source-to-image/docs/builder_image.md

sti builder image requirements

The main advantage of using sti 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 sti performance, these are:

• required image contents
• sti scripts

Required image contents

The build process consists of three fundamental elements which are combined into final docker image. These three are: sources, scripts and builder image. During the build process sti must place sources and scripts inside that builder image. To do so sti creates a tar file containing the two and then streams that file into the builder image. Before executing the assemble script, sti untars that file and places its contents into the destination specified with either the --destination flag or io.s2i.destination label from the builder image (default destination is /tmp). For this to happen your image must supply the tar archiving utility (command tar available in $PATH) and a command line interpreter (command /bin/sh). Doing so will allow your image to use the fastest possible build path, because in all other cases when either tar or /bin/sh are not available, sti build will be forced to perform an additional docker build to put both sources and scripts inside the image and only then run the usual sti build procedure (sti will do this automatically if tar or /bin/sh are not found). See the following diagram for how build workflow looks like:

* Run build's responsibility is to untar the sources, scripts and artifacts (if such exist) and invoke the assemble script. If this is the second run (due to catching any tar//bin/sh related errors) it's responsibility is only to invoke the assemble script, since both scripts and sources are already there.

sti scripts

sti expects you, as the builder image author, to supply the following scripts:

• required:
  • assemble
  • run
• optional:
  • save-artifacts
  • usage
  • test/run

All of the scripts can be written in any programming language, as long as the scripts are executable inside the builder image. STI supports multiple options providing assemble/run/save-artifacts scripts. All of these locations are checked on each build in the following order:

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

Both the io.s2i.scripts-url label specified in the image and --scripts-url flag can take one of the following forms:

• image://path_to_scripts_dir - absolute path inside the image to a directory where the STI scripts are located
• file://path_to_scripts_dir - relative or absolute path to a directory on the host where the STI scripts are located
• http(s)://path_to_scripts_dir - URL to a directory where the STI scripts are located

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

assemble

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

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

Example assemble script:

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

#!/bin/bash

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

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

# build application artifacts
pushd ${HOME}
make all

# install the artifacts
make install
popd

run

The run script is responsible for executing your application.

Example run script:

#!/bin/bash

# run the application
/opt/application/run.sh

save-artifacts

The save-artifacts script is responsible for gathering all the dependencies into a tar file and streaming it to the standard output. The existance of this can speed up the following build processes (eg. for Ruby - gems installed by Bundler, for Java - .m2 contents, etc.).

Example save-artifacts script:

#!/bin/bash

pushd ${HOME}
if [ -d deps ]; then
    # all deps contents to tar stream
    tar cf - deps
fi
popd

usage

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

Example usage script:

#!/bin/bash

# inform the user how to use the image
cat <<EOF
This is a STI sample builder image, to use it, install
https://github.com/openshift/source-to-image
EOF

test/run

The test/run script is for you, as the builder image author, to create a simple process to checks if the image is working correctly. The proposed flow of that process should be following:

1. Build the image.
2. Run the image to verify usage script.
3. Run sti build to verify assemble script.
4. (optional) Run sti 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.

NOTE The suggested place to put your test application which should be built by your test/run script is test/test-app in your image repository, see sti create.