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:
rust
struct FooV1 {
val: u32,
}
If we serialize data to files in this format, but later discover that we want
to make a change:
rust
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.
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:
```rust
impl FromVersion
This crate is still new, and these rules may evolve in the future.
The whole point of this exercise is to be able to deserialize data structures without knowing or caring what version they are. To help with this,
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 (IO interface, serialization, etc.)
let src = MyDataSource::new(...);
// Read a header struct that contains the message id and struct version
let header = src.read_header()?;
// Deserialize the message (any version of Foo) and upgrade it.
let msg = Foo::upgrade_latest(src, header.msg_ver())?;
Note that msg in this example is always the latest version of the Foo
struct family, 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.
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:
rust
enum MyProtocol {
Foo(Foo),
Bar(Bar),
}
We can derive the trait GroupDeserialize that can automatically deserialize
any of the messages in MyProtocol:
rust
let incoming_message: MyProtocol = read_message(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 field 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