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JournalDB documentation

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This commit is contained in:
arkpar 2016-03-07 07:06:55 +01:00
parent d77d9ad9d8
commit 744c4c7d8b
1 changed files with 25 additions and 2 deletions
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27
util/src/journaldb.rs
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@ -27,13 +27,37 @@ use std::env;
/// Implementation of the HashDB trait for a disk-backed database with a memory overlay /// Implementation of the HashDB trait for a disk-backed database with a memory overlay
/// and, possibly, latent-removal semantics. /// and, possibly, latent-removal semantics.
/// ///
/// If `counters` is `None`, then it behaves exactly like OverlayDB. If not it behaves /// If `journal_overlay` is `None`, then it behaves exactly like OverlayDB. If not it behaves
/// differently: /// differently:
/// ///
/// Like OverlayDB, there is a memory overlay; `commit()` must be called in order to /// Like OverlayDB, there is a memory overlay; `commit()` must be called in order to
/// write operations out to disk. Unlike OverlayDB, `remove()` operations do not take effect /// write operations out to disk. Unlike OverlayDB, `remove()` operations do not take effect
/// immediately. Rather some age (based on a linear but arbitrary metric) must pass before /// immediately. Rather some age (based on a linear but arbitrary metric) must pass before
/// the removals actually take effect. /// the removals actually take effect.
///
/// There are two memory overlays:
/// - Transaction overlay contains current transaction data. It is merged with with history
/// overlay on each `commit()`
/// - History overlay contains all data inserted during the history period. When the node
/// in the overlay becomes ancient it is written to disk on `commit()`
///
/// There is also a journal maintained in memory and on the disk as well which lists insertions
/// and removals for each commit during the history period. This is used to track
/// data nodes that go out of history scope and must be written to disk.
///
/// Commit workflow:
/// Create a new journal record from the transaction overlay.
/// Inseart each node from the transaction overlay into the History overlay increasing reference
/// count if it is already there. Note that the reference counting is managed by `MemoryDB`
/// Clear the transaction overlay.
/// For a canonical journal record that becomes ancient inserts its insertions into the disk DB
/// For each journal record that goes out of the history scope (becomes ancient) remove its
/// insertions from the history overlay, decreasing the reference counter and removing entry if
/// if reaches zero.
/// For a canonical journal record that becomes ancient delete its removals from the disk only if
/// the removed key is not present in the history overlay.
/// Delete ancient record from memory and disk.
///
pub struct JournalDB { pub struct JournalDB {
transaction_overlay: MemoryDB, transaction_overlay: MemoryDB,
backing: Arc<Database>, backing: Arc<Database>,
@ -220,7 +244,6 @@ impl JournalDB {
} }
// apply old commits' details // apply old commits' details
if let Some((end_era, canon_id)) = end { if let Some((end_era, canon_id)) = end {
let mut canon_insertions: Vec<(H256, Bytes)> = Vec::new(); let mut canon_insertions: Vec<(H256, Bytes)> = Vec::new();
let mut canon_deletions: Vec<H256> = Vec::new(); let mut canon_deletions: Vec<H256> = Vec::new();