use super::*;
/// A user of a `PageCache` needs to provide a `Materializer` which
/// handles the merging of page fragments.
pub trait Materializer {
/// The possibly fragmented page, written to log storage sequentially, and
/// read in parallel from multiple locations on disk when serving
/// a request to read the page. These will be merged to a single version
/// at read time, and possibly cached.
type PageFrag;
/// The higher-level recovery state, as
/// described by `Materializer::recover`
type Recovery;
#[doc(hidden)]
fn is_null() -> bool
where
Self: Sized,
{
false
}
/// Create a new `Materializer` with the previously recovered
/// state if any existed.
fn new(
config: Config,
recovered_state: &Option<Self::Recovery>,
) -> Self
where
Self: Sized;
/// Used to merge chains of partial pages into a form
/// that is useful for the `PageCache` owner.
fn merge(&self, frags: &[&Self::PageFrag]) -> Self::PageFrag;
/// Used to feed custom recovery information back to a higher-level abstraction
/// during startup. For example, a B-Link tree must know what the current
/// root node is before it can start serving requests.
fn recover(
&self,
frag: &Self::PageFrag,
) -> Option<Self::Recovery>;
/// Used to determine the size of the value for caching purposes.
fn size_in_bytes(&self, frag: &Self::PageFrag) -> usize;
}
/// A materializer for things that have nothing to
/// materialize or recover, like a standalone `Log`.
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct NullMaterializer;
impl Materializer for NullMaterializer {
type PageFrag = ();
type Recovery = ();
#[doc(hidden)]
fn is_null() -> bool {
true
}
fn new(_: Config, _: &Option<Self::Recovery>) -> Self {
NullMaterializer
}
fn merge(&self, _: &[&Self::PageFrag]) -> Self::PageFrag {
()
}
fn recover(&self, _: &Self::PageFrag) -> Option<Self::Recovery> {
None
}
fn size_in_bytes(&self, _: &Self::PageFrag) -> usize {
0
}
}