when executing the tests serially. The concept of pipeline being borrowed from processor design provided a sort of “optimistic speculation” by which one could effectively parallelize this testing in cases where jobs were likely to pass. It later became a tool for the automatic merging, building and testing of any new changes being made to a project, delivering an automated gated workflow pipeline. It is through this gating that new code is prevented from making it to production without passing project keeper defined unit and integration tests.
You can read additional information on Zuul here: https://vexxhost.com/blog/getting-to-know-zuul-openstacks-ci-tool/