diff --git a/swh/graph/client.py b/swh/graph/client.py
--- a/swh/graph/client.py
+++ b/swh/graph/client.py
@@ -18,6 +18,12 @@
         )
 
 
+class GraphArgumentException(Exception):
+    def __init__(self, *args, response=None):
+        super().__init__(*args)
+        self.response = response
+
+
 class RemoteGraphClient(RPCClient):
     """Client to the Software Heritage Graph."""
 
@@ -33,6 +39,13 @@
     def get_lines(self, endpoint, **kwargs):
         yield from self.raw_verb_lines("get", endpoint, **kwargs)
 
+    def raise_for_status(self, response) -> None:
+        if response.status_code // 100 == 4:
+            raise GraphArgumentException(
+                response.content.decode("ascii"), response=response
+            )
+        super().raise_for_status(response)
+
     # Web API endpoints
 
     def stats(self):
diff --git a/swh/graph/naive_client.py b/swh/graph/naive_client.py
new file mode 100644
--- /dev/null
+++ b/swh/graph/naive_client.py
@@ -0,0 +1,369 @@
+# 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
+
+import functools
+import inspect
+import re
+import statistics
+from typing import (
+    Callable,
+    Dict,
+    Iterable,
+    Iterator,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    TypeVar,
+)
+
+from swh.model.identifiers import ExtendedSWHID, ValidationError
+
+from .client import GraphArgumentException
+
+_NODE_TYPES = "ori|snp|rel|rev|dir|cnt"
+NODES_RE = re.compile(fr"(\*|{_NODE_TYPES})")
+EDGES_RE = re.compile(fr"(\*|{_NODE_TYPES}):(\*|{_NODE_TYPES})")
+
+
+T = TypeVar("T", bound=Callable)
+
+
+def check_arguments(f: T) -> T:
+    """Decorator for generic argument checking for methods of NaiveClient.
+    Checks ``src`` is a valid and known SWHID, and ``edges`` has the right format."""
+    signature = inspect.signature(f)
+
+    @functools.wraps(f)
+    def newf(*args, **kwargs):
+        __tracebackhide__ = True  # for pytest
+        try:
+            bound_args = signature.bind(*args, **kwargs)
+        except TypeError as e:
+            # rethrow the exception from here to pytest doesn't flood the terminal
+            # with signature.bind's call stack.
+            raise TypeError(*e.args) from None
+        self = bound_args.arguments["self"]
+
+        src = bound_args.arguments.get("src")
+        if src:
+            self._check_swhid(src)
+
+        edges = bound_args.arguments.get("edges")
+        if edges:
+            if edges != "*" and not EDGES_RE.match(edges):
+                raise GraphArgumentException(f"invalid edge restriction: {edges}")
+
+        return_types = bound_args.arguments.get("return_types")
+        if return_types:
+            if not NODES_RE.match(return_types):
+                raise GraphArgumentException(
+                    f"invalid return_types restriction: {return_types}"
+                )
+
+        return f(*args, **kwargs)
+
+    return newf  # type: ignore
+
+
+def filter_node_types(node_types: str, nodes: Iterable[str]) -> Iterator[str]:
+    if node_types == "*":
+        yield from nodes
+    else:
+        prefixes = tuple(f"swh:1:{type_}:" for type_ in node_types.split(","))
+        for node in nodes:
+            if node.startswith(prefixes):
+                yield node
+
+
+class NaiveClient:
+    """An alternative implementation of :class:`swh.graph.backend.Backend`,
+    written in pure-python and meant for simulating it in other components' test
+    cases.
+
+    It is NOT meant to be efficient in any way; only to be a very simple
+    implementation that provides the same behavior."""
+
+    def __init__(self, *, nodes: List[str], edges: List[Tuple[str, str]]):
+        self.graph = Graph(nodes, edges)
+
+    def _check_swhid(self, swhid):
+        try:
+            ExtendedSWHID.from_string(swhid)
+        except ValidationError as e:
+            raise GraphArgumentException(*e.args) from None
+        if swhid not in self.graph.nodes:
+            raise GraphArgumentException(f"SWHID not found: {swhid}")
+
+    def stats(self) -> Dict:
+        return {
+            "counts": {
+                "nodes": len(self.graph.nodes),
+                "edges": sum(map(len, self.graph.forward_edges.values())),
+            },
+            "ratios": {
+                "compression": 1.0,
+                "bits_per_edge": 100.0,
+                "bits_per_node": 100.0,
+                "avg_locality": 0.0,
+            },
+            "indegree": {
+                "min": min(map(len, self.graph.backward_edges.values())),
+                "max": max(map(len, self.graph.backward_edges.values())),
+                "avg": statistics.mean(map(len, self.graph.backward_edges.values())),
+            },
+            "outdegree": {
+                "min": min(map(len, self.graph.forward_edges.values())),
+                "max": max(map(len, self.graph.forward_edges.values())),
+                "avg": statistics.mean(map(len, self.graph.forward_edges.values())),
+            },
+        }
+
+    @check_arguments
+    def leaves(
+        self,
+        src: str,
+        edges: str = "*",
+        direction: str = "forward",
+        max_edges: int = 0,
+        return_types: str = "*",
+    ) -> Iterator[str]:
+        # TODO: max_edges
+        yield from filter_node_types(
+            return_types,
+            [
+                node
+                for node in self.graph.get_subgraph(src, edges, direction)
+                if not self.graph.get_filtered_neighbors(node, edges, direction)
+            ],
+        )
+
+    @check_arguments
+    def neighbors(
+        self,
+        src: str,
+        edges: str = "*",
+        direction: str = "forward",
+        max_edges: int = 0,
+        return_types: str = "*",
+    ) -> Iterator[str]:
+        # TODO: max_edges
+        yield from filter_node_types(
+            return_types, self.graph.get_filtered_neighbors(src, edges, direction)
+        )
+
+    @check_arguments
+    def visit_nodes(
+        self,
+        src: str,
+        edges: str = "*",
+        direction: str = "forward",
+        max_edges: int = 0,
+        return_types: str = "*",
+    ) -> Iterator[str]:
+        # TODO: max_edges
+        yield from filter_node_types(
+            return_types, self.graph.get_subgraph(src, edges, direction)
+        )
+
+    @check_arguments
+    def visit_edges(
+        self, src: str, edges: str = "*", direction: str = "forward", max_edges: int = 0
+    ) -> Iterator[Tuple[str, str]]:
+        if max_edges == 0:
+            max_edges = None  # type: ignore
+        else:
+            max_edges -= 1
+        yield from list(self.graph.iter_edges_dfs(direction, edges, src))[:max_edges]
+
+    @check_arguments
+    def visit_paths(
+        self, src: str, edges: str = "*", direction: str = "forward", max_edges: int = 0
+    ) -> Iterator[List[str]]:
+        # TODO: max_edges
+        for path in self.graph.iter_paths_dfs(direction, edges, src):
+            if path[-1] in self.leaves(src, edges, direction):
+                yield list(path)
+
+    @check_arguments
+    def walk(
+        self,
+        src: str,
+        dst: str,
+        edges: str = "*",
+        traversal: str = "dfs",
+        direction: str = "forward",
+        limit: Optional[int] = None,
+    ) -> Iterator[str]:
+        # TODO: implement algo="bfs"
+        # TODO: limit
+        match_path: Callable[[str], bool]
+        if ":" in dst:
+            match_path = dst.__eq__
+            self._check_swhid(dst)
+        else:
+            match_path = lambda node: node.startswith(f"swh:1:{dst}:")  # noqa
+        for path in self.graph.iter_paths_dfs(direction, edges, src):
+            if match_path(path[-1]):
+                if not limit:
+                    # 0 or None
+                    yield from path
+                elif limit > 0:
+                    yield from path[0:limit]
+                else:
+                    yield from path[limit:]
+
+    @check_arguments
+    def random_walk(
+        self,
+        src: str,
+        dst: str,
+        edges: str = "*",
+        direction: str = "forward",
+        limit: Optional[int] = None,
+    ):
+        # TODO: limit
+        yield from self.walk(src, dst, edges, "dfs", direction, limit)
+
+    @check_arguments
+    def count_leaves(
+        self, src: str, edges: str = "*", direction: str = "forward"
+    ) -> int:
+        return len(list(self.leaves(src, edges, direction)))
+
+    @check_arguments
+    def count_neighbors(
+        self, src: str, edges: str = "*", direction: str = "forward"
+    ) -> int:
+        return len(self.graph.get_filtered_neighbors(src, edges, direction))
+
+    @check_arguments
+    def count_visit_nodes(
+        self, src: str, edges: str = "*", direction: str = "forward"
+    ) -> int:
+        return len(self.graph.get_subgraph(src, edges, direction))
+
+
+class Graph:
+    def __init__(self, nodes: List[str], edges: List[Tuple[str, str]]):
+        self.nodes = nodes
+        self.forward_edges: Dict[str, List[str]] = {}
+        self.backward_edges: Dict[str, List[str]] = {}
+        for node in nodes:
+            self.forward_edges[node] = []
+            self.backward_edges[node] = []
+        for (src, dst) in edges:
+            self.forward_edges[src].append(dst)
+            self.backward_edges[dst].append(src)
+
+    def get_filtered_neighbors(
+        self, src: str, edges_fmt: str, direction: str,
+    ) -> Set[str]:
+        if direction == "forward":
+            edges = self.forward_edges
+        elif direction == "backward":
+            edges = self.backward_edges
+        else:
+            raise GraphArgumentException(f"invalid direction: {direction}")
+
+        neighbors = edges.get(src, [])
+
+        if edges_fmt == "*":
+            return set(neighbors)
+        else:
+            filtered_neighbors: Set[str] = set()
+            for edges_fmt_item in edges_fmt.split(","):
+                (src_fmt, dst_fmt) = edges_fmt_item.split(":")
+                if src_fmt != "*" and not src.startswith(f"swh:1:{src_fmt}:"):
+                    continue
+                if dst_fmt == "*":
+                    filtered_neighbors.update(neighbors)
+                else:
+                    prefix = f"swh:1:{dst_fmt}:"
+                    filtered_neighbors.update(
+                        n for n in neighbors if n.startswith(prefix)
+                    )
+            return filtered_neighbors
+
+    def get_subgraph(self, src: str, edges_fmt: str, direction: str) -> Set[str]:
+        seen = set()
+        to_visit = {src}
+        while to_visit:
+            node = to_visit.pop()
+            seen.add(node)
+            neighbors = set(self.get_filtered_neighbors(node, edges_fmt, direction))
+            new_nodes = neighbors - seen
+            to_visit.update(new_nodes)
+
+        return seen
+
+    def iter_paths_dfs(
+        self, direction: str, edges_fmt: str, src: str
+    ) -> Iterator[Tuple[str, ...]]:
+        for (path, node) in DfsSubgraphIterator(self, direction, edges_fmt, src):
+            yield path + (node,)
+
+    def iter_edges_dfs(
+        self, direction: str, edges_fmt: str, src: str
+    ) -> Iterator[Tuple[str, str]]:
+        for (path, node) in DfsSubgraphIterator(self, direction, edges_fmt, src):
+            if len(path) > 0:
+                yield (path[-1], node)
+
+
+class SubgraphIterator(Iterator[Tuple[Tuple[str, ...], str]]):
+    def __init__(self, graph: Graph, direction: str, edges_fmt: str, src: str):
+        self.graph = graph
+        self.direction = direction
+        self.edges_fmt = edges_fmt
+        self.seen: Set[str] = set()
+        self.src = src
+
+    def more_work(self) -> bool:
+        raise NotImplementedError()
+
+    def pop(self) -> Tuple[Tuple[str, ...], str]:
+        raise NotImplementedError()
+
+    def push(self, new_path: Tuple[str, ...], neighbor: str) -> None:
+        raise NotImplementedError()
+
+    def __next__(self) -> Tuple[Tuple[str, ...], str]:
+        # Stores (path, next_node)
+        if not self.more_work():
+            raise StopIteration()
+
+        (path, node) = self.pop()
+
+        new_path = path + (node,)
+
+        if node not in self.seen:
+            neighbors = self.graph.get_filtered_neighbors(
+                node, self.edges_fmt, self.direction
+            )
+
+            # We want to visit the first neighbor first, and to_visit is a stack;
+            # so we need to reversed() the list of neighbors to get it on top
+            # of the stack.
+            for neighbor in reversed(list(neighbors)):
+                self.push(new_path, neighbor)
+
+        self.seen.add(node)
+        return (path, node)
+
+
+class DfsSubgraphIterator(SubgraphIterator):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.to_visit: List[Tuple[Tuple[str, ...], str]] = [((), self.src)]
+
+    def more_work(self) -> bool:
+        return bool(self.to_visit)
+
+    def pop(self) -> Tuple[Tuple[str, ...], str]:
+        return self.to_visit.pop()
+
+    def push(self, new_path: Tuple[str, ...], neighbor: str) -> None:
+        self.to_visit.append((new_path, neighbor))
diff --git a/swh/graph/tests/conftest.py b/swh/graph/tests/conftest.py
--- a/swh/graph/tests/conftest.py
+++ b/swh/graph/tests/conftest.py
@@ -1,3 +1,9 @@
+# Copyright (C) 2019-2021  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
+
+import csv
 import multiprocessing
 from pathlib import Path
 
@@ -7,6 +13,7 @@
 from swh.graph.backend import Backend
 from swh.graph.client import RemoteGraphClient
 from swh.graph.graph import load as graph_load
+from swh.graph.naive_client import NaiveClient
 from swh.graph.server.app import make_app
 
 SWH_GRAPH_TESTS_ROOT = Path(__file__).parents[0]
@@ -33,16 +40,23 @@
             self.q.put(e)
 
 
-@pytest.fixture(scope="module")
-def graph_client():
-    queue = multiprocessing.Queue()
-    server = GraphServerProcess(queue)
-    server.start()
-    res = queue.get()
-    if isinstance(res, Exception):
-        raise res
-    yield RemoteGraphClient(str(res))
-    server.terminate()
+@pytest.fixture(scope="module", params=["remote", "naive"])
+def graph_client(request):
+    if request.param == "remote":
+        queue = multiprocessing.Queue()
+        server = GraphServerProcess(queue)
+        server.start()
+        res = queue.get()
+        if isinstance(res, Exception):
+            raise res
+        yield RemoteGraphClient(str(res))
+        server.terminate()
+    else:
+        with open(SWH_GRAPH_TESTS_ROOT / "dataset/example.nodes.csv") as fd:
+            nodes = [node for (node,) in csv.reader(fd, delimiter=" ")]
+        with open(SWH_GRAPH_TESTS_ROOT / "dataset/example.edges.csv") as fd:
+            edges = list(csv.reader(fd, delimiter=" "))
+        yield NaiveClient(nodes=nodes, edges=edges)
 
 
 @pytest.fixture(scope="module")
diff --git a/swh/graph/tests/test_api_client.py b/swh/graph/tests/test_api_client.py
--- a/swh/graph/tests/test_api_client.py
+++ b/swh/graph/tests/test_api_client.py
@@ -2,6 +2,7 @@
 from pytest import raises
 
 from swh.core.api import RemoteException
+from swh.graph.client import GraphArgumentException
 
 
 def test_stats(graph_client):
@@ -323,17 +324,19 @@
 
 
 def test_param_validation(graph_client):
-    with raises(RemoteException) as exc_info:  # SWHID not found
+    with raises(GraphArgumentException) as exc_info:  # SWHID not found
         list(graph_client.leaves("swh:1:ori:fff0000000000000000000000000000000000021"))
-    assert exc_info.value.response.status_code == 404
+    if exc_info.value.response:
+        assert exc_info.value.response.status_code == 404
 
-    with raises(RemoteException) as exc_info:  # malformed SWHID
+    with raises(GraphArgumentException) as exc_info:  # malformed SWHID
         list(
             graph_client.neighbors("swh:1:ori:fff000000zzzzzz0000000000000000000000021")
         )
-    assert exc_info.value.response.status_code == 400
+    if exc_info.value.response:
+        assert exc_info.value.response.status_code == 400
 
-    with raises(RemoteException) as exc_info:  # malformed edge specificaiton
+    with raises(GraphArgumentException) as exc_info:  # malformed edge specificaiton
         list(
             graph_client.visit_nodes(
                 "swh:1:dir:0000000000000000000000000000000000000016",
@@ -341,9 +344,10 @@
                 direction="backward",
             )
         )
-    assert exc_info.value.response.status_code == 400
+    if exc_info.value.response:
+        assert exc_info.value.response.status_code == 400
 
-    with raises(RemoteException) as exc_info:  # malformed direction
+    with raises(GraphArgumentException) as exc_info:  # malformed direction
         list(
             graph_client.visit_nodes(
                 "swh:1:dir:0000000000000000000000000000000000000016",
@@ -351,7 +355,8 @@
                 direction="notadirection",
             )
         )
-    assert exc_info.value.response.status_code == 400
+    if exc_info.value.response:
+        assert exc_info.value.response.status_code == 400
 
 
 @pytest.mark.skip(reason="currently disabled due to T1969")