# Serialization

Warning

Never load data from an untrusted source. Loading data is inherently unsafe and at this point allows arbitrary code execution on your machine. Just as you should never run a program from someone you do not trust, you should also not load their data.

Warning

Serialization development has just started and the concrete design may still change drastically. In particular the mechanism for upgrading old data to newer versions is not in place yet, so at this point we do not yet guarantee that old data can be read.

This document summarises the serialization efforts of OSCAR, how it is supposed to work, how it works and the overall goal. Serialization broadly speaking is the process of writing data to and reading data from files. There are many reasons for needing this feature in OSCAR, but the main reason is communication on mathematics by mathematicians.

## How it works

The mechanism for saving and loading is very simple. It is implemented via two methods save and load, and works in the following manner:

julia> save("/tmp/fourtitwo.json", 42);

42


As hinted by the filename, OSCAR writes a file in JSON format. The file looks as follow:

{
"_ns": {
"Oscar": [
"https://github.com/oscar-system/Oscar.jl",
{
"major": 0,
"minor": 8,
"patch": 3,
"prerelease": [
"DEV"
],
"build": []
}
]
},
"id": "-1",
"type": "Base.Int",
"data": "42"
}

It contains the version of OSCAR it was written by, its type, and the actual content, in this case as a string.

### The id

If you look at the file src/Serialization/main.jl, you will see that all save methods hand down a SerializerState, and all load methods have a DeserializerState. These two objects are very simple, they just contain dictionaries connecting objects and their id. We use this to avoid saving or loading larger objects twice (or multiple times). Consider the following example code snippet:

c = cube(3);
LP0 = LinearProgram(c, [2,2,-3]);
LP1 = LinearProgram(c, [2,2,4]);
v = [LP0, LP1];
save("vector_of_lp.json", v)

This creates two linear programs on the cube, stores them in a vector and then writes this vector to a file. It would be wasteful to store the cube twice for each linear program, instead it is only stored once and the second linear program just gets the id of the cube in its serialized form. Please take some time to look at the file written in this concrete example.

### The version number

We will use the version number for checking compatibility of the data with the current OSCAR version before attempting to load it. If the data version is lower than the OSCAR version we will provide appropriate upgrade scripts such that the data can be loaded. We will not provide scripts for attempting to downgrade data, but we will throw a warning or even error in this case. We may provide an option for attempting to load anyway in such a scenario.

### Implementation

All files for serialization can be found in the folder src/Serialization. The naming conventions of the files there follows the overall structure of OSCAR, i.e. the file src/Serialization/PolyhedralGeometry.jl contains functions for serializing objects of the polyhedral geometry section.

The file main.jl contains the core of the serialization process, namely:

• the SerializerState and DeserializerState objects;
• writing and reading versions; and
• generic functions for attempting to serialize objects that do not have their own dedicated serialization methods.

If you want to write a serialization routine for an object, the way to go is to implement the following two functions, here in the example for fmpz:

function load_internal(s::DeserializerState, ::Type{fmpz}, str::String)
return fmpz(str)
end

function save_internal(s::SerializerState, z::fmpz)
return string(z)
end

Then the main serialization methods will dispatch to load_internal and save_internal for fmpz instead of attempting the generic serialization.

Often the generic serialization will fail and it is necessary to provide a save_internal and load_internal function. In that case, please have a look at the existing functions to get an idea of how these work, and maybe use something of this as a blueprint.

## Challenges

This section documents the various challenges we (will) encounter while implementing this feature.

• OSCAR is based on several subsystems, some of which already have their own serialization. We want this to be compatible, if possible in both directions.
• Many mathematical objects need context to be understood. A polynomial needs the ring it lives in, a group element needs the surrounding group, a divisor needs the underlying variety, etc. We will need a way to store this context along the objects.
• Context should not be stored twice: A matrix of polynomials should only store the surrounding ring once.
• Support other data formats: It has been proposed to not only support JSON, but binary formats needed for HPC communication as well. It is unclear whether this needs a separate implementation.
• Versioning and upgrading: Work on OSCAR will change what its objects look like. Nevertheless, we still want to be able load data written by older versions of OSCAR. For this we intend to develop an upgrade mechanism.

Another important point is the wider mathematical context of the data and code. For data associated to a publication, this context is provided by the paper.

## Goals

The general goal is to make mathematical data FAIR, a goal for which we cooperate with the MaRDI project.

The ramifications of making mathematical data FAIR are manifold.

• It becomes easier to exchange data and code with fellow mathematicians, enhancing communication and boosting research.
• Computer experiments and new implementations require a lot of work and hence deserve to be recognized in form of a publication. Standardizing data plays an important role for this process.
• Future generations of mathematicians will be able to reuse both data and code if we establish a FAIR culture.