Docker layer caching overview - CircleCI (original) (raw)

DLC caches your Docker image layers within the container/virtual machine used to run your job.

If, for example, the first run of your job takes over two minutes to build a Docker image, and nothing changes in the Dockerfile before the second run, the Dockerfile build steps happen instantly, in zero seconds.

When none of the layers in the image change between job runs, DLC pulls the layers from the cache and reuses those instead of rebuilding the entire image.

If part of the Dockerfile changes (which changes part of the image), a subsequent run of the exact same job with the modified Dockerfile may still finish faster than rebuilding the entire image. This speed optimization happens because the cache can still be used for the first few steps that did not change in the Dockerfile. The steps that follow the change must be rerun because the Dockerfile change invalidates the cache for those layers.

If you change something in your Dockerfile, all of the later steps (from the point that the change was made) are invalidated and the layers have to be rebuilt. When some of the steps remain the same (the steps before the one you removed), those steps can be reused, therefore, it is still faster than rebuilding the entire image.

At the beginning of each job that uses DLC, there is a DLC set-up step. You are not charged for this step. At the end of each job that uses DLC, the cache upload is done asynchronously, and does not prevent the workflow from continuing to progress. This means that jobs within the same workflow are unlikely to access a cache uploaded from an upstream job. You are not charged for this DLC teardown step.

Parallelism and DLC

DLC operates in the same way for jobs that use parallelism. If a machine job using DLC is configured with parallelism: 2, two jobs run in parallel. Each virtual machine in this case will have a separate DLC cache, and whichever is saved last will be used for the next build.