How does Caching Work in the Docker Executor "Spin Up Environment" Step?

Note: This article discusses caching in the Docker Executor relating to the "Spin Up Environment" step for the main container in the job. It does not apply to Docker Layer Caching which is a feature of the Remote Docker environment.

The time it takes to spin up a docker container to run a job can vary based on several different factors, such as the size of the image and if some, or all, of the layers are already cached on the underlying Docker host machine.

Generally if you are using a more popular image, such as CircleCI Convenience Images, then cache hits are more likely for a larger number of layers. Most of our popular CircleCI images use the same base image so the majority of the base layers will be the same between images and you therefore have a greater chance of having a cache hit.

The environment has to spin up for every new job, regardless of whether it is in the same workflow or if it is a re-run/subsequent run - for security reasons, we never reuse containers. Once the job is finished the container is destroyed. We cannot guarantee jobs, even in the same workflow, will run on the same docker host machine and therefore the cache status may differ.

In all cases, cache hits are not guaranteed, but is a bonus convenience when available. With this in mind, a worst case scenario of a full image pull should be accounted for in all jobs.

Docker Layer Caching is only beneficial when building an image on CircleCI by using the remote docker environment - it does not affect spin up times for the actual docker containers used to run the job. More information about DLC can be found in our documentation.

In summary, the availability of caching is not something that can be controlled via settings or configuration, but by choosing a popular image, such as CircleCI Convenience Images, you will have more cache of hitting cached layers in the "Spin Up Environment" Step.

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