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s2i builder image requirements

The main advantage of using s2i for building reproducible docker images is ease of use for developers. To meet that criteria you, as a builder image author, should be aware of the two basic requirements for the best possible s2i performance. These are:

Required image contents

The build process consists of three fundamental elements which are combined into the final docker image. These are: source code, s2i scripts, and the builder image. During the build process s2i must place sources and scripts inside that builder image. To do so s2i creates a tar file containing the two and then streams that file into the builder image. Before executing the assemble script, s2i untars that file and places its contents into the destination specified with either the --destination flag or the value of the io.openshift.s2i.destination label set in the builder image (the default destination is /tmp). If your image does not have either tar or /bin/sh the s2i build will perform an additional docker build to place the source code and scripts into an appropriate image and then run the normal s2i build.

The following diagram illustrates the build workflow:

s2i workflow

* Run build's responsibility is to untar the sources, scripts and (optionally) artifacts and invoke the assemble script. If this is the second run after any previous runs with tar//bin/sh errors, it will only run the assemble script, since both the source and scripts are already present.

s2i scripts

s2i expects you (the builder image author) to supply the following scripts:

All of the scripts can be written in any programming language, as long as the scripts are executable inside the builder image. The build searches the following locations for these scripts in the following order:

  1. A script found at the --scripts-url URL
  2. A script found in the application source .s2i/bin directory
  3. A script found at the default image URL (io.openshift.s2i.scripts-url label)

Both the io.openshift.s2i.scripts-url label specified in the image and --scripts-url flag can be supplied in any of the following forms to indicate where the scripts are located:

  • image://path_to_scripts_dir - absolute path inside the image
  • file://path_to_scripts_dir - relative or absolute path on the host machine
  • http(s)://path_to_scripts_dir - URL to a directory

NOTE: In the case where the scripts are already placed inside the image (ie when using --scripts-url flag or the io.openshift.s2i.scripts-url with the format image:///path/in/image), then the --destination flag or the io.openshift.s2i.destination label applies only to sources and artifacts.


The assemble script is responsible for building the application artifacts from source and placing them into the appropriate directories inside the image. The workflow for the assemble script is:

  1. Restore build artifacts (in case you want to support incremental builds (if using this, make sure you define save-artifacts) as well.
  2. Place the application source code in the appropriate location.
  3. Build any application artifacts.
  4. Install the artifacts into locations appropriate for running.

In the case you need to assemble the Image using a different user than the runtime user defined in USER directive of Dockerfile, you can achive this by the following ways:

  1. use the --assemble-user in cmd line
  2. use the label io.openshift.s2i.assemble-user

Example assemble script:

NOTE: All the examples are written in Bash and it is assumed that the tar contents unpack into the /tmp directory.


# restore build artifacts
if [ "$(ls /tmp/artifacts/ 2>/dev/null)" ]; then
    mv /tmp/artifacts/* $HOME/.

# move the application source
mv /tmp/s2i/src $HOME/src

# build application artifacts
pushd ${HOME}
make all

# install the artifacts
make install


The run script is responsible for executing your application.

Example run script:


# run the application


The save-artifacts script is responsible for gathering all the dependencies into a tar file and streaming it to the standard output (eg. for Ruby - gems installed by Bundler, for Java - .m2 contents, etc.). The existence of this can speed up the following build processes. Note: it is critical that the save-artifacts script output only include the tar stream output and nothing else. This is handled by redirecting output to /dev/null in the sample script below.

Example save-artifacts script:


# Besides the tar command, all other output to standard out must 
# be surpressed.  Otherwise, the tar stream will be corrupted.
pushd ${HOME} >/dev/null
if [ -d deps ]; then
    # all deps contents to tar stream
    tar cf - deps
popd >/dev/null


The usage script is for you (as the builder image author) to inform the user how to use your image.

Example usage script:


# inform the user how to use the image
cat <<EOF
This is a S2I sample builder image.


The test/run script is for you (as the builder image author) to create a simple process to check whether the image is working correctly. The workflow of that process should be the following:

  1. Build the image.
  2. Run the image to verify the usage script.
  3. Run the s2i build to verify assemble script.
  4. (Optional) Run the s2i build once more to verify save-artifacts script and assemble's restore artifacts functionality.
  5. Run the image to verify the test application is working.
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