diff --git a/CHANGES.md b/CHANGES.md
index 888f029..a58c5fc 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,6 +2,8 @@
 
 * Change abbreviate long titles under menu tab
 * Update attr 20.2.0 (4f74fba) to 20.3.0 (f3762ba)
+* Update diskcache_py3 5.0.1 (9670fbb) to 5.1.0 (40ce0de)
+* Update diskcache_py2 4.1.0 (b0451e0) from 5.1.0 (40ce0de)
 * Update Requests library 2.24.0 (2f70990) to 2.25.0 (03957eb)
 * Update urllib3 1.25.11 (00f1769) to 1.26.1 (7675532)
 
diff --git a/lib/diskcache_py2/__init__.py b/lib/diskcache_py2/__init__.py
index a98cfee..e4d747b 100644
--- a/lib/diskcache_py2/__init__.py
+++ b/lib/diskcache_py2/__init__.py
@@ -46,8 +46,8 @@ except Exception:  # pylint: disable=broad-except
     pass
 
 __title__ = 'diskcache'
-__version__ = '5.0.3'
-__build__ = 0x050003
+__version__ = '5.1.0'
+__build__ = 0x050100
 __author__ = 'Grant Jenks'
 __license__ = 'Apache 2.0'
 __copyright__ = 'Copyright 2016-2020 Grant Jenks'
diff --git a/lib/diskcache_py2/fanout.py b/lib/diskcache_py2/fanout.py
index fcd1861..15716b7 100644
--- a/lib/diskcache_py2/fanout.py
+++ b/lib/diskcache_py2/fanout.py
@@ -1,5 +1,6 @@
 "Fanout cache automatically shards keys and values."
 
+import contextlib as cl
 import itertools as it
 import operator
 import os.path as op
@@ -69,9 +70,46 @@ class FanoutCache(object):
 
 
     def __getattr__(self, name):
+        safe_names = {'timeout', 'disk'}
+        valid_name = name in DEFAULT_SETTINGS or name in safe_names
+        assert valid_name, 'cannot access {} in cache shard'.format(name)
         return getattr(self._shards[0], name)
 
 
+    @cl.contextmanager
+    def transact(self, retry=True):
+        """Context manager to perform a transaction by locking the cache.
+
+        While the cache is locked, no other write operation is permitted.
+        Transactions should therefore be as short as possible. Read and write
+        operations performed in a transaction are atomic. Read operations may
+        occur concurrent to a transaction.
+
+        Transactions may be nested and may not be shared between threads.
+
+        Blocks until transactions are held on all cache shards by retrying as
+        necessary.
+
+        >>> cache = FanoutCache()
+        >>> with cache.transact():  # Atomically increment two keys.
+        ...     _ = cache.incr('total', 123.4)
+        ...     _ = cache.incr('count', 1)
+        >>> with cache.transact():  # Atomically calculate average.
+        ...     average = cache['total'] / cache['count']
+        >>> average
+        123.4
+
+        :return: context manager for use in `with` statement
+
+        """
+        assert retry, 'retry must be True in FanoutCache'
+        with cl.ExitStack() as stack:
+            for shard in self._shards:
+                shard_transaction = shard.transact(retry=True)
+                stack.enter_context(shard_transaction)
+            yield
+
+
     def set(self, key, value, expire=None, read=False, tag=None, retry=False):
         """Set `key` and `value` item in cache.
 
diff --git a/lib/diskcache_py3/__init__.py b/lib/diskcache_py3/__init__.py
index a98cfee..e4d747b 100644
--- a/lib/diskcache_py3/__init__.py
+++ b/lib/diskcache_py3/__init__.py
@@ -46,8 +46,8 @@ except Exception:  # pylint: disable=broad-except
     pass
 
 __title__ = 'diskcache'
-__version__ = '5.0.3'
-__build__ = 0x050003
+__version__ = '5.1.0'
+__build__ = 0x050100
 __author__ = 'Grant Jenks'
 __license__ = 'Apache 2.0'
 __copyright__ = 'Copyright 2016-2020 Grant Jenks'
diff --git a/lib/diskcache_py3/fanout.py b/lib/diskcache_py3/fanout.py
index a579a17..7e227c8 100644
--- a/lib/diskcache_py3/fanout.py
+++ b/lib/diskcache_py3/fanout.py
@@ -1,5 +1,6 @@
 "Fanout cache automatically shards keys and values."
 
+import contextlib as cl
 import functools
 import itertools as it
 import operator
@@ -58,9 +59,46 @@ class FanoutCache(object):
 
 
     def __getattr__(self, name):
+        safe_names = {'timeout', 'disk'}
+        valid_name = name in DEFAULT_SETTINGS or name in safe_names
+        assert valid_name, 'cannot access {} in cache shard'.format(name)
         return getattr(self._shards[0], name)
 
 
+    @cl.contextmanager
+    def transact(self, retry=True):
+        """Context manager to perform a transaction by locking the cache.
+
+        While the cache is locked, no other write operation is permitted.
+        Transactions should therefore be as short as possible. Read and write
+        operations performed in a transaction are atomic. Read operations may
+        occur concurrent to a transaction.
+
+        Transactions may be nested and may not be shared between threads.
+
+        Blocks until transactions are held on all cache shards by retrying as
+        necessary.
+
+        >>> cache = FanoutCache()
+        >>> with cache.transact():  # Atomically increment two keys.
+        ...     _ = cache.incr('total', 123.4)
+        ...     _ = cache.incr('count', 1)
+        >>> with cache.transact():  # Atomically calculate average.
+        ...     average = cache['total'] / cache['count']
+        >>> average
+        123.4
+
+        :return: context manager for use in `with` statement
+
+        """
+        assert retry, 'retry must be True in FanoutCache'
+        with cl.ExitStack() as stack:
+            for shard in self._shards:
+                shard_transaction = shard.transact(retry=True)
+                stack.enter_context(shard_transaction)
+            yield
+
+
     def set(self, key, value, expire=None, read=False, tag=None, retry=False):
         """Set `key` and `value` item in cache.