Improved datastore root and snapshot iteration caching

Sources/CodableDatastore/Persistence/Disk Persistence/Datastore/DatastoreIndex.swift

@@ -98,6 +98,17 @@ extension DiskPersistence.Datastore.Index {
             case .secondary(let index, let manifest):   self = .secondary(index: index, manifest: manifest)
             }
         }
+
+        init(_ id: DatastoreRootManifest.IndexManifestID) {
+            switch id {
+            case .primary(let manifest):
+                self = .primary(manifest: manifest)
+            case .direct(let index, let manifest):
+                self = .direct(index: index, manifest: manifest)
+            case .secondary(let index, let manifest):
+                self = .secondary(index: index, manifest: manifest)
+            }
+        }
     }
 }
 

Sources/CodableDatastore/Persistence/Disk Persistence/Datastore/PersistenceDatastore.swift

@@ -217,14 +217,16 @@ extension DiskPersistence.Datastore {
 
 extension DiskPersistence.Datastore {
     /// Load the root object from disk for the given identifier.
-    func loadRootObject(for rootIdentifier: DatastoreRootIdentifier) throws -> DatastoreRootManifest {
+    func loadRootObject(for rootIdentifier: DatastoreRootIdentifier, shouldCache: Bool = true) throws -> DatastoreRootManifest {
         let rootObjectURL = rootURL(for: rootIdentifier)
 
         let data = try Data(contentsOf: rootObjectURL)
 
         let root = try JSONDecoder.shared.decode(DatastoreRootManifest.self, from: data)
 
-        cachedRootObject = root
+        if shouldCache {
+            cachedRootObject = root
+        }
         return root
     }
 

Sources/CodableDatastore/Persistence/Disk Persistence/DiskPersistence.swift

@@ -291,7 +291,7 @@ extension DiskPersistence {
             return snapshot
         }
 
-        let snapshot = Snapshot(id: snapshotID, persistence: self)
+        let snapshot = Snapshot(id: snapshotID, persistence: self, isExtendedIterationCacheEnabled: !_transactionRetentionPolicy.isIndefinite)
         snapshots[snapshotID] = snapshot
 
         return snapshot

Sources/CodableDatastore/Persistence/Disk Persistence/Snapshot/Snapshot.swift

@@ -30,8 +30,9 @@ actor Snapshot<AccessMode: _AccessMode> {
     /// A cached instance of the manifest as last loaded from disk.
     var cachedManifest: SnapshotManifest?
 
-    /// A cached instance of the current iteration as last loaded from disk.
-    var cachedIteration: SnapshotIteration?
+    /// Cache for the loaded iterations as last loaded from disk. ``isExtendedIterationCacheEnabled`` controls if multiple iterations are cached or not.
+    var cachedIterations: [SnapshotIterationIdentifier : SnapshotIteration] = [:]
+    var isExtendedIterationCacheEnabled: Bool
 
     /// A pointer to the last manifest updater, so updates can be serialized after the last request.
     var lastUpdateManifestTask: Task<Sendable, Error>?
@@ -42,11 +43,13 @@ actor Snapshot<AccessMode: _AccessMode> {
     init(
         id: SnapshotIdentifier,
         persistence: DiskPersistence<AccessMode>,
-        isBackup: Bool = false
+        isBackup: Bool = false,
+        isExtendedIterationCacheEnabled: Bool = false
     ) {
         self.id = id
         self.persistence = persistence
         self.isBackup = isBackup
+        self.isExtendedIterationCacheEnabled = isExtendedIterationCacheEnabled
     }
 }
 
@@ -124,14 +127,25 @@ extension Snapshot {
         }
     }
 
+    func setExtendedIterationCacheEnabled(_ isEnabled: Bool) {
+        isExtendedIterationCacheEnabled = isEnabled
+    }
+
     /// Load an iteration from disk, or create a suitable starting value if such a file does not exist.
-    private func loadIteration(for iterationID: SnapshotIterationIdentifier) throws -> SnapshotIteration {
+    func loadIteration(for iterationID: SnapshotIterationIdentifier?) async throws -> SnapshotIteration? {
+        guard let iterationID else { return nil }
+        if let iteration = cachedIterations[iterationID] {
+            return iteration
+        }
         do {
             let data = try Data(contentsOf: iterationURL(for: iterationID))
 
             let iteration = try JSONDecoder.shared.decode(SnapshotIteration.self, from: data)
 
-            cachedIteration = iteration
+            if !isExtendedIterationCacheEnabled {
+                cachedIterations.removeAll()
+            }
+            cachedIterations[iteration.id] = iteration
             return iteration
         } catch {
             throw error
@@ -155,7 +169,7 @@ extension Snapshot {
         cachedManifest = manifest
     }
 
-    /// Write the specified iteration to the store, and cache the results in ``Snapshot/cachedIteration``.
+    /// Write the specified iteration to the store, and cache the results in ``Snapshot/cachedIterations``.
     private func write(iteration: SnapshotIteration) throws where AccessMode == ReadWrite {
         let iterationURL = iterationURL(for: iteration.id)
         /// Make sure the directories exists first.
@@ -166,7 +180,10 @@ extension Snapshot {
         try data.write(to: iterationURL, options: .atomic)
 
         /// Update the cache since we know what it should be.
-        cachedIteration = iteration
+        if !isExtendedIterationCacheEnabled {
+            cachedIterations.removeAll()
+        }
+        cachedIterations[iteration.id] = iteration
     }
 
     /// Load and update the manifest in an updater, returning the task for the updater.
@@ -200,26 +217,19 @@ extension Snapshot {
 
             /// Load the manifest so we have a fresh copy, unless we have a cached copy already.
             var manifest = try cachedManifest ?? self.loadManifest()
-            var iteration: SnapshotIteration
-            if let cachedIteration, cachedIteration.id == manifest.currentIteration {
-                iteration = cachedIteration
-            } else if let iterationID = manifest.currentIteration {
-                iteration = try self.loadIteration(for: iterationID)
-            } else {
-                let date = Date()
-                iteration = SnapshotIteration(id: SnapshotIterationIdentifier(date: date), creationDate: date)
-            }
+            let precedingIteration = try await self.loadIteration(for: manifest.currentIteration)
+            var iteration = precedingIteration ?? SnapshotIteration()
 
             /// Let the updater do something with the manifest, storing the variable on the Task Local stack.
             let returnValue = try await SnapshotTaskLocals.$manifest.withValue((manifest, iteration)) {
                 try await updater(&manifest, &iteration)
             }
 
             /// Only write to the store if we changed the manifest for any reason
-            if iteration.isMeaningfullyChanged(from: cachedIteration) {
+            if iteration.isMeaningfullyChanged(from: precedingIteration) {
                 iteration.creationDate = Date()
                 iteration.id = SnapshotIterationIdentifier(date: iteration.creationDate)
-                iteration.precedingIteration = cachedIteration?.id
+                iteration.precedingIteration = precedingIteration?.id
 
                 try write(iteration: iteration)
             }
@@ -260,15 +270,7 @@ extension Snapshot {
 
             /// Load the manifest so we have a fresh copy, unless we have a cached copy already.
             let manifest = try cachedManifest ?? self.loadManifest()
-            var iteration: SnapshotIteration
-            if let cachedIteration, cachedIteration.id == manifest.currentIteration {
-                iteration = cachedIteration
-            } else if let iterationID = manifest.currentIteration {
-                iteration = try self.loadIteration(for: iterationID)
-            } else {
-                let date = Date()
-                iteration = SnapshotIteration(id: SnapshotIterationIdentifier(date: date), creationDate: date)
-            }
+            let iteration = try await self.loadIteration(for: manifest.currentIteration) ?? SnapshotIteration()
 
             /// Let the accessor do something with the manifest, storing the variable on the Task Local stack.
             return try await SnapshotTaskLocals.$manifest.withValue((manifest, iteration)) {

Sources/CodableDatastore/Persistence/Disk Persistence/Snapshot/SnapshotIteration.swift

@@ -68,6 +68,12 @@ extension SnapshotIteration {
 }
 
 extension SnapshotIteration {
+    /// Initialize a snapshot iteration with a date
+    /// - Parameter date: The date to base the identifier and creation date off of.
+    init(date: Date = Date()) {
+        self.init(id: SnapshotIterationIdentifier(date: date), creationDate: date)
+    }
+
     /// Internal method to check if an instance should be persisted based on iff it changed significantly from a previous iteration
     /// - Parameter existingInstance: The previous iteration to check
     /// - Returns: `true` if the iteration should be persisted, `false` if it represents the same data from `existingInstance`.