TFHE-versionable

This crate provides type level versioning for serialized data. It offers a way to add backward compatibility on any data type. The versioning scheme works recursively and is independant of the chosen serialization backend.

To use it, simply define an enum that have a variant for each version of your target type.

For example, if you have defined an internal type:

struct MyStruct {
	val: u32
}

You have to define the following enum:

enum MyStructVersions {
	V0(MyStruct)
}

If at a subsequent point in time you want to add a field to this struct, the idea is to copy the previously defined version of the struct and create a new one with the added field. This mostly becomes:

struct MyStruct {
	val: u32,
	newval: u64
}

struct MyStructV0 {
	val: u32
}

enum MyStructVersions {
	V0(MyStructV0),
	V1(MyStruct)
}

You also have to implement the Upgrade trait, that tells how to go from a version to another.

To make this work recursively, this crate defines 3 derive macro that should be used on these types:

• Versionize should be used on the current version of your type, the one that is used in your code
• Version is used on every previous version of this type
• VersionsDispatch is used on the enum holding all the versions

This will implement the Versionize/Unversionize traits with their versionize and unversionize methods that should be used before/after the calls to serialize/deserialize.

The enum variants should keep their order and names between versions. The only supported operation is to add a new variant.

Complete example

use tfhe_versionable::{Unversionize, Upgrade, Version, Versionize, VersionsDispatch};

// The structure that should be versioned, as defined in your code
#[derive(Versionize)]
#[versionize(MyStructVersions)] // Link to the enum type that will holds all the versions of this type
struct MyStruct<T: Default> {
    attr: T,
    builtin: u32,
}

// To avoid polluting your code, the old versions can be defined in another module/file, along with the dispatch enum
#[derive(Version)] // Used to mark an old version of the type
struct MyStructV0 {
    builtin: u32,
}

// The Upgrade trait tells how to go from the first version to the last. During unversioning, the
// upgrade method will be called on the deserialized value enough times to go to the last variant.
impl<T: Default> Upgrade<MyStruct<T>> for MyStructV0 {
    fn upgrade(self) -> MyStruct<T> {
        MyStruct {
            attr: T::default(),
            builtin: self.builtin,
        }
    }
}

// This is the dispatch enum, that holds one variant for each version of your type.
#[derive(VersionsDispatch)]
// This enum is not directly used but serves as a template to generate new enums that will be
// serialized. This allows recursive versioning.
#[allow(unused)]
enum MyStructVersions<T: Default> {
    V0(MyStructV0),
    V1(MyStruct<T>),
}

fn main() {
    let ms = MyStruct {
        attr: 37u64,
        builtin: 1234,
    };

    let serialized = bincode::serialize(&ms.versionize()).unwrap();

    // This can be called in future versions of your application, when more variants have been added
    let _unserialized = MyStruct::<u64>::unversionize(bincode::deserialize(&serialized).unwrap());
}

See the examples folder for more usecases.