Added How To Reject Client Connections guide

docs/how-to/reject-client-connections.md

@@ -0,0 +1,56 @@
+# Rejecting Client Connections
+
+SpacetimeDB provides a way to disconnect a client during a client connection attempt.
+
+:::server-rust
+In Rust, if we returned and error (or a panic) during the `client_connected` reducer, the client will be disconnected.
+
+Here is a simple example where the server module throws an error for all incoming client connections.
+```rust
+#[reducer(client_connected)]
+pub fn client_connected(_ctx: &ReducerContext) -> Result<(), String> {
+    let client_is_rejected = true;
+    if client_is_rejected {
+        Err("The client connection was rejected. With our current code logic, all clients will be rejected.".to_string())
+    } else {
+        Ok(())
+    }
+}
+```
+
+Client behavior can vary by client type. For example:
+* **C# clients**: Client disconnection behavior is currently undefined and will generate an error reading:
+  `Unable to send subscribe message: WS sender loop has dropped its recv channel: TrySendError { kind: Disconnected }`
+
+* **Rust clients**: Client disconnection behavior is currently undefined and will generate an error reading:
+  `Unable to send subscribe message: WS sender loop has dropped its recv channel: TrySendError { kind: Disconnected }`
+
+* **TypeScript clients**: Client will receive an `Error connecting to SpacetimeDB:` and a `CloseEvent` with a code of 1006.
+
+Regardless of the client type, from the rust server's perspective, the client will be disconnected and the server module's logs will contain an entry reading:
+`ERROR: : The client connection was rejected. With our current code logic, all clients will be rejected.`
+:::
+:::server-csharp
+In C#, if we throw an exception during the `ClientConnected` reducer, the client will be disconnected.
+
+Here is a simple example where the server module throws an error for all incoming client connections.
+```csharp
+[Reducer(ReducerKind.ClientConnected)]
+// Called when a client connects to a SpacetimeDB database server
+public static void ClientConnected(ReducerContext ctx)
+{
+    throw new Exception("The client connection was rejected. With our current code logic, all clients will be rejected.");
+}
+```
+
+Client behavior can vary by client type. For example:
+* **C# clients**: Client disconnection behavior is currently undefined and will generate an error reading:
+`Disconnected abnormally: System.Net.WebSockets.WebSocketException (0x80004005): The remote party closed the WebSocket connection without completing the close handshake.`
+
+* **Rust clients**: Client will receive an `on_disconnected` event with no error message.
+
+* **TypeScript clients**: Client will receive an `Error connecting to SpacetimeDB:` and a `CloseEvent` with a code of 1006.
+
+Regardless of the client type, from the C# server's perspective, the client will be disconnected and the server module's logs will contain an entry reading:
+`ERROR: : System.Exception: The client connection was rejected. With our current code logic, all clients will be rejected.`
+:::