aversion

Versioned data structures in Rust

This crate is still under development.

The goal of this crate is to make versioned data structures easy. For example, imagine we start out with this struct:

struct FooV1 {
    val: u32,
}

If we serialize data to files in this format, but later discover that we want to make a change:

struct FooV2 {
    val: u64,
}

... then we have a bunch of work to do, if we want to support our previous files. We may need to increment a version number in the file header, and possibly go through all the different places where FooV1 was used, and decide whether to upgrade it to FooV2. Any place a FooV1 was used, we need to keep that code around, or risk breaking compatibilty.

This crate adds traits that allow us to track the version of each struct and derive traits and methods to allow upgrading any struct dynamically to the latest version. This means that most code only ever needs to interact with the latest version, while still retaining the ability to read older files.

To make this work, structs must follow a particular pattern:

• Versioned structs must follow the naming convention Name + V + {integer}, i.e. FooV1 or BarV42.
• Versions must start at 1, and be contiguous.
• There must be a type alias type Foo = FooV3 that points to the latest version.
• For each pair of versions, N and N+1, the trait FromVersion must be implemented. For example:

impl FromVersion<FooV1> for FooV2 {
    fn from_version(v1: FooV1) -> Self {
        FooV2 { val: v1.val.into() }
    }
}

This crate is still new, and these rules may evolve in the future.

Deserialization

We want to be able to deserialize data structures without knowing ahead of time what version is stored.

To do this, we use the DataSource trait, to specify our serialization format, header format, and error types.

Once the UpgradeLatest trait is implemented (there is a derive macro for this), we can quickly deserialize any version of our data structure, e.g.

// Define a data source
let src = CborData::new(...);
// Read a the next header + message, and upgrade to the latest version
let msg: Foo = data_src.expect_message()?;

Note that msg in this example is always the latest version of the Foo struct, regardless of which version was actually stored. As long as the FromVersion code is correct, the rest of the program never needs to be aware of which version was read from the file.

Message Groups

We can extend this logic to groups of different messages, to automatically build a dispatch function. For example, if we define a collection of messages:

enum MyProtocol {
    Foo(Foo),
    Bar(Bar),
}

We can derive the trait GroupDeserialize that can automatically deserialize any of the messages in MyProtocol:

let incoming_message = MyProtocol::read_message(&mut my_data_source)?;
match incoming_message {
    Foo(f) => {
        // handle the received Foo message
    }
    Bar(b) => {
        // handle the received Bar message
    }
}

Similar to the previous example, the header will tell us which message was sent (i.e. Foo or Bar), along with the version of that struct (FooV1 or FooV2) and read_message deserializes the correct version of the struct, upgrades it to the latest version, and returns it as a MyProtocol enum, for the caller to handle.

License: Apache-2.0