Prepare a Docker image for the MiniZinc Challenge 2021

• MiniZinc Challenge
• »Challenge 2021
• »Docker image preparation

Quick Summary

For those who are familiar with Docker, an image suitable for submission to the MiniZinc Challenge must comply wih the following requirements:

• Your image must use the base image of minizinc/mznc2021:latest

• Running minizinc with the default solver (i.e. without the --solver option) must run your solver. This can be achieved through one of the following:

  • If you have a solver configuration file (.msc) for your solver (recommended), you must add it to the MZN_SOLVER_PATH environment variable:

    ENV MZN_SOLVER_PATH=/path/with/msc/:${MZN_SOLVER_PATH}
    

    Then set the default solver to your solver's ID:

    RUN echo '{"tagDefaults": [["", "org.myorg.my_solver"]]}' > $HOME/.minizinc/Preferences.json
    

    Note that MiniZinc solvers using a solver configuration file will have to add --output-mode and --output-objective to the extra flags section of the configuration to declare support for these flags.

  • If you do not have a solver configuration file and are submitting a FlatZinc solver, you can place its executable at /entry_data/fzn-exec and its MiniZinc library at /entry_data/mzn-lib/. Alternatively you may modify /minizinc/mzn-exec-fd, /minizinc/mzn-exec-free and /minizinc/mzn-exec-par as appropriate for your solver's entry classes, to run your solver command.

  • If you do not have a solver configuration file and are submitting a MiniZinc solver, you must modify /minizinc/mzn-exec-fd, /minizinc/mzn-exec-free and /minizinc/mzn-exec-par as appropriate for your solver's entry classes, to run your solver command.

You can test if your submission functions as required by running the command for the entry class you are participating in:

Push your image to Docker Hub and complete the web form (which will be made available to participants at the start of the initial submission round) to submit your entry.

Solver Submission

In order to submit your solver, you must install it into a Docker image that extends a base operating system image (Ubuntu 20.04 with MiniZinc installed) which we provide. To do this, you will create a Dockerfile - a set of instructions indicating how to (build and) install your solver on to the base operating system image.

The submitted image must have your solver set as the default MiniZinc solver.

Prerequisites

1. Install Docker Engine by following the instructions here.

2. Register an account on Docker Hub to which you will upload your image so that the Challenge organisers can access it.

   Note that you can create one private repository for free, which you can share with the MiniZinc Challenge Team if you do not wish to make your image public.

MiniZinc Challenge 2021 Docker Image

The Docker base image, which all entries must extend, was created via a Dockerfile and is available at the repository minizinc/mznc2021 with the tag latest. Pull the repository via command line using this command (you should re-run this command if we announce that we have made changes to the base image).

docker pull minizinc/mznc2021:latest

The image contains a minimal installation of Ubuntu 20.04 and MiniZinc 2.5.5, which is installed in the /usr/local/bin directory. It is recommended that you create a Solver Configuration File for your solver, and set it as the default solver using a User Configuration File (the details of how to do this are in the following section), however the method of setting up scripts as in previous years is still available.

If you want to inspect the image via a bash shell, use the following command:

docker run -it --rm minizinc/mznc2021 /bin/bash

Dockerfile

Solver Entry Dockerfile

As an example, we will create a Dockerfile in order to submit an entry for Chuffed.

Chuffed is built using GCC and CMake, so our Dockerfile will have to install them in order to compile Chuffed. However, we do not want to include these tools in the final image. Therefore, we will create a separate build stage to compile Chuffed in, and then copy just the required files from this stage into the final image.

The Dockerfile will perform the following:

• In an isolated build stage:
  • Install build tools required to fetch and compile Chuffed.
  • Fetch, compile, and install Chuffed.
• Copy the installed solver to the final image (so that the compiler toolchain is not in the final image).
• Ensure Chuffed can be located by MiniZinc and is the default solver.

Create a file called Dockerfile and add the following (see comments for explanations):

# Build Chuffed in an isolated stage called builder.
#
# The FROM statement begins a new stage in the image creation process.
# You specify the base image for following statements to use.
# We're giving this one a name using AS, so that we can refer to it
# and copy files from it later on.
FROM minizinc/mznc2021:latest AS builder

# Install compiler toolchain for Chuffed.
#
# The RUN statement runs the given command in the image.
# Note the use of DEBIAN_FRONTEND=noninteractive to ensure that apt does
# not prompt for user input such as time zones.
RUN apt-get update -y && \
    DEBIAN_FRONTEND=noninteractive apt-get install -y bison build-essential cmake flex git

# Change directory to /src.
#
# WORKDIR sets the working directory for the following statements
# until the end of the stage (it creates it if it doesn't exist).
WORKDIR /src

# Clone the Chuffed git repository.
RUN git clone https://github.com/chuffed/chuffed . && \
    git checkout develop

# Build Chuffed and install it into /install.
RUN mkdir /install && mkdir build && cd build && \
    cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/install .. && \
    cmake --build . --config Release && \
    cmake --build . --config Release --target install

# Create our final image using this base.
#
# Since this is the final FROM statement in this Dockerfile,
# the contents of this stage are what constitutes the final image.
FROM minizinc/mznc2021:latest

# Copy the Chuffed installation to /chuffed in the final image.
#
# The COPY statement copies files from the local computer into the
# current stage, or from another stage into this stage.
# We want to copy from the builder stage, so we use --from=builder.
# If we omitted that, we would copy a file from the local computer.
COPY --from=builder /install /chuffed

# Add Chuffed to the MiniZinc search path.
#
# The ENV statement sets an environment variable for the image.
ENV MZN_SOLVER_PATH=/chuffed/share/minizinc/solvers:${MZN_SOLVER_PATH}

# Set Chuffed as the default solver.
#
# See https://www.minizinc.org/doc-2.5.5/en/command_line.html#user-configuration-files
RUN echo '{"tagDefaults": [["", "org.chuffed.chuffed"]]}' > $HOME/.minizinc/Preferences.json

Refer to the Dockerfile reference for more information about each of the statements.

You can then build the image with

docker build -t chuffed:latest .

And check that it functions by running

docker run --rm chuffed:latest minizinc /classes/fd.mpc /minizinc/test.mzn /minizinc/2.dzn

With your own image, replace chuffed:latest with an appropriate name for your solver in the format <repository>:<tag>.

Since Chuffed installs a solver configuration file (.msc) into share/minizinc/solvers, we can add that path to MZN_SOLVER_PATH and set it as the default solver using the $HOME/.minizinc/Preferences.json file. If your solver does not yet have a solver configuration file, you can create one by following the instructions in the next section.

Other example Dockerfiles

There are a number of other Dockerfiles and solver configuration files which have been created for the MiniZinc Challenge. They may be used as reference in writing your own.

They are available here.

In general, a Dockerfile for the MiniZinc Challenge should have (close to) the following structure:

FROM minizinc/mznc2021:latest AS builder
RUN apt-get update -y && \
    apt-get install -y <your_build_tools>

WORKDIR /src

# Replace with command to clone your solver repository, or COPY the source from
# the local computer
RUN git clone https://github.com/<your>/<repo> . && \
    git checkout <your_branch>

# Replace with command to build your solver and install it into /install
RUN mkdir /install && \
    <build_your_solver> && \
    <install_your_solver_into_/install>

FROM minizinc/mznc2021:latest

RUN apt-get update -y && \
    apt-get install -y <your_runtime_dependencies>

COPY --from=builder /install /my_solver

ENV MZN_SOLVER_PATH=/my_solver/<msc_file_dir>:${MZN_SOLVER_PATH}

RUN echo '{"tagDefaults": [["", "org.mysolver.mysolver"]]}' > $HOME/.minizinc/Preferences.json

If your solver is a prebuilt binary, you can skip the initial builder stage and simply COPY it from the local computer (or perhaps use an ADD statement to download it from a web address) in the final stage. However, there may be binary compatibility/dependency issues, so it is recommended that you build your solver from source in the Dockerfile to avoid this.

Using a solver configuration file

A solver configuration file (.msc) is a JSON configuration file specifying how to run your solver. It is the recommended way to add your solver to your image, and it is also the easiest way to allow your solver to be installed by others for use with MiniZinc.

See the documentation here for more information.

A FlatZinc solver called My Solver which is a binary file ./fzn-mysolver and a library at ./mysolver-mznlib may have the following solver configuration file:

./my-solver.msc:

{
  "name": "My Solver",
  "version": "1.0.0",
  "id": "org.myorg.my_solver",
  "mznlib": "mysolver-mznlib",
  "executable": "fzn-mysolver",
  "stdFlags": ["-a", "-f", "-p"]
}

And use the following Dockerfile:

./Dockerfile:

FROM minizinc/mznc2021
# Add solver executable to image
COPY ./fzn-mysolver /my-solver/
# Add solver library to image
COPY ./mysolver-mznlib /my-solver/
# Add solver configuration file to image
COPY ./my-solver.msc /my-solver/
# Make solver the default
RUN echo '{"tagDefaults": [["", "org.myorg.my_solver"]]}' > $HOME/.minizinc/Preferences.json
# Add to MiniZinc search path
ENV MZN_SOLVER_PATH=/my-solver:${MZN_SOLVER_PATH}

The most important parts are the mznlib and executable items for specifying the location of your MiniZinc library, and executable, respectively. These are resolved relative to the location of the solver configuration. You can also use an absolute path.

A MiniZinc solver called may use a solver configuration file:

{
  "name": "My Solver",
  "version": "1.0.0",
  "id": "org.myorg.my_solver",
  "executable": "mzn-mysolver",
  "stdFlags": ["-a", "-f", "-p"],
  "extraFlags": [
    ["--output-mode", "", "string", ""],
    ["--output-objective", "", "bool", "false"]
  ],
  "supportsMzn": true
}

Note the use of extraFlags in this case to declare support for the --output-mode and --output-objective options. This is not required for FlatZinc solvers.

Setup using scripts/fixed paths

If you do not wish to use a solver configuration file, you may use the default one provided in the image, and install your FlatZinc solver to a fixed location, or modify some scripts to execute your solver.

In the MiniZinc directory /minizinc, there are the three shell scripts mzn-exec-fd (for the FD class), mzn-exec-free (for the FREE class), and mzn-exec-par (for the PAR class) (these are the same as in previous MiniZinc Challenges). If you submit a MiniZinc solver then you need to modify them accordingly. If you submit a FlatZinc solver then you may need to add, modify, or remove parameters that are used for calling your FlatZinc solver. In the case of a FlatZinc solver, there must an executable at /entry_data/fzn-exec and the solver's MiniZinc library must be located in the folder /entry_data/mzn-lib. You should install your solver in the directory /entry_data.

You may wish to copy the scripts from the base image to your local computer, edit them, and then copy the modified scripts into your submission image.

Copy the scripts to the current directory with:

docker run --rm minizinc/mznc2021:latest cat mzn-exec-fd > mzn-exec-fd
docker run --rm minizinc/mznc2021:latest cat mzn-exec-free > mzn-exec-free
docker run --rm minizinc/mznc2021:latest cat mzn-exec-par > mzn-exec-par

Edit them in your editor of choice, and then COPY them in your Dockerfile:

FROM minizinc/mznc2021:latest

# =====================================
# Install your solver into the image...
# =====================================

# Copy modified scripts into /minizinc directory
COPY ./mzn-exec-fd /minizinc/
COPY ./mzn-exec-free /minizinc/
COPY ./mzn-exec-par /minizinc/

Creating an Image Interactively

It is also possible to create an image interactively, however this is not recommended as it makes it rather difficult to make changes to your image, and keep the final image small.

docker run -it --name solver minizinc/mznc2021:latest /bin/bash

Will bring you into a bash shell, in which you can install your solver and make it the default or modify the scripts as above. To create the final image, run:

docker commit solver <repository>:<tag>

If you need to change an environment variable in the final image, for example, if you've installed a solver configuration file, you can run docker commit with the option --change 'ENV MZN_SOLVER_PATH=/path/with/msc:${MZN_SOLVER_PATH}'.

Testing your submission

You can test if your submission functions as required by running the command for the entry class you are participating in:

It is strongly recommended that you also test your submission using -v and ensure that your MiniZinc library is being picked up correctly.

Uploading your submission

Once you have finalised your Docker image, you can push it to Docker Hub.

# If you did not include your username in the repository, you must add it with
docker tag <repository>:<tag> <username>/<name>:<tag>
# Sign into Docker Hub
docker login
# Upload the image
docker push <username>/<name>:<tag>

If you want to make your repository private then go to Docker Hub, sign into your account, click on your repository, click on settings, and then on the button Make Private. You will have to give the Docker Hub user mznc access to the repository in order to submit it.

You can then complete the web form (which will be made available to participants at the start of the initial submission round) to submit your entry.

Return to the MiniZinc Challenge 2021 home page.