This crate defines tools to implement datastructures that can live in main memory or on-disk, meaning that their natural habitat is memory-mapped files, but if that environment is threatened, they might seek refuge in lower-level environments.
One core building block of this library is the notion of virtual
memory pages, which are allocated and freed by an
externally-provided allocator (see how the sanakirja crate does
this). The particular implementation used here is meant to allow a
transactional system with readers reading the structures
concurrently with one writer at a time.
At the moment, only B trees are implemented, as well as the following general traits:
- [
LoadPage] is a trait used to get a pointer to a page. In the most basic version, this may just return a pointer to the file, offset by the requested offset. In more sophisticated versions, this can be used to encrypt and compress pages. - [
AllocPage] allocates and frees pages, because as datastructures need to be persisted on disk, we can't rely on Rust's memory management to do it for us. Users of this crate don't have to worry about this though.
Moreover, two other traits can be used to store things on pages:
[Storable] is a simple trait that all Sized + Ord types
without references can readily implement (the [direct_repr!]
macro does that). For types containing references to pages
allocated in the database, the comparison function can be
customised. Moreover, these types must supply an iterator over
these references, in order for reference-counting to work properly
when the datastructures referencing these types are forked.
Dynamically-sized types, or types that need to be represented in a
dynamically-sized way, can use the [UnsizedStorable] format.