diff --git a/setup.cfg b/setup.cfg
index eb141be..2d6dc8a 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -39,6 +39,8 @@ where = src
 redis =
     redis
     rfc3986 >= 1.2.0
+cachetools =
+    cachetools >= 4.0.0
 
 ; mypy config
 [mypy-redis]
@@ -46,3 +48,6 @@ ignore_missing_imports = True
 
 [mypy-rfc3986]
 ignore_missing_imports = True
+
+[mypy-cachetools]
+ignore_missing_imports = True
diff --git a/src/rush/exceptions.py b/src/rush/exceptions.py
index d1d02a6..8ec7930 100644
--- a/src/rush/exceptions.py
+++ b/src/rush/exceptions.py
@@ -5,6 +5,15 @@ class RushError(Exception):
     """Base class for every other Rush-generated exception."""
 
 
+class CompareAndSwapError(Exception):
+    """CAS operation failed, data out of date."""
+
+    def __init__(self, message, *, limitdata):
+        """Attach new limitdata from store."""
+        super().__init__(message)
+        self.limitdata = limitdata
+
+
 class RedisStoreError(RushError):
     """Base class for all RedisStore-related exceptions."""
 
diff --git a/src/rush/stores/local.py b/src/rush/stores/local.py
new file mode 100644
index 0000000..c42bee2
--- /dev/null
+++ b/src/rush/stores/local.py
@@ -0,0 +1,59 @@
+"""Module containing the logic for our in_memory cache stores."""
+import datetime
+import threading
+import typing
+
+import attr
+import cachetools
+
+from . import base
+from .. import exceptions
+from .. import limit_data
+
+
+@attr.s
+class TLRUCacheStore(base.BaseStore):
+    """Basic storage for testing that utilizes a TLRUCache."""
+
+    maxsize: int = attr.ib(converter=int)
+    ttl: datetime.timedelta = attr.ib()
+    store: typing.Dict[str, limit_data.LimitData] = attr.ib()
+    lock: threading.RLock = threading.RLock()
+
+    @store.default
+    def _create_store(self):
+        attr.validate(self)
+        return cachetools.TTLCache(
+            maxsize=self.maxsize, ttl=self.ttl.total_seconds()
+        )
+
+    def get(self, key: str) -> typing.Optional[limit_data.LimitData]:
+        """Retrieve the data for a given key."""
+        with self.lock:
+            data = self.store.get(key, None)
+            return data
+
+    def set(
+        self, *, key: str, data: limit_data.LimitData
+    ) -> limit_data.LimitData:
+        """Store the values for a given key."""
+        with self.lock:
+            self.store[key] = data
+        return data
+
+    def compare_and_swap(
+        self,
+        *,
+        key: str,
+        old: typing.Optional[limit_data.LimitData],
+        new: limit_data.LimitData,
+    ) -> limit_data.LimitData:
+        """Perform an atomic compare-and-swap (CAS) for a given key."""
+        with self.lock:
+            data = self.get(key)
+            if data == old:
+                return self.set(key=key, data=new)
+            raise exceptions.CompareAndSwapError(
+                "Old LimitData did not match current LimitData",
+                limitdata=data,
+            )
diff --git a/test/unit/test_tlru_cache_store.py b/test/unit/test_tlru_cache_store.py
new file mode 100644
index 0000000..3e7a5b2
--- /dev/null
+++ b/test/unit/test_tlru_cache_store.py
@@ -0,0 +1,175 @@
+"""Tests for our dictionary store."""
+import datetime
+
+import pytest
+
+from rush import exceptions
+from rush import limit_data
+from rush.stores import local
+
+
+class TestDictionaryStore:
+    """Test methods on our dictionary store."""
+
+    def test_begins_life_empty(self):
+        """Verify that by default no data exists."""
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        assert store.store.currsize == 0
+
+    def test_set(self):
+        """Verify we can add data."""
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        new_data = limit_data.LimitData(used=9999, remaining=1)
+
+        assert store.set(key="mykey", data=new_data) == new_data
+
+    def test_set_with_time_uses_now(self):
+        """Verify we can add data with the current time."""
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        new_data = limit_data.LimitData(used=9999, remaining=1)
+
+        set_data = store.set_with_time(key="mykey", data=new_data)
+        assert isinstance(set_data.time, datetime.datetime)
+        assert store.get("mykey") != {}
+
+    def test_set_with_time_uses_provided_value(self):
+        """Verify we can add data with a specific time."""
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        new_data = limit_data.LimitData(
+            used=9999,
+            remaining=1,
+            time=datetime.datetime(
+                year=2018,
+                month=12,
+                day=4,
+                hour=9,
+                minute=0,
+                second=0,
+                tzinfo=datetime.timezone.utc,
+            ),
+        )
+
+        set_data = store.set_with_time(key="mykey", data=new_data)
+        assert set_data == new_data
+        assert store.get("mykey") == new_data
+
+    def test_get(self):
+        """Verify we can retrieve data from our datastore."""
+        data = limit_data.LimitData(used=9999, remaining=1)
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        store.set(key="mykey", data=data)
+
+        assert store.get("mykey") == data
+
+    def test_get_with_time_defaults_to_now(self):
+        """Verify we can retrieve data with a default time."""
+        data = limit_data.LimitData(used=9999, remaining=1)
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        store.set(key="mykey", data=data)
+
+        dt, retrieved_data = store.get_with_time("mykey")
+        assert dt.replace(second=0, microsecond=0) == datetime.datetime.now(
+            datetime.timezone.utc
+        ).replace(second=0, microsecond=0)
+        assert retrieved_data == data.copy_with(time=dt)
+
+    def test_get_with_time_uses_existing_time(self):
+        """Verify we can retrieve data from our datastore with its time."""
+        data = limit_data.LimitData(
+            used=9999,
+            remaining=1,
+            time=datetime.datetime(
+                year=2018,
+                month=12,
+                day=4,
+                hour=9,
+                minute=0,
+                second=0,
+                tzinfo=datetime.timezone.utc,
+            ),
+        )
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        store.set(key="mykey", data=data)
+
+        dt, retrieved_data = store.get_with_time("mykey")
+        assert dt == data.time
+        assert retrieved_data == data
+
+    def test_compare_and_swap_success(self):
+        """Verify success when old is the same as new."""
+        data = limit_data.LimitData(
+            used=9999,
+            remaining=1,
+            time=datetime.datetime(
+                year=2018,
+                month=12,
+                day=4,
+                hour=9,
+                minute=0,
+                second=0,
+                tzinfo=datetime.timezone.utc,
+            ),
+        )
+        key = "mykey"
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        store.set(key="mykey", data=data)
+
+        new_data = limit_data.LimitData(
+            used=10000,
+            remaining=0,
+            time=datetime.datetime(
+                year=2018,
+                month=12,
+                day=4,
+                hour=9,
+                minute=0,
+                second=0,
+                tzinfo=datetime.timezone.utc,
+            ),
+        )
+        res = store.compare_and_swap(key=key, old=data, new=new_data)
+        assert res == new_data
+
+    def test_compare_and_swap_failure(self):
+        """Verify correct exception raised when old is not the same as new."""
+        data = limit_data.LimitData(
+            used=9999,
+            remaining=1,
+            time=datetime.datetime(
+                year=2018,
+                month=12,
+                day=4,
+                hour=9,
+                minute=0,
+                second=0,
+                tzinfo=datetime.timezone.utc,
+            ),
+        )
+        key = "mykey"
+        ttl = datetime.timedelta(seconds=10)
+        store = local.TLRUCacheStore(maxsize=1, ttl=ttl)
+        store.set(key="mykey", data=data)
+
+        new_data = limit_data.LimitData(
+            used=10000,
+            remaining=0,
+            time=datetime.datetime(
+                year=2018,
+                month=12,
+                day=4,
+                hour=9,
+                minute=0,
+                second=0,
+                tzinfo=datetime.timezone.utc,
+            ),
+        )
+        with pytest.raises(exceptions.CompareAndSwapError):
+            store.compare_and_swap(key=key, old=new_data, new=new_data)
diff --git a/tox.ini b/tox.ini
index bd022e0..236d6ad 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,6 +9,7 @@ deps =
     coverage
 extras =
     redis
+    cachetools
 commands =
     coverage run --parallel-mode -m pytest {posargs}
     coverage combine
@@ -19,6 +20,7 @@ recreate = true
 basepython = python3
 extras =
     redis
+    cachetools
 commands = {posargs:python}
 
 [testenv:commitlint]
@@ -50,6 +52,7 @@ deps =
     -rdoc/source/requirements.txt
 extras =
     redis
+    cachetools
 commands =
     doc8 doc/source/
     sphinx-build -E -W -c doc/source/ -b html doc/source/ doc/build/html