diff --git a/clustinator/receiver.py b/clustinator/receiver.py
index 5bbf989..8950d41 100644
--- a/clustinator/receiver.py
+++ b/clustinator/receiver.py
@@ -26,6 +26,7 @@ def __init__(self, rabbitmq_host, rabbitmq_port, elastic_host, timeout):
         
             connection = pika.BlockingConnection(pika.ConnectionParameters(host=rabbitmq_host, port=rabbitmq_port, heartbeat=timeout))
             channel = connection.channel()
+            channel.basic_qos(prefetch_count = 1) # receive at most 1 unacked message at a time
             self.channel = channel
             
             clustering_queue = 'continuity.clustinator.task.clustinator.cluster'
diff --git a/clustinator/session_matrix_creator.py b/clustinator/session_matrix_creator.py
index 2292549..a43f2cc 100644
--- a/clustinator/session_matrix_creator.py
+++ b/clustinator/session_matrix_creator.py
@@ -31,11 +31,18 @@ def create(self):
         
         return matrix
     
-    def update_group_ids(self, sessions_per_cluster):
+    def update_group_ids(self, sessions_per_cluster, chunk_size = 2000):
         """
         Updates the group IDs of the sessions as specified in the passed dict.
         :param sessions_per_cluster: Dict of session IDs per group (cluster) ID.
+        :param chunk_size: The number of sessions to update with one request. Defaults to 2000.
         """
         
         for beh_id, session_ids in sessions_per_cluster.items():
-            self._elastic.set_group_ids(beh_id, session_ids)
+            
+            for chunk in [
+                session_ids[i * chunk_size:(i + 1) * chunk_size]
+                for i in range((len(session_ids) + chunk_size - 1) // chunk_size )
+                ]:
+                
+                self._elastic.set_group_ids(beh_id, chunk)