diff --git a/swh/counters/__init__.py b/swh/counters/__init__.py
--- a/swh/counters/__init__.py
+++ b/swh/counters/__init__.py
@@ -14,6 +14,7 @@
 COUNTERS_IMPLEMENTATIONS = {
     "redis": ".redis.Redis",
     "remote": ".api.client.RemoteCounters",
+    "memory": ".in_memory.InMemory",
 }
 
 HISTORY_IMPLEMENTATIONS = {
diff --git a/swh/counters/in_memory.py b/swh/counters/in_memory.py
new file mode 100644
--- /dev/null
+++ b/swh/counters/in_memory.py
@@ -0,0 +1,31 @@
+# Copyright (C) 2021  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from collections import defaultdict
+from typing import Any, Dict, Iterable, List
+
+
+class InMemory:
+    """InMemory implementation of the counters.
+       Naive implementation using a Dict[str, Set]"""
+
+    def __init__(self):
+        self.counters = defaultdict(set)
+
+    def check(self):
+        return "OK"
+
+    def add(self, collection: str, keys: Iterable[Any]) -> None:
+        for value in keys:
+            self.counters[collection].add(value)
+
+    def get_count(self, collection: str) -> int:
+        return len(self.counters.get(collection, []))
+
+    def get_counts(self, collections: List[str]) -> Dict[str, int]:
+        return {coll: self.get_count(coll) for coll in collections}
+
+    def get_counters(self) -> Iterable[str]:
+        return list(self.counters.keys())
diff --git a/swh/counters/tests/test_init.py b/swh/counters/tests/test_init.py
--- a/swh/counters/tests/test_init.py
+++ b/swh/counters/tests/test_init.py
@@ -10,12 +10,14 @@
 from swh.counters import get_counters, get_history
 from swh.counters.api.client import RemoteCounters
 from swh.counters.history import History
+from swh.counters.in_memory import InMemory
 from swh.counters.interface import CountersInterface
 from swh.counters.redis import Redis
 
 COUNTERS_IMPLEMENTATIONS = [
     ("remote", RemoteCounters, {"url": "localhost"}),
     ("redis", Redis, {"host": "localhost"}),
+    ("memory", InMemory, {}),
 ]
 
 
diff --git a/swh/counters/tests/test_inmemory.py b/swh/counters/tests/test_inmemory.py
new file mode 100644
--- /dev/null
+++ b/swh/counters/tests/test_inmemory.py
@@ -0,0 +1,54 @@
+# Copyright (C) 2021  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from swh.counters.in_memory import InMemory
+
+
+def test__inmemory__add():
+    im = InMemory()
+
+    im.add("counter1", ["val1"])
+    im.add("counter2", ["val1"])
+    im.add("counter1", [1])
+
+    assert im.get_count("counter1") == 2
+    assert im.get_count("counter2") == 1
+    assert im.get_count("inexisting") == 0
+
+
+def test__inmemory_getcounters():
+    im = InMemory()
+
+    assert len(im.get_counters()) == 0
+
+    counters = ["c1", "c2", "c3"]
+
+    count = 0
+
+    for counter in counters:
+        im.add(counter, [1, 2])
+        count += 1
+        assert count == len(im.get_counters())
+
+    results = im.get_counters()
+    assert results == counters
+
+
+def test__inmemory_getcounts():
+    im = InMemory()
+
+    expected = {"c1": 1, "c2": 2, "c3": 0}
+
+    im.add("c1", ["v1"])
+    im.add("c2", ["v1", "v2"])
+
+    result = im.get_counts(["c1", "c2", "c3"])
+    assert result == expected
+
+
+def test__inmemory_check():
+    im = InMemory()
+
+    assert im.check() == "OK"