diff --git a/marconi/queues/storage/mongodb/messages.py b/marconi/queues/storage/mongodb/messages.py
index 63cad68af..ccb830815 100644
--- a/marconi/queues/storage/mongodb/messages.py
+++ b/marconi/queues/storage/mongodb/messages.py
@@ -24,6 +24,7 @@ Field Mappings:
 import datetime
 import time
 
+from bson import objectid
 import pymongo.errors
 import pymongo.read_preferences
 
@@ -66,6 +67,20 @@ ACTIVE_INDEX_FIELDS = [
     (PROJ_QUEUE, 1),  # Project will be unique, so put first
     ('k', 1),  # Used for sorting and paging, must come before range queries
     ('c.e', 1),  # Used for filtering out claimed messages
+
+    # NOTE(kgriffs): We do not include 'u' and 'tx' here on
+    # purpose. It was found experimentally that adding 'u' did
+    # not improve performance, and so it was left out in order
+    # to reduce index size and make updating the index
+    # faster. When 'tx' was added, it was assumed that it would
+    # follow a similar performance pattern to 'u', since by
+    # the time you traverse the index down past the fields
+    # listed above, there is very little left to scan, esp.
+    # considering all queries are limited (limit=) to a fairly
+    # small number.
+    #
+    # TODO(kgriffs): The extrapolation wrt 'tx' needs to be
+    # proven empirically.
 ]
 
 # For counting
@@ -88,6 +103,10 @@ MARKER_INDEX_FIELDS = [
     ('k', 1),
 ]
 
+TRANSACTION_INDEX_FIELDS = [
+    ('tx', 1),
+]
+
 
 class MessageController(storage.MessageBase):
     """Implements message resource operations using MongoDB.
@@ -95,14 +114,16 @@ class MessageController(storage.MessageBase):
     Messages are scoped by project + queue.
 
     Messages:
-        Name        Field
-        -----------------
-        scope    ->   p_q
-        expires  ->     e
-        ttl      ->     t
-        uuid     ->     u
-        claim    ->     c
-        marker   ->     k
+        Name                Field
+        -------------------------
+        scope            ->   p_q
+        ttl              ->     t
+        expires          ->     e
+        marker           ->     k
+        body             ->     b
+        claim            ->     c
+        client uuid      ->     u
+        transaction      ->    tx
     """
 
     def __init__(self, *args, **kwargs):
@@ -163,6 +184,10 @@ class MessageController(storage.MessageBase):
                                 unique=True,
                                 background=True)
 
+        collection.ensure_index(TRANSACTION_INDEX_FIELDS,
+                                name='transaction',
+                                background=True)
+
     def _collection(self, queue_name, project=None):
         """Get a partitioned collection instance."""
         return self._collections[utils.get_partition(self._num_partitions,
@@ -237,9 +262,15 @@ class MessageController(storage.MessageBase):
         now = timeutils.utcnow_ts()
 
         query = {
-            # Messages must belong to this
-            # queue and project
+            # Messages must belong to this queue and project.
             PROJ_QUEUE: utils.scope_queue_name(queue_name, project),
+
+            # NOTE(kgriffs): Messages must be finalized (i.e., must not
+            # be part of an unfinalized transaction).
+            #
+            # See also the note wrt 'tx' within the definition
+            # of ACTIVE_INDEX_FIELDS.
+            'tx': None,
         }
 
         if not echo:
@@ -279,8 +310,15 @@ class MessageController(storage.MessageBase):
             they haven't been GC'd yet. This is done for performance.
         """
         query = {
-            # Messages must belong to this queue
+            # Messages must belong to this queue and project.
             PROJ_QUEUE: utils.scope_queue_name(queue_name, project),
+
+            # NOTE(kgriffs): Messages must be finalized (i.e., must not
+            # be part of an unfinalized transaction).
+            #
+            # See also the note wrt 'tx' within the definition
+            # of ACTIVE_INDEX_FIELDS.
+            'tx': None,
         }
 
         if not include_claimed:
@@ -458,20 +496,30 @@ class MessageController(storage.MessageBase):
         # Set the next basis marker for the first attempt.
         next_marker = self._queue_ctrl._get_counter(queue_name, project)
 
+        # Unique transaction ID to facilitate atomic batch inserts
+        transaction = objectid.ObjectId()
+
         prepared_messages = [
             {
-                't': message['ttl'],
                 PROJ_QUEUE: utils.scope_queue_name(queue_name, project),
+                't': message['ttl'],
                 'e': now_dt + datetime.timedelta(seconds=message['ttl']),
                 'u': client_uuid,
                 'c': {'id': None, 'e': now},
                 'b': message['body'] if 'body' in message else {},
                 'k': next_marker + index,
+                'tx': transaction,
             }
 
             for index, message in enumerate(messages)
         ]
 
+        # NOTE(kgriffs): Don't take the time to do a 2-phase insert
+        # if there is no way for it to partially succeed.
+        if len(prepared_messages) == 1:
+            transaction = None
+            prepared_messages[0]['tx'] = None
+
         # Use a retry range for sanity, although we expect
         # to rarely, if ever, reach the maximum number of
         # retries.
@@ -506,11 +554,18 @@ class MessageController(storage.MessageBase):
                 self._queue_ctrl._inc_counter(queue_name, project,
                                               amount=len(ids))
 
+                # NOTE(kgriffs): Finalize the insert once we can say that
+                # all the messages made it. This makes bulk inserts
+                # atomic, assuming queries filter out any non-finalized
+                # messages.
+                if transaction is not None:
+                    collection.update({'tx': transaction},
+                                      {'$set': {'tx': None}},
+                                      upsert=False, multi=True)
+
                 return map(str, ids)
 
             except pymongo.errors.DuplicateKeyError as ex:
-                # Try again with the remaining messages
-
                 # TODO(kgriffs): Record stats of how often retries happen,
                 # and how many attempts, on average, are required to insert
                 # messages.
@@ -593,10 +648,6 @@ class MessageController(storage.MessageBase):
                     message['k'] = next_marker + index
 
             except Exception as ex:
-                # TODO(kgriffs): Query the DB to get the last marker that
-                # made it, and extrapolate from there to figure out what
-                # needs to be retried.
-
                 LOG.exception(ex)
                 raise