source-to-image (s2i)

Source-to-image (s2i) is a tool for building reproducible Docker images. s2i produces ready-to-run images by injecting source code into a Docker image and assembling a new Docker image which incorporates the builder image and built source. The result is then ready to use with docker run. s2i supports incremental builds which re-use previously downloaded dependencies, previously built artifacts, etc.

Interested in learning more? Read on!

Want to just get started now? Check out the instructions.

Philosophy

Simplify the process of application source + builder image -> usable image for most use cases (the 80%)
Define and implement a workflow for incremental builds that eventually uses only Docker primitives
Develop tooling that can assist in verifying that two different builder images result in the same docker run outcome for the same input
Use native Docker primitives to accomplish this - map out useful improvements to Docker that benefit all image builders

Anatomy of a builder image

Creating builder images is easy. s2i looks for you to supply the following scripts to use with an image:

assemble - builds and/or deploys the source
run- runs the assembled artifacts
save-artifacts (optional) - captures the artifacts from a previous build into the next incremental build
usage (optional) - displays builder image usage information

Additionally for the best user experience and optimized s2i operation we suggest images to have /bin/sh and tar commands available.

See a practical tutorial on how to create a builder image here and read this for a detailed description of the requirements and scripts along with examples of builder images.

Build workflow

The s2i build workflow is:

s2i creates a container based on the build image and passes it a tar file that contains:
1. The application source in src, excluding any files selected by .s2iignore
2. The build artifacts in artifacts (if applicable - see incremental builds)
s2i sets the environment variables from .s2i/environment (optional)
s2i starts the container and runs its assemble script
s2i waits for the container to finish
s2i commits the container, setting the CMD for the output image to be the run script and tagging the image with the name provided.

Filtering the contents of the source tree is possible if the user supplies a .s2iignore file in the root directory of the source repository, where .s2iignore contains regular expressions that capture the set of files and directories you want filtered from the image s2i produces.

Specifically:

Specify one rule per line, with each line terminating in \n.
Filepaths are appended to the absolute path of the root of the source tree (either the local directory supplied, or the target destination of the clone of the remote source repository s2i creates).
Wildcards and globbing (file name expansion) leverage Go's filepath.Match and filepath.Glob functions.
Search is not recursive. Subdirectory paths must be specified (though wildcards and regular expressions can be used in the subdirectory specifications).
If the first character is the # character, the line is treated as a comment.
If the first character is the !, the rule is an exception rule, and can undo candidates selected for filtering by prior rules (but only prior rules).

Here are some examples to help illustrate:

With specifying subdirectories, the */temp* rule prevents the filtering of any files starting with temp that are in any subdirectory that is immediately (or one level) below the root directory. And the */*/temp* rule prevents the filtering of any files starting with temp that are in any subdirectory that is two levels below the root directory.

Next, to illustrate exception rules, first consider the following example snippet of a .s2iignore file:

*.md
!README.md

With this exception rule example, README.md will not be filtered, and remain in the image s2i produces. However, with this snippet:

!README.md
*.md

README.md, if filtered by any prior rules, but then put back in by !README.md, would be filtered, and not part of the resulting image s2i produces. Since *.md follows !README.md, *.md takes precedence.

Users can also set extra environment variables in the application source code. They are passed to the build, and the assemble script consumes them. All environment variables are also present in the output application image. These variables are defined in the .s2i/environment file inside the application sources. The format of this file is a simple key-value, for example:

FOO=bar

In this case, the value of FOO environment variable will be set to bar.

Using ONBUILD images

In case you want to use one of the official Docker language stack images for your build you don't have do anything extra. S2I is capable of recognizing the Docker image with ONBUILD instructions and choosing the OnBuild strategy. This strategy will trigger all ONBUILD instructions and execute the assemble script (if it exists) as the last instruction.

Since the ONBUILD images usually don't provide any entrypoint, in order to use this build strategy you will have to provide one. You can either include the 'run', 'start' or 'execute' script in your application source root folder or you can specify a valid S2I script URL and the 'run' script will be fetched and set as an entrypoint in that case.

Incremental builds

s2i automatically detects:

Whether a builder image is compatible with incremental building
Whether a previous image exists, with the same name as the output name for this build

If a save-artifacts script exists, a prior image already exists, and the --incremental=true option is used, the workflow is as follows:

s2i creates a new Docker container from the prior build image
s2i runs save-artifacts in this container - this script is responsible for streaming out a tar of the artifacts to stdout
s2i builds the new output image:
1. The artifacts from the previous build will be in the artifacts directory of the tar passed to the build
2. The build image's assemble script is responsible for detecting and using the build artifacts

NOTE: The save-artifacts script is responsible for streaming out dependencies in a tar file.

Dependencies

Docker >= 1.6
Go >= 1.4
(optional) Git

Installation

Assuming Go and Docker are installed and configured, execute the following commands:

$ go get github.com/openshift/source-to-image
$ cd ${GOPATH}/src/github.com/openshift/source-to-image
$ export PATH=$PATH:${GOPATH}/src/github.com/openshift/source-to-image/_output/local/bin/linux/amd64/
$ hack/build-go.sh

Security

Since the s2i command uses the Docker client library, it has to run in the same security context as the docker command. For some systems, it is enough to add yourself into the 'docker' group to be able to work with Docker as 'non-root'. In the latest versions of Fedora/RHEL, it is recommended to use the sudo command as this way is more auditable and secure.

If you are using the sudo docker command already, then you will have to also use sudo s2i to give S2I permission to work with Docker directly.

Be aware that being a member of the 'docker' group effectively grants root access, as described here.

Getting Started

You can start using s2i right away (see releases) with the following test sources and publicly available images:

$ s2i build git://github.com/pmorie/simple-ruby openshift/ruby-20-centos7 test-ruby-app
$ docker run --rm -i -p :8080 -t test-ruby-app

$ s2i build git://github.com/bparees/openshift-jee-sample openshift/wildfly-100-centos7 test-jee-app
$ docker run --rm -i -p :8080 -t test-jee-app

Interested in more advanced s2i usage? See here for detailed descriptions of the different CLI commands with examples.

Running into some issues and need some advice debugging? Peruse here for some tips.

mnagy/source-to-image