add two exercises

content/sharing.rst

@@ -160,7 +160,91 @@ Use version control and public registries
 - Link the repo to the public registry
 
 
-Exercise
---------
-
-(work in progress - Radovan working on this part)
+Exercises
+---------
+
+.. exercise:: Exercise Sharing-1: Time-travel with containers
+
+   Imagine the following situation: A researcher has written and published their research code which
+   requires a number of libraries and system dependencies. They ran their code
+   on a Linux computer (Ubuntu). One very nice thing they did was to publish
+   also a container image with all dependencies included, as well as the
+   definition file (below) to create the container image.
+
+   Now we travel 3 years into the future and want to reuse their work and adapt
+   it for our data. The container registry where they uploaded the container
+   image however no longer exists. But luckily (!) we still have the definition
+   file (below). From this we should be able to create a new container image.
+
+   - Can you anticipate problems using the definition file here 3 years after its
+     creation? Which possible problems can you point out?
+   - Discuss possible take-aways for creating more reusable containers.
+
+   .. tabs::
+
+      .. tab:: Python project using virtual environment
+
+         .. literalinclude:: sharing/bad-example-python.def
+            :language: singularity
+            :linenos:
+
+         .. solution::
+
+            - Line 2: "ubuntu:latest" will mean something different 3 years in future.
+            - Lines 11-12: The compiler gcc and the library libgomp1 will have evolved.
+            - Line 30: The container uses requirements.txt to build the virtual environment but we don't see
+              here what libraries the code depends on.
+            - Line 33: Data is copied in from the hard disk of the person who created it. Hopefully we can find the data somewhere.
+            - Line 35: The library fancylib has been built outside the container and copied in but we don't see here how it was done.
+            - Python version will be different then and hopefully the code still runs then.
+            - Singularity/Apptainer will have also evolved by then. Hopefully this definition file then still works.
+            - No help text.
+            - No contact address to ask more questions about this file.
+            - (Can you find more? Please contribute more points.)
+
+            .. literalinclude:: sharing/bad-example-python.def
+               :language: singularity
+               :linenos:
+               :emphasize-lines: 2, 11-12, 30, 33, 35
+
+      .. tab:: C++ project
+
+         This definition files has potential problems 3 years later. Further
+         down on this page we show a better and real version.
+
+         .. literalinclude:: sharing/bad-example-cxx.def
+            :language: singularity
+            :linenos:
+
+         .. solution::
+
+            - Line 2: "ubuntu:latest" will mean something different 3 years in future.
+            - Lines 9: The libraries will have evolved.
+            - Line 11: We clone a Git repository recursively and that repository might evolve until we build the container image the next time.
+              here what libraries the code depends on.
+            - Line 18: The library fancylib has been built outside the container and copied in but we don't see here how it was done.
+            - Singularity/Apptainer will have also evolved by then. Hopefully this definition file then still works.
+            - No help text.
+            - No contact address to ask more questions about this file.
+            - (Can you find more? Please contribute more points.)
+
+            .. literalinclude:: sharing/bad-example-cxx.def
+               :language: singularity
+               :linenos:
+               :emphasize-lines: 2, 9, 11, 18
+
+
+.. exercise:: Exercise Sharing-2: Building a container on GitHub
+
+   You can build a container on GitHub (using GitHub Actions) or GitLab (using
+   GitLab CI) and host the image it on GitHub/GitLab. This has the following
+   advantages:
+    - You don't need to host it yourself.
+    - But the image stays close to its sources and is not on a different service.
+    - Anybody can inspect the recipe and how it was built.
+    - Every time you make a change to the recipe, it builds a new image.
+
+   If you want to try this out:
+    - Take `this repository <https://github.com/bast/html2pdf>`_ as starting point and inspiration.
+    - Don't focus too much on what this container does, but rather `how it is built <https://github.com/bast/html2pdf/tree/main/.github/workflows>`_.
+    - To build a new version, one needs to send a pull request which updates ``VERSION`` and modifies the definition file.

content/sharing/bad-example-cxx.def

@@ -0,0 +1,26 @@
+Bootstrap: docker
+From: ubuntu:latest
+
+%post
+    export DEBIAN_FRONTEND=noninteractive
+    apt-get update -y
+
+    apt install -y git build-essential pkg-config
+    apt install -y libz-dev libbz2-dev liblzma-dev libcurl4-openssl-dev libssl-dev libgsl-dev
+
+    git clone --recursive https://github.com/someuser/sometool.git
+    cd sometool
+
+    make
+
+%files
+    # Workaround to fix dependency on fancylib
+    /home/myself/fancylib /usr/lib/fancylib
+
+%environment
+    export LC_ALL=C
+
+%runscript
+    export PATH=/sometool:$PATH
+
+    $@

content/sharing/bad-example-python.def

@@ -0,0 +1,46 @@
+Bootstrap: docker
+From: ubuntu:latest
+
+%post
+    # Set environment variables
+    export VIRTUAL_ENV=/app/venv
+
+    # Install system dependencies and Python 3
+    apt-get update && \
+    apt-get install -y --no-install-recommends \
+        gcc \
+        libgomp1 \
+        python3 \
+        python3-venv \
+        python3-distutils \
+        python3-pip && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
+    # Set up the virtual environment
+    python3 -m venv $VIRTUAL_ENV
+    . $VIRTUAL_ENV/bin/activate
+
+    # Install Python libraries
+    pip install --no-cache-dir --upgrade pip && \
+    pip install --no-cache-dir -r /app/requirements.txt
+
+%files
+    # Copy project files
+    ./requirements.txt /app/requirements.txt
+    ./app.py /app/app.py
+    # Copy data
+    /home/myself/data /app/data
+    # Workaround to fix dependency on fancylib
+    /home/myself/fancylib /usr/lib/fancylib
+
+%environment
+    # Set the environment variables
+    export LANG=C.UTF-8 LC_ALL=C.UTF-8
+    export VIRTUAL_ENV=/app/venv
+
+%runscript
+    # Activate the virtual environment
+    . $VIRTUAL_ENV/bin/activate
+    # Run the application
+    python /app/app.py