diff --git a/.gitignore b/.gitignore
index 07febb3..181b2a7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -45,3 +45,6 @@ logs/
 # Sphinx
 docs/_build/
 docs/ref/modules/
+
+# known clients
+config/known_clients.yaml
diff --git a/config/.gitkeep b/config/.gitkeep
new file mode 100644
index 0000000..e69de29
diff --git a/src/judas_server/__init__.py b/src/judas_server/__init__.py
index c66c5db..5bcf5d2 100644
--- a/src/judas_server/__init__.py
+++ b/src/judas_server/__init__.py
@@ -1 +1 @@
-__version__: str = "0.1.0"
+__version__: str = "0.5.0"
diff --git a/src/judas_server/__main__.py b/src/judas_server/__main__.py
index d03289a..e850375 100644
--- a/src/judas_server/__main__.py
+++ b/src/judas_server/__main__.py
@@ -12,6 +12,8 @@ if __name__ == "__main__":
         format="%(asctime)s : [%(levelname)s] : %(threadName)s : %(name)s :: %(message)s",
     )
 
+    lg.getLogger("werkzeug").setLevel(lg.WARNING)
+
     ladygaga_logger = lg.getLogger(f"{__name__}.LAGA_DYGA")
     ladygaga_logger.info(LADY_GAGA)
 
diff --git a/src/judas_server/backend/__init__.py b/src/judas_server/backend/__init__.py
index 8e31517..60e15d3 100644
--- a/src/judas_server/backend/__init__.py
+++ b/src/judas_server/backend/__init__.py
@@ -1,3 +1,5 @@
 from .backend_server import BackendServer
+from .client import Client
+from .client_status import ClientStatus
 
-__all__ = ["BackendServer"]
+__all__ = ["BackendServer", "Client", "ClientStatus"]
diff --git a/src/judas_server/backend/backend_server.py b/src/judas_server/backend/backend_server.py
index b1bea71..cce28a7 100644
--- a/src/judas_server/backend/backend_server.py
+++ b/src/judas_server/backend/backend_server.py
@@ -6,16 +6,24 @@ import selectors
 import socket
 import threading
 import time
+from typing import TYPE_CHECKING, Any, Final
+
 import yaml
-
-from typing import Any
-
 from judas_protocol import Category, ControlAction, Message
 
 from judas_server.backend.client import Client, ClientStatus
+from judas_server.backend.handler.hello_handler import HelloHandler
+
+if TYPE_CHECKING:
+    from typing import Callable
+
+    from judas_protocol import ActionType
 
 
 class BackendServer:
+    ACK_TIMEOUT: Final[float] = 5.0  # seconds
+    HELLO_TIMEOUT: Final[float] = 3.0  # seconds
+
     def __init__(self, host: str = "0.0.0.0", port: int = 3692) -> None:
         """Initialize the backend server.
 
@@ -28,27 +36,6 @@ class BackendServer:
         )
         self.logger.debug("Initializing Server...")
 
-        self.known_clients: dict[str, dict[str, str | float]] = {}
-        try:
-            with open("cache/known_clients.yaml", "r") as f:
-                self.known_clients = (
-                    yaml.safe_load(f).get("known_clients", {}) or {}
-                )
-                self.logger.debug(
-                    f"Loaded known clients: {self.known_clients}"
-                )
-                self.logger.info(
-                    f"Loaded {len(self.known_clients)} known clients"
-                )
-        except FileNotFoundError:
-            self.logger.warning(
-                "known_clients.yaml not found, creating empty known clients list"
-            )
-            with open("cache/known_clients.yaml", "w") as f:
-                yaml.safe_dump({"known_clients": {}}, f)
-        except Exception as e:
-            self.logger.error(f"Error loading known clients: {e}")
-
         self.selector = selectors.DefaultSelector()
         self.server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         self.server_socket.setsockopt(
@@ -63,20 +50,85 @@ class BackendServer:
 
         self.clients: dict[str, Client] = {}
 
-        if self.known_clients:
-            for client_id in self.known_clients:
-                client = Client(id=client_id, addr=None, socket=None)
-                client.status = ClientStatus.OFFLINE
-                client.last_seen = float(
-                    self.known_clients[client_id].get("last_seen", 0.0)
-                )
-                self.clients[client_id] = client
+        self.known_clients: dict[str, dict[str, str | float]] = {}
+        self.known_clients = self._load_known_clients()
+
+        self.message_handlers: dict[
+            tuple[Category, ActionType], Callable[[Client, Message], None]
+        ] = {}
+        self._initialize_handlers()
+
+        self.pending_acks: list[tuple[Client, Message, float]] = []
+        self.pending_hello: dict[Client, float] = {}
 
         self.running: bool = False
 
+    def _initialize_handlers(self) -> None:
+        """Initialize message handlers."""
+
+        hello_handler = HelloHandler(self)
+
+        self.message_handlers[(Category.CONTROL, ControlAction.HELLO)] = (
+            hello_handler.handle
+        )
+
+    def _load_known_clients(self) -> dict[str, dict[str, str | float]]:
+        """Load the list of known clients from a YAML file and validate."""
+        known_clients: dict[str, dict[str, str | float]] = {}
+
+        try:
+            with open("config/known_clients.yaml", "r") as f:
+                data = yaml.safe_load(f)
+
+                if not isinstance(data, dict):
+                    raise ValueError("YAML root must be a dict")
+
+                known_clients = data.get("known_clients", {}) or {}
+
+                if not isinstance(known_clients, dict):
+                    raise ValueError("'known_clients' must be a dict")
+
+                for client_id, client_data in known_clients.items():
+                    if not isinstance(client_data, dict):
+                        raise ValueError(
+                            f"Client {client_id} data must be a dict"
+                        )
+                    last_seen = client_data.get("last_seen", 0.0)
+                    if not isinstance(last_seen, (float, int)):
+                        raise ValueError(
+                            f"Client {client_id} 'last_seen' must be a float or int"
+                        )
+
+                self.logger.debug(f"Loaded known clients: {known_clients}")
+                self.logger.info(f"Loaded {len(known_clients)} known clients")
+
+            for client_id in known_clients:
+                client = Client(id=client_id, addr=None, socket=None)
+                client.status = ClientStatus.OFFLINE
+                client.last_seen = float(
+                    known_clients[client_id].get("last_seen", 0.0)
+                )
+                self.clients[client_id] = client
+
+        except FileNotFoundError:
+            self.logger.warning(
+                "known_clients.yaml not found, creating empty known clients list"
+            )
+            self._save_known_clients()
+        except Exception as e:
+            self.logger.error(f"Error loading known clients: {e}")
+            raise
+
+        return known_clients
+
     def _save_known_clients(self) -> None:
         """Save the list of known clients to a YAML file."""
-        with open("cache/known_clients.yaml", "w") as f:
+        with open("config/known_clients.yaml", "w") as f:
+            f.write(
+                "# This file is automatically generated by BackendServer.\n"
+                + "# Do not edit manually.\n"
+                + f"# Generated at: {time.strftime('%Y-%m-%d %H:%M:%S', time.localtime())}\n\n"
+            )
             yaml.safe_dump({"known_clients": self.known_clients}, f)
         self.logger.debug("Saved known clients")
 
@@ -99,6 +151,22 @@ class BackendServer:
                 )
                 time.sleep(1)
 
+    def send(self, client: Client, msg: Message) -> None:
+        """Send a message to a client.
+
+        Args:
+            client (Client): The client to send the message to.
+            msg (Message): The message to send.
+        """
+        msg_bytes: bytes = msg.to_bytes()
+        self.logger.info(
+            f"[>] Sending message {msg.id} to {client}, category: {msg.category}, action: {msg.action}, ack_required: {msg.ack_required}"
+        )
+        self.logger.debug(f"[>] Message bytes: {msg_bytes!r}")
+        if msg.ack_required:
+            self.pending_acks.append((client, msg, time.time()))
+        client.outbound += msg_bytes
+
     def send_ack(self, client: Client, target_id: str) -> None:
         """Send an ACK message to a client.
 
@@ -106,9 +174,9 @@ class BackendServer:
             client (Client): The client to send the ACK to.
             target_id (str): The id of the ACK'd message.
         """
-        ack: bytes = Message.ack(target_id=target_id).to_bytes()
+        ack: Message = Message.Control.ack(target_id=target_id)
         self.logger.info(f"[>] Sending ACK to {client}")
-        client.outbound += ack
+        self.send(client, ack)
 
     def send_close(self, client: Client) -> None:
         """Send a CLOSE message to a client.
@@ -116,9 +184,9 @@ class BackendServer:
         Args:
             client (Client): The client to send the CLOSE message to.
         """
-        close_msg: bytes = Message.close().to_bytes()
+        close_msg: Message = Message.Control.close()
         self.logger.info(f"[>] Sending CLOSE to {client}")
-        client.outbound += close_msg
+        self.send(client, close_msg)
 
     def _accept_connection(self, sock: socket.socket) -> None:
         """Accept a new client connection.
@@ -134,6 +202,8 @@ class BackendServer:
         events = selectors.EVENT_READ | selectors.EVENT_WRITE
         self.selector.register(conn, events, data=client)
 
+        self.pending_hello[client] = time.time()
+
         self.logger.info(f"[+] Registered client {client}, HELLO pending...")
 
     def _disconnect(self, client: Client) -> None:
@@ -144,6 +214,12 @@ class BackendServer:
         """
         self.logger.info(f"[-] Disconnecting {client}...")
 
+        if client.socket is None or client.socket._closed:
+            self.logger.warning(
+                f"Client {client} has no socket, nothing to disconnect."
+            )
+            return
+
         try:
             self.selector.unregister(client.socket)
         except Exception as e:
@@ -202,51 +278,6 @@ class BackendServer:
         try:
             if mask & selectors.EVENT_READ:
                 self._receive_inbound(sock, client)
-                if not client.inbound:
-                    self._disconnect(client)
-                    return
-
-                if client.id is None:
-                    # expect HELLO message
-                    try:
-                        msg = Message.from_bytes(client.inbound)
-                    except Exception as e:
-                        self.logger.error(
-                            f"Failed to parse HELLO message from {client}: {e}"
-                        )
-                        self._disconnect(client)
-                        return
-
-                    if (
-                        msg.category == Category.CONTROL
-                        and msg.action == ControlAction.HELLO
-                        and msg.payload.get("id") is not None
-                    ):
-                        client.id = msg.payload["id"]
-                        if (
-                            client.id in self.clients
-                            and self.clients[client.id].status == "connected"
-                        ):
-                            old_client: Client = self.clients[client.id]
-                            self.logger.warning(
-                                f"Client {client.id} is already connected from {old_client.addr}, disconnecting old client..."
-                            )
-                            self.send_close(old_client)
-
-                        self.clients[client.id] = client
-                        self.known_clients[client.id] = {
-                            "last_seen": client.last_seen
-                        }
-                        self._save_known_clients()
-                        client.status = ClientStatus.ONLINE
-
-                        self.logger.info(f"[+] Registered new client {client}")
-                    else:
-                        self.logger.error(
-                            f"Expected HELLO message from {client}, got {msg}"
-                        )
-                        self._disconnect(client)
-                        return
 
                 while b"\n" in client.inbound:
                     line, client.inbound = client.inbound.split(b"\n", 1)
@@ -256,13 +287,40 @@ class BackendServer:
                     try:
                         msg = Message.from_bytes(line)
                         self.logger.info(f"[.] Parsed message {msg.id}")
+
+                        if client.id is None:
+                            self.logger.debug(
+                                f"Client {client} has no ID, expecting HELLO message..."
+                            )
+                            if (
+                                msg.category != Category.CONTROL
+                                or msg.action != ControlAction.HELLO
+                            ):
+                                self.logger.warning(
+                                    f"First message from {client} must be HELLO, disconnecting..."
+                                )
+                                self._disconnect(client)
+                                continue
+
+                        handler: Callable[[Client, Message], None] | None = (
+                            self.message_handlers.get(
+                                (msg.category, msg.action), None
+                            )
+                        )
+                        if handler is not None:
+                            handler(client, msg)
+                        else:
+                            self.logger.warning(
+                                f"No handler for message {msg.id} with category {msg.category} and action {msg.action}"
+                            )
+                            continue
+
                         if msg.ack_required:
                             self.send_ack(client, target_id=msg.id)
                     except Exception as e:
                         self.logger.error(
                             f"Failed to parse message from {client}: {e}"
                         )
-                        self._disconnect(client)
                         return
 
             if mask & selectors.EVENT_WRITE and client.outbound:
@@ -302,6 +360,25 @@ class BackendServer:
                         and now - client.last_seen > 60 * 60 * 24  # 24 hours
                     ):
                         self.clients[client.id].status = ClientStatus.STALE
+
+                # check pending ACKs
+                for client, msg, timestamp in self.pending_acks[:]:
+                    if time.time() - timestamp > self.ACK_TIMEOUT:
+                        self.logger.warning(
+                            f"ACK timeout for message {msg.id} to {client}, resending..."
+                        )
+                        self.send(client, msg)
+                        self.pending_acks.remove((client, msg, timestamp))
+
+                # check pending HELLOs
+                for client, timestamp in list(self.pending_hello.items()):
+                    if time.time() - timestamp > self.HELLO_TIMEOUT:
+                        self.logger.warning(
+                            f"HELLO timeout for {client}, disconnecting..."
+                        )
+                        self._disconnect(client)
+                        del self.pending_hello[client]
+
                 time.sleep(0.001)  # prevent 100% CPU usage
 
         except Exception as e:
diff --git a/src/judas_server/backend/client.py b/src/judas_server/backend/client.py
index 500f9f4..1c20fa2 100644
--- a/src/judas_server/backend/client.py
+++ b/src/judas_server/backend/client.py
@@ -5,24 +5,19 @@ from __future__ import annotations
 
 import logging as lg
 import socket
-from enum import Enum
 import time
 
-
-class ClientStatus(str, Enum):
-    """Enumeration of client connection statuses."""
-
-    ONLINE = "online"
-    PENDING = "pending"
-    OFFLINE = "offline"
-    STALE = "stale"
+from judas_server.backend.client_status import ClientStatus
 
 
 class Client:
     """Represents a client."""
 
     def __init__(
-        self, id: str | None, addr: tuple[str, int], socket: socket.socket
+        self,
+        id: str | None,
+        addr: tuple[str, int] | None,
+        socket: socket.socket | None,
     ) -> None:
         """Initialize the client.
 
@@ -41,13 +36,15 @@ class Client:
         self.last_seen: float = 0.0  # unix timestanp of last inbound message
         self.status: ClientStatus = ClientStatus.PENDING
 
-        self.socket: socket.socket = socket
-        self.addr: tuple[str, int] = addr
+        self.socket: socket.socket | None = socket
+        self.addr: tuple[str, int] | None = addr
         self.inbound: bytes = b""
         self.outbound: bytes = b""
 
     def __str__(self) -> str:
-        return f"Client({self.id} ({self.addr[0]}:{self.addr[1]}))"
+        if self.addr:
+            return f"Client({self.id} ({self.addr[0]}:{self.addr[1]}))"
+        return f"Client({self.id} (not connected))"
 
     def __repr__(self) -> str:
         return f"Client({self.id}, {self.addr})"
@@ -55,6 +52,11 @@ class Client:
     def disconnect(self) -> None:
         """Disconnect the client and close the socket."""
         self.logger.debug(f"Disconnecting Client {self}...")
+        if self.socket is None:
+            self.logger.warning(
+                f"Client {self} not connected, nothing to disconnect."
+            )
+            return
         try:
             self.socket.close()
         except Exception as e:
diff --git a/src/judas_server/backend/client_status.py b/src/judas_server/backend/client_status.py
new file mode 100644
index 0000000..1dabba6
--- /dev/null
+++ b/src/judas_server/backend/client_status.py
@@ -0,0 +1,11 @@
+# -*- coding: utf-8 -*-
+from enum import Enum
+
+
+class ClientStatus(str, Enum):
+    """Enumeration of client connection statuses."""
+
+    ONLINE = "online"
+    PENDING = "pending"
+    OFFLINE = "offline"
+    STALE = "stale"
diff --git a/src/judas_server/backend/handler/__init__.py b/src/judas_server/backend/handler/__init__.py
new file mode 100644
index 0000000..dc07972
--- /dev/null
+++ b/src/judas_server/backend/handler/__init__.py
@@ -0,0 +1,5 @@
+from .base_handler import BaseHandler
+from .hello_handler import HelloHandler
+from .ack_handler import AckHandler
+
+__all__ = ["BaseHandler", "HelloHandler", "AckHandler"]
diff --git a/src/judas_server/backend/handler/ack_handler.py b/src/judas_server/backend/handler/ack_handler.py
new file mode 100644
index 0000000..8204be0
--- /dev/null
+++ b/src/judas_server/backend/handler/ack_handler.py
@@ -0,0 +1,27 @@
+# -*- coding: utf-8 -*-
+
+from typing import TYPE_CHECKING
+
+from .base_handler import BaseHandler
+
+if TYPE_CHECKING:
+    from judas_protocol import Message
+
+    from judas_server.backend import BackendServer, Client
+
+
+class AckHandler(BaseHandler):
+    def __init__(self, backend_server: BackendServer) -> None:
+        super().__init__(backend_server)
+
+    def handle(self, client: Client, message: Message) -> None:
+        pending_acks = self.backend_server.pending_acks
+        if message.id in pending_acks:
+            del pending_acks[message.id]
+            self.logger.debug(
+                f"[*] Received ACK for message {message.id} from {client}."
+            )
+        else:
+            self.logger.warning(
+                f"[!] Received ACK for unknown (or ACK'd) message {message.id} from {client}."
+            )
diff --git a/src/judas_server/backend/handler/base_handler.py b/src/judas_server/backend/handler/base_handler.py
new file mode 100644
index 0000000..e7dfa8c
--- /dev/null
+++ b/src/judas_server/backend/handler/base_handler.py
@@ -0,0 +1,41 @@
+# -*- coding: utf-8 -*-
+from __future__ import annotations
+
+import logging as lg
+from typing import TYPE_CHECKING
+
+from judas_server.backend.client import Client
+
+if TYPE_CHECKING:
+    from judas_protocol import Message
+
+    from judas_server.backend import BackendServer
+
+
+class BaseHandler:
+    """BaseHandler is the base class for all message handlers in the backend server.
+
+    It defines the interface for handling messages and provides common functionality for all handlers.
+    """
+
+    def __init__(self, backend_server: BackendServer) -> None:
+        """Initialize the BaseHandler with a reference to the backend server.
+
+        Args:
+            backend_server (BackendServer): The backend server instance that this handler belongs to.
+        """
+        self.logger: lg.Logger = lg.getLogger(
+            f"{__name__}.{self.__class__.__name__}"
+        )
+        self.backend_server: BackendServer = backend_server
+
+    def handle(self, client: Client, message: Message) -> None:
+        """Handle a message from a client.
+
+        This method must be implemented by subclasses to define the specific handling logic for different message types.
+
+        Args:
+            client (Client): The client that sent the message.
+            message (Message): The message to be handled.
+        """
+        raise NotImplementedError("handle() must be implemented by subclasses")
diff --git a/src/judas_server/backend/handler/hello_handler.py b/src/judas_server/backend/handler/hello_handler.py
new file mode 100644
index 0000000..a9d78a1
--- /dev/null
+++ b/src/judas_server/backend/handler/hello_handler.py
@@ -0,0 +1,66 @@
+# -*- coding: utf-8 -*-
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, override
+
+from judas_protocol import Category, ControlAction, Message
+
+from judas_server.backend.client import ClientStatus
+from judas_server.backend.handler import BaseHandler
+
+if TYPE_CHECKING:
+    from judas_server.backend.backend_server import BackendServer
+    from judas_server.backend.client import Client
+
+
+class HelloHandler(BaseHandler):
+    def __init__(self, backend_server: BackendServer) -> None:
+        super().__init__(backend_server)
+
+    @override
+    def handle(self, client: Client, message: Message) -> None:
+        if client.id is not None:
+            return
+
+        if (
+            message.category != Category.CONTROL
+            or message.action != ControlAction.HELLO
+        ):
+            self.logger.error(
+                f"Expected HELLO message from {client}, got {message}, disconnecting client..."
+            )
+            self.backend_server._disconnect(client)
+            return
+
+        if message.payload.get("id") is None:
+            self.logger.error(
+                f"HELLO message from {client} missing 'id' field, disconnecting client..."
+            )
+            self.backend_server._disconnect(client)
+            return
+
+        client.id = message.payload["id"]
+
+        # check if client already connected, if so disconnect old client and register new one
+        if (
+            client.id in self.backend_server.clients
+            and self.backend_server.clients[client.id].status
+            == ClientStatus.ONLINE
+        ):
+            old_client: Client = self.backend_server.clients[client.id]
+            self.backend_server.logger.warning(
+                f"Client {client.id} is already connected from {old_client.addr}, disconnecting old client..."
+            )
+            self.backend_server.send_close(old_client)
+            return
+
+        self.backend_server.clients[client.id] = client  # type: ignore
+        self.backend_server.known_clients[client.id] = {  # type: ignore
+            "last_seen": client.last_seen
+        }
+
+        del self.backend_server.pending_hello[client]
+        self.backend_server._save_known_clients()
+        client.status = ClientStatus.ONLINE
+
+        self.logger.info(f"[+] Registered new client {client}")
diff --git a/uv.lock b/uv.lock
index a96478f..1c9901a 100644
--- a/uv.lock
+++ b/uv.lock
@@ -358,8 +358,8 @@ wheels = [
 
 [[package]]
 name = "judas-protocol"
-version = "0.6.0"
-source = { git = "https://gitea.pufereq.pl/judas/judas_protocol.git#d16c1914ba343aed300f1c5fae0201370c3274de" }
+version = "0.8.0"
+source = { git = "https://gitea.pufereq.pl/judas/judas_protocol.git#a805ccf38edffadc1b8c8b276e60758c86516cd3" }
 
 [[package]]
 name = "judas-server"