fraiseql_server/routes/

subscriptions.rs

//! `WebSocket` subscription handler with protocol negotiation.
//!
//! Supports both the modern `graphql-transport-ws` protocol and the legacy
//! `graphql-ws` (Apollo subscriptions-transport-ws) protocol. Protocol
//! selection happens during the `WebSocket` upgrade via the `Sec-WebSocket-Protocol`
//! header.
//!
//! # Lifecycle Hooks
//!
//! Configurable callbacks are invoked at key points in the subscription
//! lifecycle: `on_connect`, `on_disconnect`, `on_subscribe`, `on_unsubscribe`.
//!
//! # Example
//!
//! ```text
//! // Requires: running server with initialized subscription manager.
//! use fraiseql_server::routes::subscriptions::{subscription_handler, SubscriptionState};
//!
//! let state = SubscriptionState::new(subscription_manager);
//!
//! let app = Router::new()
//!     .route("/ws", get(subscription_handler))
//!     .with_state(state);
//! ```

use std::{
    collections::HashMap,
    sync::{
        Arc,
        atomic::{AtomicU64, Ordering},
    },
    time::Duration,
};

use axum::{
    extract::{
        State,
        ws::{Message, WebSocket, WebSocketUpgrade},
    },
    http::HeaderMap,
    response::IntoResponse,
};
use fraiseql_core::{
    runtime::{
        SubscriptionId, SubscriptionManager, SubscriptionPayload,
        protocol::{
            ClientMessage, ClientMessageType, CloseCode, GraphQLError, ServerMessage,
            SubscribePayload,
        },
    },
    security::{Authorizer, OperationKind, SecurityContext, authorizer::enforce_authz},
};
use futures::{SinkExt, StreamExt};
use tokio::sync::broadcast;
use tracing::{debug, error, info, warn};

use crate::{
    extractors::OptionalSecurityContext,
    routes::graphql::DomainRegistry,
    subscriptions::{
        lifecycle::SubscriptionLifecycle,
        protocol::{ProtocolCodec, WsProtocol},
    },
};

// ── Subscription metrics (module-level atomics) ──────────────────────

static WS_CONNECTIONS_ACCEPTED: AtomicU64 = AtomicU64::new(0);
static WS_CONNECTIONS_REJECTED: AtomicU64 = AtomicU64::new(0);
static WS_SUBSCRIPTIONS_ACCEPTED: AtomicU64 = AtomicU64::new(0);
static WS_SUBSCRIPTIONS_REJECTED: AtomicU64 = AtomicU64::new(0);

/// Subscription metrics for Prometheus export.
#[must_use]
pub fn subscription_metrics() -> SubscriptionMetrics {
    SubscriptionMetrics {
        connections_accepted:   WS_CONNECTIONS_ACCEPTED.load(Ordering::Relaxed),
        connections_rejected:   WS_CONNECTIONS_REJECTED.load(Ordering::Relaxed),
        subscriptions_accepted: WS_SUBSCRIPTIONS_ACCEPTED.load(Ordering::Relaxed),
        subscriptions_rejected: WS_SUBSCRIPTIONS_REJECTED.load(Ordering::Relaxed),
    }
}

/// Reset all subscription counters to zero.
///
/// Call this at the start of each test that checks counter values to avoid
/// cross-test interference from the module-level statics.
#[cfg(test)]
pub fn reset_metrics_for_test() {
    WS_CONNECTIONS_ACCEPTED.store(0, Ordering::SeqCst);
    WS_CONNECTIONS_REJECTED.store(0, Ordering::SeqCst);
    WS_SUBSCRIPTIONS_ACCEPTED.store(0, Ordering::SeqCst);
    WS_SUBSCRIPTIONS_REJECTED.store(0, Ordering::SeqCst);
}

/// Snapshot of subscription counters.
pub struct SubscriptionMetrics {
    /// Total `WebSocket` connections accepted (after `on_connect`).
    pub connections_accepted:   u64,
    /// Total `WebSocket` connections rejected by lifecycle hook.
    pub connections_rejected:   u64,
    /// Total subscriptions accepted (after `on_subscribe`).
    pub subscriptions_accepted: u64,
    /// Total subscriptions rejected (by hook or limit).
    pub subscriptions_rejected: u64,
}

/// Connection initialization timeout (5 seconds per graphql-ws spec).
const CONNECTION_INIT_TIMEOUT: Duration = Duration::from_secs(5);

/// Ping/keepalive interval.
const PING_INTERVAL: Duration = Duration::from_secs(30);

/// State for subscription `WebSocket` handler.
#[derive(Clone)]
pub struct SubscriptionState {
    /// Subscription manager.
    pub manager: Arc<SubscriptionManager>,
    /// Lifecycle hooks.
    pub lifecycle: Arc<dyn SubscriptionLifecycle>,
    /// Maximum subscriptions per connection (`None` = unlimited).
    pub max_subscriptions_per_connection: Option<u32>,
    /// Subscription fields owned by remote subgraphs.
    ///
    /// Maps root subscription field name to the subgraph `WebSocket` URL.
    /// Empty when federation is disabled or no remote subscription fields are declared.
    pub remote_subscription_fields: Arc<HashMap<String, String>>,
    /// Host-header → tenant-key domain registry. `None` until a host binary
    /// installs one (mirrors `AppState::domain_registry`).
    pub domain_registry: Option<Arc<DomainRegistry>>,
    /// Reject conflicting tenant sources (JWT vs `X-Tenant-ID` vs Host) on the
    /// upgrade. Driven by `schema.has_rls_configured()`, mirroring the GraphQL
    /// handler's strict tenant validation.
    pub strict_tenant_validation: bool,
    /// Optional operation-level authorizer (#422). When set, each subscription is
    /// authorized at establishment with [`OperationKind::Subscription`], the
    /// subscription field name, and the connection's principal. `None` until a host
    /// binary installs one (from `Executor::config().authorizer`).
    pub authorizer: Option<Arc<dyn Authorizer>>,
}

impl SubscriptionState {
    /// Create new subscription state.
    pub fn new(manager: Arc<SubscriptionManager>) -> Self {
        Self {
            manager,
            lifecycle: Arc::new(crate::subscriptions::lifecycle::NoopLifecycle),
            max_subscriptions_per_connection: None,
            remote_subscription_fields: Arc::new(HashMap::new()),
            domain_registry: None,
            strict_tenant_validation: false,
            authorizer: None,
        }
    }

    /// Install the operation-level authorizer (#422). When set, every subscription is
    /// authorized at establishment; a `Deny` (or any policy error) rejects the
    /// subscription with a `FORBIDDEN` GraphQL-WS error. Typically populated from
    /// `Executor::config().authorizer`.
    #[must_use]
    pub fn with_authorizer(mut self, authorizer: Option<Arc<dyn Authorizer>>) -> Self {
        self.authorizer = authorizer;
        self
    }

    /// Install the tenant-resolution context — the Host-header domain registry
    /// and strict-validation flag — so the subscription upgrade dispatches the
    /// tenant key the same way the GraphQL handler does (JWT `tenant_id` >
    /// `X-Tenant-ID` header > Host domain, with cross-source conflict rejection
    /// when `strict_tenant_validation` is set). See
    /// [`crate::routes::graphql::TenantKeyResolver`].
    #[must_use]
    pub fn with_tenant_context(
        mut self,
        domain_registry: Arc<DomainRegistry>,
        strict_tenant_validation: bool,
    ) -> Self {
        self.domain_registry = Some(domain_registry);
        self.strict_tenant_validation = strict_tenant_validation;
        self
    }

    /// Set lifecycle hooks.
    #[must_use]
    pub fn with_lifecycle(mut self, lifecycle: Arc<dyn SubscriptionLifecycle>) -> Self {
        self.lifecycle = lifecycle;
        self
    }

    /// Set maximum subscriptions per connection.
    #[must_use]
    pub const fn with_max_subscriptions(mut self, max: Option<u32>) -> Self {
        self.max_subscriptions_per_connection = max;
        self
    }

    /// Set remote subscription fields (federation passthrough).
    ///
    /// Maps subscription field names to the owning subgraph's `WebSocket` URL.
    #[must_use]
    pub fn with_remote_subscription_fields(mut self, fields: HashMap<String, String>) -> Self {
        self.remote_subscription_fields = Arc::new(fields);
        self
    }
}

/// `WebSocket` upgrade handler for subscriptions.
///
/// Negotiates the `WebSocket` sub-protocol from the `Sec-WebSocket-Protocol`
/// header. Supports `graphql-transport-ws` (modern) and `graphql-ws` (legacy).
/// Defaults to `graphql-transport-ws` when no header is present.
/// Returns `400 Bad Request` for unrecognised protocols.
pub async fn subscription_handler(
    headers: HeaderMap,
    OptionalSecurityContext(security_context): OptionalSecurityContext,
    ws: WebSocketUpgrade,
    State(state): State<SubscriptionState>,
) -> impl IntoResponse {
    let protocol_header = headers.get("sec-websocket-protocol").and_then(|v| v.to_str().ok());

    let protocol = match protocol_header {
        None => WsProtocol::GraphqlTransportWs,
        Some(header) => {
            if let Some(p) = WsProtocol::from_header(Some(header)) {
                p
            } else {
                warn!(header = %header, "Unknown WebSocket sub-protocol requested");
                return axum::http::StatusCode::BAD_REQUEST.into_response();
            }
        },
    };

    // Resolve the tenant key exactly as the GraphQL handler does: the JWT
    // `tenant_id` (trusted) takes precedence over the `X-Tenant-ID` header and
    // the Host-domain lookup, and conflicting sources are rejected when strict
    // validation is enabled. Previously this passed `None, None, false`,
    // silently dropping JWT precedence, ignoring an installed domain registry,
    // and disabling strict cross-source validation (#331).
    let tenant_id = match resolve_subscription_tenant(security_context.as_ref(), &headers, &state) {
        Ok(tenant_id) => tenant_id,
        Err(e) => {
            warn!(error = %e, "Subscription tenant resolution rejected the upgrade");
            return axum::http::StatusCode::BAD_REQUEST.into_response();
        },
    };

    ws.protocols([protocol.as_str()])
        .on_upgrade(move |socket| {
            handle_subscription_connection(socket, state, protocol, tenant_id, security_context)
        })
        .into_response()
}

/// Resolve the subscription's tenant key, mirroring the GraphQL handler's
/// dispatch (`routes/graphql/handler.rs`): JWT `tenant_id` > `X-Tenant-ID`
/// header > Host-domain registry, with cross-source conflict rejection when the
/// schema has RLS configured (`strict_tenant_validation`).
///
/// # Errors
///
/// Returns `FraiseQLError::Validation` when the `X-Tenant-ID` header is invalid,
/// or — under `strict_tenant_validation` — when the resolved sources disagree.
fn resolve_subscription_tenant(
    security_context: Option<&SecurityContext>,
    headers: &HeaderMap,
    state: &SubscriptionState,
) -> fraiseql_error::Result<Option<String>> {
    super::graphql::TenantKeyResolver::resolve(
        security_context,
        headers,
        state.domain_registry.as_deref(),
        state.strict_tenant_validation,
    )
}

/// Handle a `WebSocket` subscription connection.
#[allow(clippy::cognitive_complexity)] // Reason: WebSocket protocol state machine with message routing and lifecycle management
async fn handle_subscription_connection(
    socket: WebSocket,
    state: SubscriptionState,
    protocol: WsProtocol,
    tenant_id: Option<String>,
    principal: Option<SecurityContext>,
) {
    let connection_id = uuid::Uuid::new_v4().to_string();
    let codec = ProtocolCodec::new(protocol);
    info!(
        connection_id = %connection_id,
        protocol = %protocol.as_str(),
        "WebSocket connection established"
    );

    let (mut sender, mut receiver) = socket.split();

    // Wait for connection_init with timeout
    let init_result = tokio::time::timeout(CONNECTION_INIT_TIMEOUT, async {
        while let Some(msg) = receiver.next().await {
            match msg {
                Ok(Message::Text(text)) => {
                    if let Ok(client_msg) = codec.decode(&text) {
                        if client_msg.parsed_type() == Some(ClientMessageType::ConnectionInit) {
                            return Some(client_msg);
                        }
                    }
                },
                Ok(Message::Close(_)) => return None,
                Err(e) => {
                    error!(error = %e, "WebSocket error during init");
                    return None;
                },
                _ => {},
            }
        }
        None
    })
    .await;

    // Handle init timeout or failure
    let _init_payload = match init_result {
        Ok(Some(msg)) => {
            // Call lifecycle on_connect hook
            let params = msg.payload.clone().unwrap_or(serde_json::json!({}));
            if let Err(reason) = state.lifecycle.on_connect(&params, &connection_id).await {
                warn!(
                    connection_id = %connection_id,
                    reason = %reason,
                    "Lifecycle on_connect rejected connection"
                );
                WS_CONNECTIONS_REJECTED.fetch_add(1, Ordering::Relaxed);
                // Best-effort: connection is already being terminated.
                let _ = sender
                    .send(Message::Close(Some(axum::extract::ws::CloseFrame {
                        code:   4400,
                        reason: reason.into(),
                    })))
                    .await;
                return;
            }

            // Send connection_ack
            let ack = ServerMessage::connection_ack(None);
            if let Err(send_err) = send_server_message(&codec, &mut sender, ack).await {
                error!(connection_id = %connection_id, error = %send_err, "Failed to send connection_ack");
                return;
            }
            WS_CONNECTIONS_ACCEPTED.fetch_add(1, Ordering::Relaxed);
            info!(connection_id = %connection_id, "Connection initialized");
            msg.payload
        },
        Ok(None) => {
            warn!(connection_id = %connection_id, "Connection closed during init");
            return;
        },
        Err(_) => {
            warn!(connection_id = %connection_id, "Connection init timeout");
            // Best-effort: connection is already being terminated.
            let _ = sender
                .send(Message::Close(Some(axum::extract::ws::CloseFrame {
                    code:   CloseCode::ConnectionInitTimeout.code(),
                    reason: CloseCode::ConnectionInitTimeout.reason().into(),
                })))
                .await;
            return;
        },
    };

    // Track active operations (operation_id -> subscription_id)
    let mut active_operations: HashMap<String, SubscriptionId> = HashMap::new();

    // Remote subscription message output channel.
    //
    // Forwarder tasks (federation feature) send pre-encoded ServerMessage values here.
    // The channel is always present so the select! loop has a uniform branch regardless
    // of whether any remote subscriptions are active.
    let (remote_msg_tx, mut remote_msg_rx) = tokio::sync::mpsc::channel::<ServerMessage>(64);

    // Subscribe to event broadcast
    let mut event_receiver = state.manager.receiver();

    // Ping/keepalive timer
    let mut ping_interval = tokio::time::interval(PING_INTERVAL);
    ping_interval.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);

    // A44 — Token expiry re-check on long-lived subscriptions.
    //
    // JWTs validated at ConnectionInit may expire while the WebSocket is open.
    // The check below should be added when the auth layer surfaces expiry data:
    //
    //   1. At ConnectionInit, extract the `exp` claim from the JWT and store it: `let
    //      token_expires_at: Option<std::time::Instant> = extract_exp(&init_payload);`
    //
    //   2. In the select! loop (before processing each client message or broadcast event), check
    //      expiry: ```rust,ignore if token_expires_at.is_some_and(|exp| std::time::Instant::now()
    //      >= exp) { warn!(connection_id = %connection_id, "Token expired; closing WebSocket"); let
    //      _ = sender.send(Message::Close(Some(axum::extract::ws::CloseFrame { code:
    //      CloseCode::Unauthorized.code(), reason: "Token expired".into(), }))).await; break; } ```
    //
    // This requires the lifecycle `on_connect` hook or the JWT middleware to return
    // the expiry time, which is not yet threaded through `SubscriptionState`.
    // Tracked as A44 in the remediation plan.

    // Main message loop
    loop {
        tokio::select! {
            msg = receiver.next() => {
                match msg {
                    Some(Ok(Message::Text(text))) => {
                        if let Err(close_code) = handle_client_message(
                            &text,
                            &connection_id,
                            &state,
                            &codec,
                            &mut active_operations,
                            remote_msg_tx.clone(),
                            &mut sender,
                            tenant_id.as_deref(),
                            principal.as_ref(),
                        ).await {
                            // Best-effort: connection is already being closed.
                            let _ = sender.send(Message::Close(Some(axum::extract::ws::CloseFrame {
                                code: close_code.code(),
                                reason: close_code.reason().into(),
                            }))).await;
                            break;
                        }
                    }
                    Some(Ok(Message::Ping(data))) => {
                        // Best-effort: if the connection is already dead the pong will fail.
                        let _ = sender.send(Message::Pong(data)).await;
                    }
                    Some(Ok(Message::Close(_))) => {
                        info!(connection_id = %connection_id, "Client closed connection");
                        break;
                    }
                    Some(Err(e)) => {
                        error!(connection_id = %connection_id, error = %e, "WebSocket error");
                        break;
                    }
                    None => {
                        info!(connection_id = %connection_id, "WebSocket stream ended");
                        break;
                    }
                    _ => {}
                }
            }

            event = event_receiver.recv() => {
                match event {
                    Ok(payload) => {
                        // Defense-in-depth tenant guard: when both the connection and the
                        // event carry an explicit tenant_id they must agree. Primary
                        // isolation is already guaranteed by subscription_id UUIDs, but
                        // this check catches any future path that introduces deterministic
                        // subscription IDs (which could collide across tenants).
                        let tenant_matches = match (
                            tenant_id.as_deref(),
                            payload.event.tenant_id.as_deref(),
                        ) {
                            (Some(conn_tid), Some(evt_tid)) => conn_tid == evt_tid,
                            _ => true, // either side absent → no conflict
                        };
                        if tenant_matches {
                            if let Some((op_id, _)) = active_operations
                                .iter()
                                .find(|(_, sub_id)| **sub_id == payload.subscription_id)
                            {
                                let msg = create_next_message(op_id, &payload);
                                if send_server_message(&codec, &mut sender, msg).await.is_err() {
                                    warn!(connection_id = %connection_id, "Failed to send event");
                                    break;
                                }
                            }
                        }
                    }
                    Err(broadcast::error::RecvError::Lagged(n)) => {
                        warn!(connection_id = %connection_id, lagged = n, "Event receiver lagged");
                    }
                    Err(broadcast::error::RecvError::Closed) => {
                        error!(connection_id = %connection_id, "Event channel closed");
                        break;
                    }
                }
            }

            remote_msg = remote_msg_rx.recv() => {
                // Remote subscription event forwarded by a federation forwarder task.
                // The channel is always present; it only carries messages when the
                // federation feature is enabled and remote subscriptions are active.
                if let Some(msg) = remote_msg {
                    if send_server_message(&codec, &mut sender, msg).await.is_err() {
                        warn!(connection_id = %connection_id, "Failed to send remote subscription message");
                        break;
                    }
                }
            }

            _ = ping_interval.tick() => {
                let msg = ServerMessage::ping(None);
                if send_server_message(&codec, &mut sender, msg).await.is_err() {
                    warn!(connection_id = %connection_id, "Failed to send ping/keepalive");
                    break;
                }
            }
        }
    }

    // Cleanup
    state.manager.unsubscribe_connection(&connection_id);
    state.lifecycle.on_disconnect(&connection_id).await;
    info!(connection_id = %connection_id, "WebSocket connection closed");
}

/// Handle a client message.
///
/// Returns `Ok(())` on success, or `Err(CloseCode)` if the connection should be closed.
///
/// `remote_msg_tx` is used by federation forwarder tasks to send pre-encoded
/// `ServerMessage` values back to the client connection loop.
#[allow(clippy::cognitive_complexity)] // Reason: WebSocket message dispatch with subscribe/unsubscribe/query protocol handling
#[allow(clippy::too_many_arguments)] // Reason: WebSocket handler needs connection state, protocol codec, and tenant context
async fn handle_client_message(
    text: &str,
    connection_id: &str,
    state: &SubscriptionState,
    codec: &ProtocolCodec,
    active_operations: &mut HashMap<String, SubscriptionId>,
    remote_msg_tx: tokio::sync::mpsc::Sender<ServerMessage>,
    sender: &mut futures::stream::SplitSink<WebSocket, Message>,
    tenant_id: Option<&str>,
    principal: Option<&SecurityContext>,
) -> Result<(), CloseCode> {
    // remote_msg_tx is only consumed inside the #[cfg(feature = "federation")] block below.
    // When the feature is disabled the parameter goes unused; suppress the warning.
    #[cfg(not(feature = "federation"))]
    let _ = &remote_msg_tx;

    let client_msg: ClientMessage = codec.decode(text).map_err(|e| {
        warn!(error = %e, "Failed to parse client message");
        CloseCode::ProtocolError
    })?;

    match client_msg.parsed_type() {
        Some(ClientMessageType::Ping) => {
            let pong = ServerMessage::pong(client_msg.payload);
            // Best-effort: if the connection is already dead the pong will fail.
            let _ = send_server_message(codec, sender, pong).await;
        },

        Some(ClientMessageType::Pong) => {
            debug!(connection_id = %connection_id, "Received pong");
        },

        Some(ClientMessageType::Subscribe) => {
            let payload: SubscribePayload = client_msg.subscription_payload().ok_or_else(|| {
                warn!("Invalid subscribe payload");
                CloseCode::ProtocolError
            })?;

            let op_id = client_msg.id.ok_or_else(|| {
                warn!("Subscribe message missing operation ID");
                CloseCode::ProtocolError
            })?;

            // Check for duplicate operation ID
            if active_operations.contains_key(&op_id) {
                warn!(operation_id = %op_id, "Duplicate operation ID");
                return Err(CloseCode::SubscriberAlreadyExists);
            }

            // Enforce per-connection subscription limit
            if let Some(max) = state.max_subscriptions_per_connection {
                if active_operations.len() >= max as usize {
                    warn!(
                        connection_id = %connection_id,
                        active = active_operations.len(),
                        max = max,
                        "Subscription limit reached"
                    );
                    WS_SUBSCRIPTIONS_REJECTED.fetch_add(1, Ordering::Relaxed);
                    let error = ServerMessage::error(
                        &op_id,
                        vec![GraphQLError::with_code(
                            format!("Maximum subscriptions per connection ({max}) reached"),
                            "SUBSCRIPTION_LIMIT_REACHED",
                        )],
                    );
                    if let Err(e) = send_server_message(codec, sender, error).await {
                        debug!(connection_id = %connection_id, error = %e, "Could not send subscription limit error to client");
                    }
                    return Ok(());
                }
            }

            // Extract subscription name from query
            let Some(subscription_name) = extract_subscription_name(&payload.query) else {
                let error = ServerMessage::error(
                    &op_id,
                    vec![GraphQLError::with_code(
                        "Could not parse subscription query",
                        "PARSE_ERROR",
                    )],
                );
                if let Err(e) = send_server_message(codec, sender, error).await {
                    debug!(connection_id = %connection_id, error = %e, "Could not send parse error to client");
                }
                return Ok(());
            };

            // Call lifecycle on_subscribe hook
            // HashMap<String, Value> serialization is infallible; the error path cannot occur.
            let variables_value = serde_json::to_value(&payload.variables)
                .expect("HashMap<String, serde_json::Value> serialization is infallible");

            // #422: operation-level authorization at subscription establishment.
            // The per-event delivery does not route through the executor, so the
            // subscription is authorized once, here, with the connection's principal
            // (or `None` when anonymous). Fail-closed: a `Deny` or any policy error
            // rejects the subscription with a `FORBIDDEN` GraphQL-WS error.
            if let Some(authorizer) = state.authorizer.as_ref() {
                let ops = [(OperationKind::Subscription, subscription_name.clone())];
                if let Err(err) =
                    enforce_authz(authorizer.as_ref(), principal, &ops, Some(&variables_value))
                {
                    warn!(
                        connection_id = %connection_id,
                        subscription = %subscription_name,
                        "Operation authorizer denied the subscription"
                    );
                    WS_SUBSCRIPTIONS_REJECTED.fetch_add(1, Ordering::Relaxed);
                    let error = ServerMessage::error(
                        &op_id,
                        vec![GraphQLError::with_code(err.to_string(), "FORBIDDEN")],
                    );
                    if let Err(e) = send_server_message(codec, sender, error).await {
                        debug!(connection_id = %connection_id, error = %e, "Could not send authorization denial to client");
                    }
                    return Ok(());
                }
            }

            if let Err(reason) = state
                .lifecycle
                .on_subscribe(&subscription_name, &variables_value, connection_id)
                .await
            {
                warn!(
                    connection_id = %connection_id,
                    subscription = %subscription_name,
                    reason = %reason,
                    "Lifecycle on_subscribe rejected subscription"
                );
                WS_SUBSCRIPTIONS_REJECTED.fetch_add(1, Ordering::Relaxed);
                let error = ServerMessage::error(
                    &op_id,
                    vec![GraphQLError::with_code(reason, "SUBSCRIPTION_REJECTED")],
                );
                if let Err(e) = send_server_message(codec, sender, error).await {
                    debug!(connection_id = %connection_id, error = %e, "Could not send subscription rejection to client");
                }
                return Ok(());
            }

            // Forward to remote subgraph when the subscription field is owned remotely.
            #[cfg(feature = "federation")]
            if let Some(subgraph_url) = state.remote_subscription_fields.get(&subscription_name) {
                use fraiseql_federation::subscription_forwarder::{
                    ForwardedEvent, SubscriptionForwarder,
                };

                match SubscriptionForwarder::new(subgraph_url) {
                    Ok(forwarder) => {
                        // Create a channel so the forwarder task can send us raw events.
                        let (event_tx, mut event_rx) =
                            tokio::sync::mpsc::channel::<ForwardedEvent>(32);

                        // Task 1: run the WebSocket forwarder (sends ForwardedEvent to event_tx).
                        let fwd_op = op_id.clone();
                        let fwd_query = payload.query.clone();
                        let fwd_vars = variables_value.clone();
                        tokio::spawn(async move {
                            if let Err(e) =
                                forwarder.forward(&fwd_op, &fwd_query, fwd_vars, event_tx).await
                            {
                                warn!(error = %e, "Remote subscription forwarder failed");
                            }
                        });

                        // Task 2: relay ForwardedEvent → ServerMessage → client.
                        let relay_op = op_id.clone();
                        let relay_tx = remote_msg_tx.clone();
                        tokio::spawn(async move {
                            while let Some(event) = event_rx.recv().await {
                                let server_msg = match event {
                                    ForwardedEvent::Next(data) => {
                                        ServerMessage::next(&relay_op, data)
                                    },
                                    ForwardedEvent::Error(errors) => {
                                        let errors_vec = errors.as_array().map_or_else(
                                            || {
                                                vec![GraphQLError::with_code(
                                                    errors.to_string(),
                                                    "REMOTE_ERROR",
                                                )]
                                            },
                                            |arr| {
                                                arr.iter()
                                                    .map(|e| {
                                                        GraphQLError::with_code(
                                                            e.get("message")
                                                                .and_then(|v| v.as_str())
                                                                .unwrap_or("Remote subgraph error"),
                                                            "REMOTE_ERROR",
                                                        )
                                                    })
                                                    .collect()
                                            },
                                        );
                                        ServerMessage::error(&relay_op, errors_vec)
                                    },
                                    ForwardedEvent::Complete => ServerMessage::complete(&relay_op),
                                };
                                if relay_tx.send(server_msg).await.is_err() {
                                    break; // Client disconnected
                                }
                            }
                        });

                        WS_SUBSCRIPTIONS_ACCEPTED.fetch_add(1, Ordering::Relaxed);
                        info!(
                            connection_id = %connection_id,
                            operation_id = %op_id,
                            subscription = %subscription_name,
                            "Subscription forwarded to remote subgraph"
                        );
                        return Ok(());
                    },
                    Err(e) => {
                        let error = ServerMessage::error(
                            &op_id,
                            vec![GraphQLError::with_code(e.to_string(), "SUBSCRIPTION_ERROR")],
                        );
                        if let Err(send_err) = send_server_message(codec, sender, error).await {
                            debug!(connection_id = %connection_id, error = %send_err, "Could not send forwarding error to client");
                        }
                        return Ok(());
                    },
                }
            }

            // Validate client-provided tenant variable against server-resolved
            if let Some(server_tid) = tenant_id {
                if let Some(client_tid) = variables_value.get("tenant_id").and_then(|v| v.as_str())
                {
                    if client_tid != server_tid {
                        let error = ServerMessage::error(
                            &op_id,
                            vec![GraphQLError::with_code(
                                format!(
                                    "Tenant mismatch: client provided '{client_tid}', server resolved '{server_tid}'"
                                ),
                                "TENANT_MISMATCH",
                            )],
                        );
                        if let Err(send_err) = send_server_message(codec, sender, error).await {
                            debug!(connection_id = %connection_id, error = %send_err, "Could not send tenant mismatch error to client");
                        }
                        return Ok(());
                    }
                }
            }

            // Build context with server-resolved tenant_id
            let mut context = serde_json::json!({});
            if let Some(tid) = tenant_id {
                context["tenant_id"] = serde_json::Value::String(tid.to_string());
            }

            // Subscribe locally (field is owned by this subgraph)
            match state.manager.subscribe(
                &subscription_name,
                context,
                variables_value,
                connection_id,
            ) {
                Ok(sub_id) => {
                    active_operations.insert(op_id.clone(), sub_id);
                    WS_SUBSCRIPTIONS_ACCEPTED.fetch_add(1, Ordering::Relaxed);
                    info!(
                        connection_id = %connection_id,
                        operation_id = %op_id,
                        subscription = %subscription_name,
                        "Subscription started"
                    );
                },
                Err(e) => {
                    let error = ServerMessage::error(
                        &op_id,
                        vec![GraphQLError::with_code(e.to_string(), "SUBSCRIPTION_ERROR")],
                    );
                    if let Err(send_err) = send_server_message(codec, sender, error).await {
                        debug!(connection_id = %connection_id, error = %send_err, "Could not send subscription error to client");
                    }
                },
            }
        },

        Some(ClientMessageType::Complete) => {
            let op_id = client_msg.id.ok_or_else(|| {
                warn!("Complete message missing operation ID");
                CloseCode::ProtocolError
            })?;

            if let Some(sub_id) = active_operations.remove(&op_id) {
                if let Err(e) = state.manager.unsubscribe(sub_id) {
                    warn!(connection_id = %connection_id, operation_id = %op_id, error = %e, "Failed to unsubscribe; subscription may be leaked");
                }
                state.lifecycle.on_unsubscribe(&op_id, connection_id).await;
                info!(
                    connection_id = %connection_id,
                    operation_id = %op_id,
                    "Subscription completed"
                );
            }
        },

        Some(ClientMessageType::ConnectionInit) => {
            warn!(connection_id = %connection_id, "Duplicate connection_init");
            return Err(CloseCode::TooManyInitRequests);
        },

        None => {
            warn!(message_type = %client_msg.message_type, "Unknown message type");
        },
        // Reason: non_exhaustive requires catch-all for cross-crate matches
        _ => {
            warn!(message_type = %client_msg.message_type, "Unrecognized message type");
        },
    }

    Ok(())
}

/// Send a server message through the codec, handling protocol translation.
async fn send_server_message(
    codec: &ProtocolCodec,
    sender: &mut futures::stream::SplitSink<WebSocket, Message>,
    msg: ServerMessage,
) -> Result<(), String> {
    match codec.encode(&msg) {
        Ok(Some(json)) => sender.send(Message::Text(json.into())).await.map_err(|e| e.to_string()),
        Ok(None) => Ok(()), // Message suppressed by codec (e.g. pong in legacy mode)
        Err(e) => Err(e.to_string()),
    }
}

/// Create a "next" message for a subscription event.
fn create_next_message(operation_id: &str, payload: &SubscriptionPayload) -> ServerMessage {
    let data = serde_json::json!({
        payload.subscription_name.clone(): payload.data
    });
    ServerMessage::next(operation_id, data)
}

/// Extract subscription name from a GraphQL subscription query.
pub(crate) fn extract_subscription_name(query: &str) -> Option<String> {
    let query = query.trim();

    let sub_idx = query.find("subscription")?;
    let after_sub = &query[sub_idx + "subscription".len()..];

    let brace_idx = after_sub.find('{')?;
    let after_brace = after_sub[brace_idx + 1..].trim_start();

    let name_end = after_brace
        .find(|c: char| !c.is_alphanumeric() && c != '_')
        .unwrap_or(after_brace.len());

    if name_end == 0 {
        return None;
    }

    Some(after_brace[..name_end].to_string())
}

#[cfg(test)]
mod tests;