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//! Background sync task for CrdtDoc.
//!
//! Handles three event sources via `tokio::select!`:
//! 1. Local updates from Loro's `subscribe_local_update` callback
//! 2. Incoming sync messages from peers (namespace `"crdt:{doc_id}"`)
//! 3. Peer join/leave events (for sync-on-join and cleanup-on-leave)
use std::sync::Arc;
use tokio::sync::{broadcast, mpsc};
use crate::network::NetworkProvider;
use crate::node::Node;
use crate::session::PeerEvent;
use super::backend::CrdtBackend;
use super::types::{CrdtDocEvent, CrdtSyncMessage};
/// Compaction threshold in bytes. When accumulated local updates exceed this
/// size, the document is compacted into a snapshot on disk.
const COMPACTION_THRESHOLD: usize = 256 * 1024; // 256 KB
/// Spawn the background sync task.
///
/// Returns a `JoinHandle` that can be aborted to stop the task.
pub(super) fn spawn_sync_task<N: NetworkProvider + 'static>(
node: Arc<Node<N>>,
doc: Arc<std::sync::Mutex<loro::LoroDoc>>,
doc_id: String,
backend: Arc<dyn CrdtBackend>,
event_tx: broadcast::Sender<CrdtDocEvent>,
mut local_update_rx: mpsc::UnboundedReceiver<Vec<u8>>,
mut compact_rx: mpsc::UnboundedReceiver<()>,
) -> tokio::task::JoinHandle<()> {
let namespace = format!("crdt:{doc_id}");
tokio::spawn(async move {
let mut msg_rx = node.subscribe(&namespace);
let mut peer_rx = node.on_peer_change();
let mut accumulated_update_bytes: usize = 0;
tracing::info!(doc_id = doc_id.as_str(), "crdt_doc: sync task started");
loop {
tokio::select! {
// ── Arm 1: local updates from Loro subscribe_local_update ──
Some(bytes) = local_update_rx.recv() => {
// Persist the update.
backend.save_update(&doc_id, &bytes);
accumulated_update_bytes += bytes.len();
// Broadcast to peers.
let sync_msg = CrdtSyncMessage::Update { data: bytes };
if let Ok(payload) = serde_json::to_value(&sync_msg) {
node.broadcast_typed(&namespace, "update", &payload).await;
}
// Compact if accumulated updates exceed threshold.
if accumulated_update_bytes > COMPACTION_THRESHOLD {
let snapshot = {
let doc = doc.lock().unwrap();
doc.export(loro::ExportMode::Snapshot)
};
match snapshot {
Ok(data) => {
backend.save_snapshot(&doc_id, &data);
accumulated_update_bytes = 0;
tracing::debug!(
doc_id = doc_id.as_str(),
"crdt_doc: compacted to snapshot"
);
}
Err(e) => {
tracing::warn!(
doc_id = doc_id.as_str(),
"crdt_doc: compaction failed: {e}"
);
}
}
}
let _ = event_tx.send(CrdtDocEvent::LocalChange);
}
// ── Arm 2: incoming sync messages from peers ──
result = msg_rx.recv() => {
match result {
Ok(msg) => {
let sync_msg: CrdtSyncMessage = match serde_json::from_value(msg.payload) {
Ok(m) => m,
Err(e) => {
tracing::warn!(
doc_id = doc_id.as_str(),
from = msg.from.as_str(),
"crdt_doc: failed to parse sync message: {e}"
);
continue;
}
};
match sync_msg {
CrdtSyncMessage::Update { data } => {
let import_result = {
let doc = doc.lock().unwrap();
doc.import(&data)
};
match import_result {
Ok(_) => {
let _ = event_tx.send(CrdtDocEvent::RemoteChange {
from: msg.from.clone(),
});
}
Err(e) => {
tracing::warn!(
doc_id = doc_id.as_str(),
from = msg.from.as_str(),
"crdt_doc: failed to import update: {e}"
);
}
}
}
CrdtSyncMessage::SyncRequest { version_vector } => {
// Compute delta from their version vector.
let their_vv = match loro::VersionVector::decode(&version_vector) {
Ok(vv) => vv,
Err(e) => {
tracing::warn!(
doc_id = doc_id.as_str(),
from = msg.from.as_str(),
"crdt_doc: failed to decode version vector: {e}"
);
continue;
}
};
let delta = {
let doc = doc.lock().unwrap();
doc.export(loro::ExportMode::updates(&their_vv))
};
match delta {
Ok(data) => {
let response = CrdtSyncMessage::SyncResponse { data };
if let Ok(payload) = serde_json::to_value(&response) {
if let Err(e) = node.send_typed(
&msg.from,
&namespace,
"sync_response",
&payload,
).await {
tracing::warn!(
doc_id = doc_id.as_str(),
peer = msg.from.as_str(),
"crdt_doc: failed to send SyncResponse: {e}"
);
}
}
}
Err(e) => {
tracing::warn!(
doc_id = doc_id.as_str(),
"crdt_doc: failed to export delta: {e}"
);
// Fall back to sending a full snapshot.
let snapshot = {
let doc = doc.lock().unwrap();
doc.export(loro::ExportMode::Snapshot)
};
if let Ok(data) = snapshot {
let snap_msg = CrdtSyncMessage::Snapshot { data };
if let Ok(payload) = serde_json::to_value(&snap_msg) {
let _ = node.send_typed(
&msg.from,
&namespace,
"snapshot",
&payload,
).await;
}
}
}
}
}
CrdtSyncMessage::SyncResponse { data } | CrdtSyncMessage::Snapshot { data } => {
let import_result = {
let doc = doc.lock().unwrap();
doc.import(&data)
};
match import_result {
Ok(_) => {
let _ = event_tx.send(CrdtDocEvent::PeerSynced {
peer_id: msg.from.clone(),
});
}
Err(e) => {
tracing::warn!(
doc_id = doc_id.as_str(),
from = msg.from.as_str(),
"crdt_doc: failed to import sync response: {e}"
);
}
}
}
}
}
Err(broadcast::error::RecvError::Lagged(n)) => {
tracing::warn!(
doc_id = doc_id.as_str(),
missed = n,
"crdt_doc: message receiver lagged"
);
}
Err(broadcast::error::RecvError::Closed) => {
tracing::debug!(doc_id = doc_id.as_str(), "crdt_doc: message channel closed");
break;
}
}
}
// ── Arm 3: peer join/leave events ──
result = peer_rx.recv() => {
match result {
Ok(PeerEvent::Joined(state)) => {
// Send a SyncRequest with our version vector.
let vv_bytes = {
let doc = doc.lock().unwrap();
doc.state_vv().encode()
};
let request = CrdtSyncMessage::SyncRequest {
version_vector: vv_bytes,
};
if let Ok(payload) = serde_json::to_value(&request) {
if let Err(e) = node.send_typed(
&state.id,
&namespace,
"sync_request",
&payload,
).await {
tracing::warn!(
doc_id = doc_id.as_str(),
peer = state.id.as_str(),
"crdt_doc: failed to send SyncRequest to new peer: {e}"
);
}
}
}
Ok(PeerEvent::Left(peer_id)) => {
let _ = event_tx.send(CrdtDocEvent::PeerLeft {
peer_id: peer_id.clone(),
});
tracing::debug!(
doc_id = doc_id.as_str(),
peer = peer_id.as_str(),
"crdt_doc: peer left"
);
}
Ok(_) => {} // Updated, WsConnected, WsDisconnected, AuthRequired — ignore
Err(broadcast::error::RecvError::Lagged(n)) => {
tracing::warn!(
doc_id = doc_id.as_str(),
missed = n,
"crdt_doc: peer event receiver lagged"
);
}
Err(broadcast::error::RecvError::Closed) => {
tracing::debug!(doc_id = doc_id.as_str(), "crdt_doc: peer event channel closed");
break;
}
}
}
// ── Arm 4: manual compaction notification ──
Some(()) = compact_rx.recv() => {
accumulated_update_bytes = 0;
tracing::debug!(
doc_id = doc_id.as_str(),
"crdt_doc: sync task reset accumulated_update_bytes after manual compact"
);
}
}
}
tracing::info!(doc_id = doc_id.as_str(), "crdt_doc: sync task stopped");
})
}