From d3722abdbd1e7e0ccb377f070dce4b7114f635ae Mon Sep 17 00:00:00 2001
From: Somtochi Onyekwere <somtochionyekwere@gmail.com>
Date: Thu, 4 Jul 2024 15:40:55 +0100
Subject: [PATCH] order nodes chosen for sync by last sync time also

---
 crates/corro-agent/src/agent/handlers.rs | 22 +++++++++++++++++++---
 crates/corro-types/src/broadcast.rs      |  2 +-
 crates/corro-types/src/members.rs        |  8 ++++++++
 3 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/crates/corro-agent/src/agent/handlers.rs b/crates/corro-agent/src/agent/handlers.rs
index e4d7e4c1..6a7e079d 100644
--- a/crates/corro-agent/src/agent/handlers.rs
+++ b/crates/corro-agent/src/agent/handlers.rs
@@ -792,8 +792,15 @@ pub async fn handle_sync(
                     **id != agent.actor_id() && state.cluster_id == agent.cluster_id()
                 })
                 // Grab a ring-buffer index to the member RTT range
-                .map(|(id, state)| (*id, state.ring.unwrap_or(255), state.addr))
-                .collect::<Vec<(ActorId, u8, SocketAddr)>>()
+                .map(|(id, state)| {
+                    (
+                        *id,
+                        state.ring.unwrap_or(255),
+                        state.addr,
+                        state.last_sync_ts,
+                    )
+                })
+                .collect::<Vec<(ActorId, u8, SocketAddr, Option<Timestamp>)>>()
         };
 
         if candidates.is_empty() {
@@ -816,6 +823,8 @@ pub async fn handle_sync(
             sync_state
                 .need_len_for_actor(&b.0)
                 .cmp(&sync_state.need_len_for_actor(&a.0))
+                // if equal, look at last sync time
+                .then_with(|| a.3.cmp(&b.3))
                 // if equal, look at proximity (via `ring`)
                 .then_with(|| a.1.cmp(&b.1))
         });
@@ -823,7 +832,7 @@ pub async fn handle_sync(
         choices.truncate(desired_count);
         choices
             .into_iter()
-            .map(|(actor_id, _, addr)| (actor_id, addr))
+            .map(|(actor_id, _, addr, _)| (actor_id, addr))
             .collect()
     };
 
@@ -852,6 +861,7 @@ pub async fn handle_sync(
         info!(
             "synced {n} changes w/ {} in {}s @ {} changes/s",
             chosen
+                .clone()
                 .into_iter()
                 .map(|(actor_id, _)| actor_id.to_string())
                 .collect::<Vec<_>>()
@@ -859,6 +869,12 @@ pub async fn handle_sync(
             elapsed.as_secs_f64(),
             n as f64 / elapsed.as_secs_f64()
         );
+
+        let ts = Timestamp::from(agent.clock().new_timestamp());
+        for (actor_id, _) in chosen {
+            let mut members = agent.members().write();
+            members.update_sync_ts(actor_id, ts);
+        }
     }
     Ok(())
 }
diff --git a/crates/corro-types/src/broadcast.rs b/crates/corro-types/src/broadcast.rs
index ee859ad1..382bfce5 100644
--- a/crates/corro-types/src/broadcast.rs
+++ b/crates/corro-types/src/broadcast.rs
@@ -264,7 +264,7 @@ pub enum TimestampParseError {
     Parse(ParseNTP64Error),
 }
 
-#[derive(Debug, Default, Clone, Copy, Serialize, Deserialize, Eq, PartialOrd, Hash)]
+#[derive(Debug, Default, Clone, Copy, Serialize, Deserialize, Eq, PartialOrd, Ord, Hash)]
 #[serde(transparent)]
 pub struct Timestamp(pub NTP64);
 
diff --git a/crates/corro-types/src/members.rs b/crates/corro-types/src/members.rs
index c5c77fff..753b2df5 100644
--- a/crates/corro-types/src/members.rs
+++ b/crates/corro-types/src/members.rs
@@ -16,6 +16,7 @@ pub struct MemberState {
     pub cluster_id: ClusterId,
 
     pub ring: Option<u8>,
+    pub last_sync_ts: Option<Timestamp>,
 }
 
 impl MemberState {
@@ -25,6 +26,7 @@ impl MemberState {
             ts,
             cluster_id,
             ring: None,
+            last_sync_ts: None,
         }
     }
 
@@ -59,6 +61,12 @@ impl Members {
         self.states.get(id)
     }
 
+    pub fn update_sync_ts(&mut self, actor_id: ActorId, ts: Timestamp) {
+        if let Some(state) = self.states.get_mut(&actor_id) {
+            state.last_sync_ts = Some(ts);
+        }
+    }
+
     // A result of `true` means that the effective list of
     // cluster member addresses has changed
     pub fn add_member(&mut self, actor: &Actor) -> MemberAddedResult {