Serialize

Types implement Serialize separately from Archive. Serialize creates a resolver for some object, then Archive turns the value and that resolver into an archived type. Having a separate Serialize trait is necessary because although a type may have only one archived representation, it may support many different types of serializers which fulfill its requirements.

The Serialize trait is parameterized over the serializer. The serializer is just a mutable object that helps the type serialize itself. The most basic types like u32 or char don't bound their serializer type because they can serialize themselves with any kind of serializer. More complex types like Box and String require a serializer that implements Writer, and even more complex types like Rc and Vec require a serializer that additionally implements Sharing or Allocator.

Unlike Serialize, Archive doesn't parameterize over the serializer used to make it. It shouldn't matter what serializer a resolver was made with, only that it's made correctly.

Serializer

rkyv provides default serializers which can serialize all standard library types, as well as components which can be combined into custom-built serializers. By combining rkyv's provided components, serializers can be customized for high-performance, no-std, and custom allocation.

When using the high-level API, a HighSerializer provides a good balance of flexibility and performance by default. When using the low-level API, a LowSerializer does the same without any allocations. You can make custom serializers using the Serializer combinator, or by writing your own from scratch.

rkyv comes with a few primary serializer traits built-in:

Positional

This core serializer trait provides positional information during serialization. Because types need to know the relative distance between objects, the Positional trait provides the current position of the "write head" of the serializer. Resolvers will often store the position of some serialized data so that a relative pointer can be calculated to it during resolve.

Writer

Writer accepts byte slices and writes them to some output. It is similar to the standard library's Write trait, but rkyv's Writer trait works in no-std contexts. In rkyv, writers are always write-forward - they never backtrack and rewrite data later. This makes it possible for writers to eagerly sink bytes to disk or the network without having to first buffer the entire message.

Several kinds of Writers are supported by default:

Vec<u8>
AlignedVec, which is a highly-aligned vector of bytes. This is the writer rkyv uses by default in most cases.
Buffer, which supports no-std use cases (for example, writing into fixed-size stack memory).
Types which implement std::io::Write can be adapted into a Writer by wrapping them in the IoWriter type.

Allocator

Many types require temporarily-allocated space during serialization. This space is used temporarily, and then returned to the serializer before serialization finishes. For example, Vec might request a dynamically-sized allocation to store the resolvers for its elements until it finishes serializing all of them. Allocating memory from the serializer allows the same bytes to be efficiently reused many times, which reduces the number of slow memory allocations performed during serialization.

rkyv serializes shared pointers like Rc and Arc and can control whether they are de-duplicated. The Sharing trait provides some mutable state on the serializer which keeps track of which shared pointers have been serialized so far, and can instruct repeated shared pointers to point to a previously-serialized instance. This also allows rkyv to preserve shared pointers during zero-copy access and deserialization.