diff --git a/rust/numaflow-core/src/tracker.rs b/rust/numaflow-core/src/tracker.rs
index 2c23cef99..6f5a08937 100644
--- a/rust/numaflow-core/src/tracker.rs
+++ b/rust/numaflow-core/src/tracker.rs
@@ -23,7 +23,7 @@ use crate::Result;
 #[derive(Debug)]
 struct TrackerEntry {
     ack_send: oneshot::Sender<ReadAck>,
-    count: u32,
+    count: usize,
     eof: bool,
     callback_info: Option<CallbackInfo>,
 }
@@ -42,6 +42,7 @@ enum ActorMessage {
     UpdateMany {
         offset: Bytes,
         responses: Vec<Option<Vec<String>>>,
+        count: usize,
         eof: bool,
     },
     UpdateEOF {
@@ -170,15 +171,16 @@ impl Tracker {
             ActorMessage::UpdateMany {
                 offset,
                 responses,
+                count,
                 eof,
             } => {
-                self.handle_update_many(offset, responses, eof);
+                self.handle_update_many(offset, responses, count, eof);
             }
             ActorMessage::UpdateEOF { offset } => {
-                self.handle_update_eof(offset);
+                self.handle_update_eof(offset).await;
             }
             ActorMessage::Delete { offset } => {
-                self.handle_delete(offset);
+                self.handle_delete(offset).await;
             }
             ActorMessage::Discard { offset } => {
                 self.handle_discard(offset);
@@ -223,20 +225,9 @@ impl Tracker {
             .callback_info
             .as_mut()
             .map(|cb| cb.responses.push(responses));
-
-        // if the count is zero, we can send an ack immediately
-        // this is case where map stream will send eof true after
-        // receiving all the messages.
-        if entry.count == 0 {
-            let entry = self.entries.remove(&offset).unwrap();
-            entry
-                .ack_send
-                .send(ReadAck::Ack)
-                .expect("Failed to send ack");
-        }
     }
 
-    fn handle_update_eof(&mut self, offset: Bytes) {
+    async fn handle_update_eof(&mut self, offset: Bytes) {
         let Some(entry) = self.entries.get_mut(&offset) else {
             return;
         };
@@ -246,10 +237,7 @@ impl Tracker {
         // receiving all the messages.
         if entry.count == 0 {
             let entry = self.entries.remove(&offset).unwrap();
-            entry
-                .ack_send
-                .send(ReadAck::Ack)
-                .expect("Failed to send ack");
+            self.ack_message(entry).await;
         }
     }
 
@@ -257,34 +245,24 @@ impl Tracker {
         &mut self,
         offset: Bytes,
         responses: Vec<Option<Vec<String>>>,
+        count: usize,
         eof: bool,
     ) {
         let Some(entry) = self.entries.get_mut(&offset) else {
             return;
         };
 
-        entry.count += responses.len() as u32;
+        entry.count += count;
         entry.eof = eof;
         entry
             .callback_info
             .as_mut()
             .map(|cb| cb.responses.extend(responses));
-
-        // if the count is zero, we can send an ack immediately
-        // this is case where map stream will send eof true after
-        // receiving all the messages.
-        if entry.count == 0 {
-            let entry = self.entries.remove(&offset).unwrap();
-            entry
-                .ack_send
-                .send(ReadAck::Ack)
-                .expect("Failed to send ack");
-        }
     }
 
     /// Removes an entry from the tracker and sends an acknowledgment if the count is zero
     /// and the entry is marked as EOF.
-    fn handle_delete(&mut self, offset: Bytes) {
+    async fn handle_delete(&mut self, offset: Bytes) {
         let Some(mut entry) = self.entries.remove(&offset) else {
             return;
         };
@@ -292,10 +270,7 @@ impl Tracker {
             entry.count -= 1;
         }
         if entry.count == 0 && entry.eof {
-            entry
-                .ack_send
-                .send(ReadAck::Ack)
-                .expect("Failed to send ack");
+            self.ack_message(entry).await;
         } else {
             self.entries.insert(offset, entry);
         }
@@ -321,6 +296,37 @@ impl Tracker {
                 .expect("Failed to send nak");
         }
     }
+
+    async fn ack_message(&self, entry: TrackerEntry) {
+        let TrackerEntry {
+            ack_send,
+            callback_info,
+            ..
+        } = entry;
+
+        ack_send.send(ReadAck::Ack).expect("Failed to send ack");
+
+        let Some(ref callback_handler) = self.callback_handler else {
+            return;
+        };
+        let Some(callback_info) = callback_info else {
+            tracing::error!("Callback is enabled, but Tracker doesn't contain callback info");
+            return;
+        };
+
+        let id = callback_info.id.clone();
+        let result = callback_handler
+            .callback(
+                callback_info.id,
+                callback_info.callback_url,
+                callback_info.from_vertex,
+                callback_info.responses,
+            )
+            .await;
+        if let Err(e) = result {
+            tracing::error!(?e, id, "Failed to send callback");
+        }
+    }
 }
 
 /// TrackerHandle provides an interface to interact with the Tracker.
@@ -419,6 +425,7 @@ impl TrackerHandle {
         let message = ActorMessage::UpdateMany {
             offset,
             responses,
+            count: messages.len(),
             eof,
         };
         self.sender