Add per peer watermark store for shared sync

- Introduce PeerWatermarkStore to track max HLC received per peer for
shared resource incremental sync - Initialize the
peer_received_watermarks table at startup - Persist received watermarks
during backfill and after backfill completion - Query the max watermark
across peers via PeerWatermarkStore - Expose the new module and export
PeerWatermarkStore from sync

core/src/infra/sync/mod.rs

@@ -18,6 +18,7 @@ pub mod event_bus;
 pub mod fk_mapper;
 pub mod hlc;
 pub mod peer_log;
+pub mod peer_watermarks;
 pub mod registry;
 pub mod syncable;
 pub mod transaction;
@@ -48,3 +49,4 @@ pub use syncable::Syncable;
 pub use transaction::{BulkOperation, BulkOperationMetadata, TransactionManager, TxError};
 pub use transport::NetworkTransport;
 pub use watermarks::{ResourceWatermarkStore, WatermarkError};
+pub use peer_watermarks::PeerWatermarkStore;

core/src/infra/sync/peer_log.rs

@@ -102,6 +102,11 @@ impl PeerLog {
 			.await
 			.map_err(|e| PeerLogError::QueryError(e.to_string()))?;
 
+		// peer_received_watermarks table (shared resource incremental sync)
+		super::peer_watermarks::PeerWatermarkStore::init_table(conn)
+			.await
+			.map_err(|e| PeerLogError::QueryError(e.to_string()))?;
+
 		// backfill_checkpoints table (resumable backfill)
 		super::checkpoints::BackfillCheckpointStore::init_table(conn)
 			.await

core/src/infra/sync/peer_watermarks.rs

@@ -0,0 +1,364 @@
+//! Peer received watermark tracking for shared resource incremental sync
+//!
+//! Tracks the maximum HLC received from each peer for shared resources (tags, metadata, etc).
+//! This enables incremental catch-up by requesting only entries newer than the watermark.
+
+use crate::infra::sync::hlc::HLC;
+use sea_orm::{ConnectionTrait, DbBackend, Statement};
+use std::collections::HashMap;
+use uuid::Uuid;
+
+/// Peer watermark tracking for shared resource incremental sync
+///
+/// Manages per-peer watermarks in sync.db to track the maximum HLC received
+/// from each peer. This prevents re-syncing shared resources that have already
+/// been received.
+pub struct PeerWatermarkStore {
+	device_uuid: Uuid,
+}
+
+impl PeerWatermarkStore {
+	/// Create a new peer watermark store for a device
+	pub fn new(device_uuid: Uuid) -> Self {
+		Self { device_uuid }
+	}
+
+	/// Initialize the peer_received_watermarks table in sync.db
+	pub async fn init_table<C: ConnectionTrait>(conn: &C) -> Result<(), WatermarkError> {
+		// Create main table
+		conn.execute(Statement::from_string(
+			DbBackend::Sqlite,
+			r#"
+			CREATE TABLE IF NOT EXISTS peer_received_watermarks (
+				device_uuid TEXT NOT NULL,
+				peer_device_uuid TEXT NOT NULL,
+				max_received_hlc TEXT NOT NULL,
+				updated_at TEXT NOT NULL,
+				PRIMARY KEY (device_uuid, peer_device_uuid)
+			)
+			"#
+			.to_string(),
+		))
+		.await
+		.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+		// Create index for efficient peer queries
+		conn.execute(Statement::from_string(
+			DbBackend::Sqlite,
+			"CREATE INDEX IF NOT EXISTS idx_peer_received_watermarks_peer
+			 ON peer_received_watermarks(peer_device_uuid)"
+				.to_string(),
+		))
+		.await
+		.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+		Ok(())
+	}
+
+	/// Get max HLC received from a specific peer
+	pub async fn get<C: ConnectionTrait>(
+		&self,
+		conn: &C,
+		peer_device_uuid: Uuid,
+	) -> Result<Option<HLC>, WatermarkError> {
+		let row = conn
+			.query_one(Statement::from_sql_and_values(
+				DbBackend::Sqlite,
+				"SELECT max_received_hlc FROM peer_received_watermarks
+				 WHERE device_uuid = ? AND peer_device_uuid = ?",
+				vec![
+					self.device_uuid.to_string().into(),
+					peer_device_uuid.to_string().into(),
+				],
+			))
+			.await
+			.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+		match row {
+			Some(row) => {
+				let hlc_str: String = row
+					.try_get("", "max_received_hlc")
+					.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+				let hlc = hlc_str
+					.parse()
+					.map_err(|e: crate::infra::sync::hlc::HLCError| {
+						WatermarkError::ParseError(e.to_string())
+					})?;
+
+				Ok(Some(hlc))
+			}
+			None => Ok(None),
+		}
+	}
+
+	/// Update max HLC received from peer (only if newer)
+	pub async fn upsert<C: ConnectionTrait>(
+		&self,
+		conn: &C,
+		peer_device_uuid: Uuid,
+		received_hlc: HLC,
+	) -> Result<(), WatermarkError> {
+		// Prevent self-watermarks
+		if peer_device_uuid == self.device_uuid {
+			tracing::warn!(
+				device_uuid = %self.device_uuid,
+				peer_device_uuid = %peer_device_uuid,
+				"Attempted to track received HLC from self - skipping"
+			);
+			return Ok(());
+		}
+
+		// Check if newer before updating
+		let existing = self.get(conn, peer_device_uuid).await?;
+
+		if let Some(existing_hlc) = existing {
+			if received_hlc <= existing_hlc {
+				return Ok(());
+			}
+		}
+
+		// Upsert
+		conn.execute(Statement::from_sql_and_values(
+			DbBackend::Sqlite,
+			r#"
+			INSERT INTO peer_received_watermarks
+			(device_uuid, peer_device_uuid, max_received_hlc, updated_at)
+			VALUES (?, ?, ?, ?)
+			ON CONFLICT (device_uuid, peer_device_uuid)
+			DO UPDATE SET
+				max_received_hlc = excluded.max_received_hlc,
+				updated_at = excluded.updated_at
+			"#,
+			vec![
+				self.device_uuid.to_string().into(),
+				peer_device_uuid.to_string().into(),
+				received_hlc.to_string().into(),
+				chrono::Utc::now().to_rfc3339().into(),
+			],
+		))
+		.await
+		.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+		Ok(())
+	}
+
+	/// Get all received watermarks (for diagnostics)
+	pub async fn get_all<C: ConnectionTrait>(
+		&self,
+		conn: &C,
+	) -> Result<HashMap<Uuid, HLC>, WatermarkError> {
+		let rows = conn
+			.query_all(Statement::from_sql_and_values(
+				DbBackend::Sqlite,
+				"SELECT peer_device_uuid, max_received_hlc FROM peer_received_watermarks
+				 WHERE device_uuid = ?",
+				vec![self.device_uuid.to_string().into()],
+			))
+			.await
+			.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+		let mut results = HashMap::new();
+		for row in rows {
+			let peer_str: String = row
+				.try_get("", "peer_device_uuid")
+				.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+			let hlc_str: String = row
+				.try_get("", "max_received_hlc")
+				.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+			if let (Ok(peer_uuid), Ok(hlc)) = (Uuid::parse_str(&peer_str), hlc_str.parse()) {
+				results.insert(peer_uuid, hlc);
+			}
+		}
+
+		Ok(results)
+	}
+
+	/// Get the maximum HLC received from any peer (for catch-up decisions)
+	pub async fn get_max_across_all_peers<C: ConnectionTrait>(
+		&self,
+		conn: &C,
+	) -> Result<Option<HLC>, WatermarkError> {
+		let row = conn
+			.query_one(Statement::from_sql_and_values(
+				DbBackend::Sqlite,
+				"SELECT MAX(max_received_hlc) as max_hlc FROM peer_received_watermarks
+				 WHERE device_uuid = ?",
+				vec![self.device_uuid.to_string().into()],
+			))
+			.await
+			.map_err(|e| WatermarkError::QueryError(e.to_string()))?;
+
+		match row {
+			Some(row) => {
+				let hlc_str: Option<String> = row.try_get("", "max_hlc").ok();
+
+				match hlc_str {
+					Some(s) => {
+						let hlc = s.parse().map_err(|e: crate::infra::sync::hlc::HLCError| {
+							WatermarkError::ParseError(e.to_string())
+						})?;
+						Ok(Some(hlc))
+					}
+					None => Ok(None),
+				}
+			}
+			None => Ok(None),
+		}
+	}
+}
+
+/// Watermark errors
+#[derive(Debug, thiserror::Error)]
+pub enum WatermarkError {
+	#[error("Database query error: {0}")]
+	QueryError(String),
+
+	#[error("Parse error: {0}")]
+	ParseError(String),
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use sea_orm::Database;
+	use tempfile::TempDir;
+
+	async fn create_test_db() -> (sea_orm::DatabaseConnection, TempDir) {
+		let temp_dir = TempDir::new().unwrap();
+		let db_path = temp_dir.path().join("test_peer_watermarks.db");
+		let database_url = format!("sqlite://{}?mode=rwc", db_path.display());
+		let conn = Database::connect(&database_url).await.unwrap();
+
+		PeerWatermarkStore::init_table(&conn).await.unwrap();
+
+		(conn, temp_dir)
+	}
+
+	#[tokio::test]
+	async fn test_peer_watermark_persistence() {
+		let (conn, _temp) = create_test_db().await;
+
+		let device_uuid = Uuid::new_v4();
+		let peer = Uuid::new_v4();
+		let store = PeerWatermarkStore::new(device_uuid);
+
+		let hlc = HLC::now(peer);
+
+		// Store watermark
+		store.upsert(&conn, peer, hlc).await.unwrap();
+
+		// Query back
+		let retrieved = store.get(&conn, peer).await.unwrap();
+		assert_eq!(retrieved, Some(hlc));
+	}
+
+	#[tokio::test]
+	async fn test_prevent_self_watermark() {
+		let (conn, _temp) = create_test_db().await;
+
+		let device_uuid = Uuid::new_v4();
+		let store = PeerWatermarkStore::new(device_uuid);
+
+		let hlc = HLC::now(device_uuid);
+
+		// Attempt to track self
+		let result = store.upsert(&conn, device_uuid, hlc).await;
+		assert!(result.is_ok());
+
+		// Verify not stored
+		let retrieved = store.get(&conn, device_uuid).await.unwrap();
+		assert_eq!(retrieved, None);
+	}
+
+	#[tokio::test]
+	async fn test_watermark_only_advances() {
+		let (conn, _temp) = create_test_db().await;
+
+		let device_uuid = Uuid::new_v4();
+		let peer = Uuid::new_v4();
+		let store = PeerWatermarkStore::new(device_uuid);
+
+		// Store HLC(1000)
+		let hlc1 = HLC {
+			timestamp: 1000,
+			counter: 0,
+			device_id: peer,
+		};
+		store.upsert(&conn, peer, hlc1).await.unwrap();
+
+		// Try to store older HLC(500)
+		let hlc2 = HLC {
+			timestamp: 500,
+			counter: 0,
+			device_id: peer,
+		};
+		store.upsert(&conn, peer, hlc2).await.unwrap();
+
+		// Should still be HLC(1000)
+		let retrieved = store.get(&conn, peer).await.unwrap();
+		assert_eq!(retrieved.unwrap().timestamp, 1000);
+	}
+
+	#[tokio::test]
+	async fn test_get_max_across_all_peers() {
+		let (conn, _temp) = create_test_db().await;
+
+		let device_uuid = Uuid::new_v4();
+		let store = PeerWatermarkStore::new(device_uuid);
+
+		let peer1 = Uuid::new_v4();
+		let peer2 = Uuid::new_v4();
+		let peer3 = Uuid::new_v4();
+
+		// Store different HLCs from different peers
+		let hlc1 = HLC {
+			timestamp: 1000,
+			counter: 0,
+			device_id: peer1,
+		};
+		let hlc2 = HLC {
+			timestamp: 2000,
+			counter: 0,
+			device_id: peer2,
+		};
+		let hlc3 = HLC {
+			timestamp: 1500,
+			counter: 0,
+			device_id: peer3,
+		};
+
+		store.upsert(&conn, peer1, hlc1).await.unwrap();
+		store.upsert(&conn, peer2, hlc2).await.unwrap();
+		store.upsert(&conn, peer3, hlc3).await.unwrap();
+
+		// Get max should return peer2's HLC(2000)
+		let max = store.get_max_across_all_peers(&conn).await.unwrap();
+		assert!(max.is_some());
+		assert_eq!(max.unwrap().timestamp, 2000);
+	}
+
+	#[tokio::test]
+	async fn test_get_all() {
+		let (conn, _temp) = create_test_db().await;
+
+		let device_uuid = Uuid::new_v4();
+		let store = PeerWatermarkStore::new(device_uuid);
+
+		let peer1 = Uuid::new_v4();
+		let peer2 = Uuid::new_v4();
+
+		let hlc1 = HLC::now(peer1);
+		let hlc2 = HLC::now(peer2);
+
+		store.upsert(&conn, peer1, hlc1).await.unwrap();
+		store.upsert(&conn, peer2, hlc2).await.unwrap();
+
+		let all = store.get_all(&conn).await.unwrap();
+		assert_eq!(all.len(), 2);
+		assert_eq!(all.get(&peer1), Some(&hlc1));
+		assert_eq!(all.get(&peer2), Some(&hlc2));
+	}
+}

core/src/service/sync/backfill.rs

@@ -159,7 +159,7 @@ impl BackfillManager {
 		self.peer_sync.transition_to_ready().await?;
 
 		// Phase 5: Set initial watermarks from actual received data (not local DB query)
-		self.set_initial_watermarks_after_backfill(final_state_checkpoint, max_shared_hlc)
+		self.set_initial_watermarks_after_backfill(selected_peer, final_state_checkpoint, max_shared_hlc)
 			.await?;
 
 		// Record metrics
@@ -222,7 +222,7 @@ impl BackfillManager {
 			.await?;
 
 		// Update watermarks from actual received data (not local DB query)
-		self.set_initial_watermarks_after_backfill(final_state_checkpoint, max_shared_hlc)
+		self.set_initial_watermarks_after_backfill(peer, final_state_checkpoint, max_shared_hlc)
 			.await?;
 
 		// Record metrics
@@ -1074,11 +1074,12 @@ impl BackfillManager {
 	/// Uses actual checkpoints from received data, not local database queries
 	async fn set_initial_watermarks_after_backfill(
 		&self,
+		peer: Uuid,
 		final_state_checkpoint: Option<String>,
 		max_shared_hlc: Option<crate::infra::sync::HLC>,
 	) -> Result<()> {
 		self.peer_sync
-			.set_initial_watermarks(final_state_checkpoint, max_shared_hlc)
+			.set_initial_watermarks(peer, final_state_checkpoint, max_shared_hlc)
 			.await
 	}
 }

core/src/service/sync/peer.rs

@@ -327,7 +327,7 @@ impl PeerSync {
 	/// Query watermarks from sync.db (per-resource aggregation)
 	///
 	/// For state watermark: Returns the maximum (most recent) timestamp across all resources
-	/// For shared watermark: Returns the maximum HLC from peer log
+	/// For shared watermark: Returns the maximum HLC received from any peer
 	async fn query_device_watermarks(
 		device_id: Uuid,
 		peer_log: &Arc<crate::infra::sync::PeerLog>,
@@ -348,14 +348,18 @@ impl PeerSync {
 			}
 		};
 
-		// Get max shared watermark from peer log
+		// Get max shared watermark from peer_received_watermarks (not our peer log!)
-		let shared_watermark = match peer_log.get_max_hlc().await {
+		// This tracks what we've RECEIVED from peers, not what we've SENT
+		let shared_watermark = match crate::infra::sync::PeerWatermarkStore::new(device_id)
+			.get_max_across_all_peers(peer_log.conn())
+			.await
+		{
 			Ok(max) => max,
 			Err(e) => {
 				warn!(
 					device_id = %device_id,
 					error = %e,
-					"Failed to query max HLC from peer log"
+					"Failed to query received HLC watermarks from peers"
 				);
 				None
 			}
@@ -368,7 +372,7 @@ impl PeerSync {
 	///
 	/// Returns (state_watermark, shared_watermark) aggregated from sync.db.
 	/// State watermark: Maximum timestamp across all per-resource watermarks.
-	/// Shared watermark (HLC): Maximum HLC from peer log.
+	/// Shared watermark (HLC): Maximum HLC received from any peer.
 	pub async fn get_watermarks(&self) -> (Option<chrono::DateTime<chrono::Utc>>, Option<HLC>) {
 		Self::query_device_watermarks(self.device_id, &self.peer_log).await
 	}
@@ -389,16 +393,18 @@ impl PeerSync {
 		Ok(())
 	}
 
-	/// Mark backfill complete by updating last_sync_at
+	/// Mark backfill complete by updating last_sync_at and persisting shared watermark
 	///
 	/// Note: Per-resource watermarks are now tracked automatically as data is received.
-	/// This method only updates the last_sync_at timestamp.
+	/// This method updates the last_sync_at timestamp and persists the shared watermark.
 	pub async fn set_initial_watermarks(
 		&self,
+		peer: Uuid,
 		_final_state_checkpoint: Option<String>,
 		max_shared_hlc: Option<crate::infra::sync::HLC>,
 	) -> Result<()> {
 		use crate::infra::db::entities;
+		use crate::infra::sync::PeerWatermarkStore;
 		use sea_orm::{ActiveModelTrait, ActiveValue::Set, ColumnTrait, EntityTrait, QueryFilter};
 
 		let now = chrono::Utc::now();
@@ -410,6 +416,18 @@ impl PeerSync {
 				hlc = %hlc,
 				"Updated HLC generator from backfill max HLC"
 			);
+
+			// Persist the received watermark to survive restarts
+			PeerWatermarkStore::new(self.device_id)
+				.upsert(self.peer_log.conn(), peer, hlc)
+				.await
+				.map_err(|e| anyhow::anyhow!("Failed to persist peer watermark: {}", e))?;
+
+			info!(
+				peer = %peer,
+				max_received_hlc = %hlc,
+				"Persisted shared watermark for peer"
+			);
 		}
 
 		// Update last_sync_at to mark backfill complete