Introduction

As explained before, Descartes off-chain processing is performed by a Cartesi Machine, which is capable of running a Linux operating system and executing arbitrary computations in a verifiable and reproducible way. Up to this point, the machines we have built have always used the default Linux kernel and root file-system provided by Cartesi, which is more than sufficient for executing a wide variety of tasks. Indeed, several common Linux tools are already included in this distribution, such as the bc calculator tool used in the previous tutorial or the sh command interpreter. Even a Lua interpreter is already included in the package.

Nevertheless, in real-world scenarios it is expected that specific tools and libraries will be required, and in such cases we will need to use a mechanism to include those additional resources in the Cartesi Machine.

In this tutorial, we will explore how to create and use a custom root file-system, which in our case will include the Python3 interpreter along with the PyJWT library for encoding and decoding JWT tokens.

Building a custom root file-system

Essentially, our goal is to create an ext2 file with the customized full root file-system, including our specific additional libraries and tools. Later on, when building the Cartesi Machine, we will be able to refer to that file so that our machine has access to all the desired resources.

The process of building a custom root file-system is fully documented in the Cartesi Machine target perspective section. However, we can speed things up by taking advantage of the default cartesi/rootfs Docker image provided by Cartesi, and building on top of it.

First of all, let's create a cartesi-machine subdirectory and cd into it, as we did for the previous DApp tutorials:

mkdir cartesi-machine
cd cartesi-machine

Then, pull the cartesi/rootfs Docker image and tag it as cartesi/rootfs:devel:

docker pull cartesi/rootfs:0.4.0
docker tag cartesi/rootfs:0.4.0 cartesi/rootfs:devel

After that, clone Cartesi's machine-emulator-sdk repository along with its submodules:

git clone --recurse-submodules git@github.com:cartesi/machine-emulator-sdk.git

or using the http address:

git clone --recurse-submodules https://github.com/cartesi/machine-emulator-sdk.git

After the repository and submodules are downloaded, switch to the machine-emulator-sdk/fs subdirectory and start up the configuration tool to select the desired packages using a textual menu interface:

cd machine-emulator-sdk/fs
make config

For this project, select Target packages and then Interpreter languages and scripting. Select the python3 entry, then navigate into External python modules and select python-pyjwt.

After that, simply exit the configuration interface and answer y when prompted to build the root file-system. This process may take some minutes to complete, after which a file called rootfs.ext will be generated.

Finally, move the generated root file-system file back to our project's cartesi-machine directory:

mv rootfs.ext2 ../../rootfs-python-jwt.ext2 
cd ../../

And as such we have created our own customized root file-system, and can now focus on using it to build our Cartesi Machine, as discussed in the next section.