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S2I Workflow and Logic

Source-to-Image (S2I) is an automation tool for building images from source code. S2I injects source code into an Image Builder for compiling and then automatically packages the compiled code into a Docker image.

For more information about how to use S2I in KubeSphere, refer to Source to Image: Publish an App without a Dockerfile. Besides, you can refer to the code repositories S2IOperator and S2IRun for more details.

S2I Workflow and Logic

Image Builder

For interpreted languages like Python and Ruby, the build-time and runtime environments for an application are typically the same. For example, a Ruby-based Image Builder usually contains Bundler, Rake, Apache, GCC, and other packages needed to set up a runtime environment. The following diagram describes the build workflow.


How S2I works

S2I performs the following steps:

  1. Start a container from the Image Builder with the application source code injected into a specified directory.
  2. Execute the assemble script from the Image Builder to build that source code into a ready-to-run application by installing dependencies and moving the source code into a working directory.
  3. Set the run script provided by the Image Builder as the image entrypoint for starting the container, and then commit a new image as the application image to meet user needs.

See the S2I workflow chart as below.


Runtime Image

For compiled languages like Go, C, C++, or Java, the dependencies necessary for compiling will increase the size of resulting images. To build slimmer images, S2I uses a phased build workflow with unnecessary files removed from images. An artifact, which is an executable like a Jar file or binary file, will be extracted when building finishes in the Image Builder, and then injected into a Runtime Image for execution.

See the building workflow as below.