feat(meta): align splits with source actor with an upstream

src/meta/src/barrier/command.rs

@@ -461,7 +461,7 @@ impl CommandContext {
                         // Find the actors of the upstream fragment.
                         let upstream_actor_ids = self
                             .fragment_manager
-                            .get_running_actors_of_fragment(upstream_fragment_id)
+                            .get_running_actor_ids_of_fragment(upstream_fragment_id)
                             .await?;
 
                         // Record updates for all actors.
@@ -495,7 +495,7 @@ impl CommandContext {
                         // Find the actors of the downstream fragment.
                         let downstream_actor_ids = self
                             .fragment_manager
-                            .get_running_actors_of_fragment(downstream_fragment_id)
+                            .get_running_actor_ids_of_fragment(downstream_fragment_id)
                             .await?;
 
                         // Downstream removed actors should be skipped

src/meta/src/manager/catalog/fragment.rs

@@ -1034,7 +1034,7 @@ impl FragmentManager {
     }
 
     /// Get the actor ids of the fragment with `fragment_id` with `Running` status.
-    pub async fn get_running_actors_of_fragment(
+    pub async fn get_running_actor_ids_of_fragment(
         &self,
         fragment_id: FragmentId,
     ) -> MetaResult<HashSet<ActorId>> {
@@ -1055,6 +1055,29 @@ impl FragmentManager {
         bail!("fragment not found: {}", fragment_id)
     }
 
+    pub async fn get_running_actors_and_upstream_fragment_of_fragment(
+        &self,
+        fragment_id: FragmentId,
+    ) -> MetaResult<(Vec<StreamActor>, Vec<FragmentId>)> {
+        let map = &self.core.read().await.table_fragments;
+
+        for table_fragment in map.values() {
+            if let Some(fragment) = table_fragment.fragments.get(&fragment_id) {
+                let running_actors = fragment
+                    .actors
+                    .iter()
+                    .filter(|a| {
+                        table_fragment.actor_status[&a.actor_id].state == ActorState::Running as i32
+                    })
+                    .cloned()
+                    .collect();
+                return Ok((running_actors, fragment.upstream_fragment_ids.clone()));
+            }
+        }
+
+        bail!("fragment not found: {}", fragment_id)
+    }
+
     /// Add the newly added Actor to the `FragmentManager`
     pub async fn pre_apply_reschedules(
         &self,

src/meta/src/stream/source_manager.rs

@@ -273,28 +273,63 @@ impl SourceManagerCore {
             };
 
             let Some(discovered_splits) = handle.discovered_splits().await else {
-                return Ok(split_assignment);
+                continue;
             };
             if discovered_splits.is_empty() {
                 tracing::warn!("No splits discovered for source {}", source_id);
             }
 
+            let mut source_fragments = vec![];
+            let mut backfill_fragments = vec![];
+
             for fragment_id in fragment_ids {
-                let actor_ids = match self
+                let (actors, upstream_fragment_ids) = match self
                     .fragment_manager
-                    .get_running_actors_of_fragment(*fragment_id)
+                    .get_running_actors_and_upstream_fragment_of_fragment(*fragment_id)
                     .await
                 {
-                    Ok(actor_ids) => actor_ids,
+                    Ok((actors, upstream_fragment_ids)) => {
+                        if actors.is_empty() {
+                            tracing::warn!("No actors found for fragment {}", fragment_id);
+                            continue;
+                        }
+                        (actors, upstream_fragment_ids)
+                    }
                     Err(err) => {
                         tracing::warn!("Failed to get the actor of the fragment {}, maybe the fragment doesn't exist anymore", err.to_string());
                         continue;
                     }
                 };
 
-                let prev_actor_splits: HashMap<_, _> = actor_ids
+                if !upstream_fragment_ids.is_empty() {
+                    debug_assert!(
+                        upstream_fragment_ids.len() == 1,
+                        "source backfill fragment should have exactly one upstream fragment, fragment_id: {fragment_id}, upstream_fragment_ids: {upstream_fragment_ids:?}"
+                    );
+                    for actor in &actors {
+                        debug_assert!(
+                            actor.upstream_actor_id.len() == 1,
+                            "source backfill actor should have exactly one upstream actor, fragment_id: {fragment_id}, actor: {actor:?}"
+                        );
+                    }
+                    backfill_fragments.push((*fragment_id, upstream_fragment_ids[0], actors));
+                } else {
+                    for actor in &actors {
+                        debug_assert!(
+                            actor.upstream_actor_id.is_empty(),
+                            "source actor should not have upstream actors, fragment_id: {fragment_id}, actor: {actor:?}"
+                        );
+                    }
+                    source_fragments.push((*fragment_id, actors));
+                }
+            }
+
+            // assign splits for source fragments first
+            for (fragment_id, actors) in source_fragments {
+                let prev_actor_splits: HashMap<_, _> = actors
                     .into_iter()
-                    .map(|actor_id| {
+                    .map(|actor| {
+                        let actor_id = actor.actor_id;
                         (
                             actor_id,
                             self.actor_splits
@@ -306,16 +341,40 @@ impl SourceManagerCore {
                     .collect();
 
                 if let Some(new_assignment) = reassign_splits(
-                    *fragment_id,
+                    fragment_id,
                     prev_actor_splits,
                     &discovered_splits,
                     SplitDiffOptions {
                         enable_scale_in: handle.enable_scale_in,
                     },
                 ) {
-                    split_assignment.insert(*fragment_id, new_assignment);
+                    split_assignment.insert(fragment_id, new_assignment);
                 }
             }
+
+            // align splits for backfill fragments with its upstream source fragment
+            for (fragment_id, upstream_fragment_id, actors) in backfill_fragments {
+                let upstream_assignment = split_assignment
+                    .get(&upstream_fragment_id)
+                    .unwrap_or_else(||panic!(
+                        "source backfill fragment's upstream fragment should have assignment, fragment_id: {fragment_id}, upstream_fragment_id: {upstream_fragment_id}, split_assignment: {split_assignment:?}"));
+                split_assignment.insert(
+                    fragment_id,
+                    actors
+                        .into_iter()
+                        .map(|a| {
+                            let actor_id = a.actor_id;
+                            (
+                                actor_id,
+                                upstream_assignment
+                                    .get(&actor_id)
+                                    .cloned()
+                                    .unwrap_or_else(||panic!("source backfill actor should have upstream actor, fragment_id: {fragment_id}, upstream_fragment_id: {upstream_fragment_id}, actor: {a:?}, upstream_assignment: {upstream_assignment:?}")),
+                            )
+                        })
+                        .collect(),
+                );
+            }
         }
 
         Ok(split_assignment)
@@ -430,9 +489,12 @@ impl Default for SplitDiffOptions {
     }
 }
 
-/// Reassigns splits if there are new splits or dropped splits,
+/// Reassigns splits (for a fragment's actors) if there are new splits or dropped splits,
 /// i.e., `actor_splits` and `discovered_splits` differ.
 ///
+/// If an actor has an upstream actor, it should be a backfill executor,
+/// and its splits should be aligned with the upstream actor. `reassign_splits` should not be used in this case.
+///
 /// - `fragment_id`: just for logging
 fn reassign_splits<T>(
     fragment_id: FragmentId,