diff --git a/python/array_record_data_source.py b/python/array_record_data_source.py
index 117d244..90bab86 100644
--- a/python/array_record_data_source.py
+++ b/python/array_record_data_source.py
@@ -39,6 +39,7 @@ def __getitem__(self, record_keys: Sequence[int]) -> Sequence[T]:
 import os
 import pathlib
 import re
+import threading
 import typing
 from typing import Any, Callable, Iterator, List, Mapping, Protocol, Sequence, SupportsIndex, Tuple, TypeVar, Union
 
@@ -272,6 +273,7 @@ def __init__(
     self._paths = [ri.filename for ri in self._read_instructions]
     # We open readers lazily when we need to read from them.
     self._readers = [None] * len(self._read_instructions)
+    self._lock = threading.Lock()
     self._num_records = sum(
         map(lambda x: x.num_records, self._read_instructions)
     )
@@ -333,10 +335,13 @@ def _ensure_reader_exists(self, reader_idx: int) -> None:
     """Threadsafe method to create corresponding reader if it doesn't exist."""
     if self._readers[reader_idx] is not None:
       return
-    filename = self._read_instructions[reader_idx].filename
-    reader = _create_reader(filename, self._reader_options_string)
-    _check_group_size(filename, reader)
-    self._readers[reader_idx] = reader
+    with self._lock:
+      if self._readers[reader_idx] is not None:
+        return
+      filename = self._read_instructions[reader_idx].filename
+      reader = _create_reader(filename, self._reader_options_string)
+      _check_group_size(filename, reader)
+      self._readers[reader_idx] = reader
 
   def __getitem__(self, record_key: SupportsIndex) -> bytes:
     reader_idx, position = self._reader_idx_and_position(record_key)
@@ -391,6 +396,7 @@ def __getstate__(self):
     logging.debug("__getstate__ for ArrayRecordDataSource is called.")
     state = self.__dict__.copy()
     del state["_readers"]
+    del state["_lock"]
     return state
 
   def __setstate__(self, state):
@@ -399,6 +405,7 @@ def __setstate__(self, state):
     # We open readers lazily when we need to read from them. Thus, we don't
     # need to re-open the same files as before pickling.
     self._readers = [None] * len(self._read_instructions)
+    self._lock = threading.Lock()
 
   def __repr__(self) -> str:
     """Storing a hash of paths since paths can be a very long list."""
diff --git a/python/array_record_data_source_test.py b/python/array_record_data_source_test.py
index 8977a27..3b06410 100644
--- a/python/array_record_data_source_test.py
+++ b/python/array_record_data_source_test.py
@@ -17,6 +17,7 @@
 import dataclasses
 import os
 import pathlib
+import time
 from unittest import mock
 
 from absl import flags