# Gateway Sessions

The sessions subsystem owns the in-memory representation of an active gateway-to-worker pairing and coordinates its lifecycle from open through close. Each `GatewaySession` corresponds to exactly one MXAccess worker process connected over a dedicated named pipe.

## Overview

A session is the gateway-side handle that callers use to invoke worker commands, stream worker events, and tear the worker down. The subsystem is split between the per-session state machine (`GatewaySession`), an in-memory directory (`SessionRegistry`), the orchestrator that opens and closes sessions (`SessionManager`), the worker construction step (`SessionWorkerClientFactory`), and a hosted service that drains sessions during host shutdown (`SessionShutdownHostedService`).

All four interfaces (`ISessionManager`, `ISessionRegistry`, `ISessionWorkerClientFactory`) plus `SessionShutdownHostedService` are wired as singletons by `SessionServiceCollectionExtensions.AddGatewaySessions`.

## Key Types

### GatewaySession

`GatewaySession` is a sealed class that holds the identity, configured timeouts, worker client reference, and current `SessionState` for one session. State is protected by a private `_syncRoot` lock so that property reads and transitions are observed atomically by concurrent gRPC calls and the lease sweeper.

The session id is an opaque string in the form `session-{guid:N}` and the per-session pipe name is `mxaccess-gateway-{ProcessId}-{SessionId}`. Encoding the gateway PID into the pipe name avoids collisions when an old gateway process leaks pipes that the OS has not yet reclaimed.

`SessionState` itself is the protobuf-generated enum from `MxGateway.Contracts.Proto`, so it is shared between the gateway and clients on the wire.

```csharp
public void TransitionTo(SessionState nextState)
{
    lock (_syncRoot)
    {
        if (_state is SessionState.Closed)
        {
            return;
        }

        if (_state is SessionState.Faulted && nextState is not SessionState.Closed)
        {
            return;
        }

        _state = nextState;
    }
}
```

`Closed` is terminal and `Faulted` only allows a transition to `Closed`. This guards against late callbacks (worker exit, heartbeat timeout) re-animating a session that is already torn down.

### SessionManager (ISessionManager)

`SessionManager` is the orchestrator. It exposes `OpenSessionAsync`, `TryGetSession`, `InvokeAsync`, `ReadEventsAsync`, `CloseSessionAsync`, `CloseExpiredLeasesAsync`, and `ShutdownAsync`. It composes `ISessionRegistry`, `ISessionWorkerClientFactory`, `GatewayMetrics`, and `GatewayOptions`.

Concurrency is bounded by a `SemaphoreSlim` initialized to `GatewayOptions.Sessions.MaxSessions`. Open requests that exceed the bound throw `SessionManagerException` with `SessionLimitExceeded` rather than queuing; the caller is expected to retry.

```csharp
private void EnsureSessionCapacity()
{
    if (!_sessionSlots.Wait(0))
    {
        throw new SessionManagerException(
            SessionManagerErrorCode.SessionLimitExceeded,
            $"Gateway session limit {_options.Sessions.MaxSessions} has been reached.");
    }
}
```

`SessionManager` also defines three close-reason constants — `DefaultCloseReason` (`"client-close"`), `GatewayShutdownReason` (`"gateway-shutdown"`), and `LeaseExpiredReason` (`"lease-expired"`) — so that the metrics and worker shutdown paths agree on a fixed vocabulary.

### SessionRegistry (ISessionRegistry)

`SessionRegistry` is a thin wrapper over a `ConcurrentDictionary<string, GatewaySession>` keyed by session id with `StringComparer.Ordinal`. `Snapshot` materializes the values into an array so iteration callers (lease sweeper, shutdown) do not race with concurrent `TryAdd` and `TryRemove` calls.

`ActiveCount` filters out sessions whose state is `Closed`; this is consumed by metrics and the dashboard, where `Count` would otherwise momentarily over-report during teardown.

### SessionWorkerClientFactory (ISessionWorkerClientFactory)

`SessionWorkerClientFactory.CreateAsync` is the only path that builds an `IWorkerClient`. It drives the session through the protobuf `SessionState` substates in order: `StartingWorker`, `WaitingForPipe`, `Handshaking`, `InitializingWorker`. The substates are wire-visible so the dashboard and clients can render startup progress.

A linked `CancellationTokenSource` enforces `session.StartupTimeout`. If startup fails or times out, the factory either kills the partially-constructed `WorkerClient` or, if the client was never built, kills the launched process and disposes the named pipe before rethrowing. A pure timeout is rewritten as `TimeoutException` so callers can distinguish it from caller-driven cancellation:

```csharp
if (exception is OperationCanceledException
    && startupCancellation.IsCancellationRequested
    && !cancellationToken.IsCancellationRequested)
{
    throw new TimeoutException(
        $"Worker session {session.SessionId} did not complete startup within {session.StartupTimeout}.",
        exception);
}
```

The named pipe is created with `maxNumberOfServerInstances: 1` so a second worker cannot connect to the same pipe name even if the first launch is still pending. Combined with the per-session nonce passed to the worker, this is the gateway's defense against a foreign process answering a pipe.

### SessionShutdownHostedService

`SessionShutdownHostedService` is an `IHostedService` whose only job is to call `ISessionManager.ShutdownAsync` from `StopAsync`. It catches `OperationCanceledException` triggered by the host shutdown timeout and logs a warning so that an over-running shutdown does not surface as an unhandled exception.

### SessionOpenRequest

`SessionOpenRequest` is the gateway-internal record passed to `OpenSessionAsync`. It is constructed from the wire-level `OpenSessionRequest` via `SessionOpenRequest.FromContract`. Keeping a separate internal record means the gRPC layer can normalize input (defaulting backend, sanitizing strings) without leaking generated proto types into `SessionManager`.

```csharp
public sealed record SessionOpenRequest(
    string? RequestedBackend,
    string? ClientSessionName,
    string? ClientCorrelationId,
    Duration? CommandTimeout)
{
    public static SessionOpenRequest FromContract(OpenSessionRequest request)
    {
        ArgumentNullException.ThrowIfNull(request);

        return new SessionOpenRequest(
            request.RequestedBackend,
            request.ClientSessionName,
            request.ClientCorrelationId,
            request.CommandTimeout);
    }
}
```

### SessionCloseResult

`SessionCloseResult` is the record returned from a successful close. `AlreadyClosed` distinguishes "this call closed the session" from "the session was already closed when we acquired the close lock", which the metrics layer uses to avoid double-counting.

```csharp
public sealed record SessionCloseResult(
    string SessionId,
    SessionState FinalState,
    bool AlreadyClosed);
```

### SessionCloseStartedException

`SessionCloseStartedException` is `internal` and is only thrown from inside `GatewaySession.CloseAsync` when the close path has already begun mutating worker state and a subsequent step fails. `SessionManager.CloseSessionCoreAsync` catches it, marks the session faulted, increments the close-failed metric, removes the session from the registry, and rethrows it wrapped as `SessionManagerException` with `CloseFailed`. The intermediate type exists so the public API surface only ever exposes `SessionManagerException`.

### SessionManagerException and SessionManagerErrorCode

`SessionManagerException` is the single public error type emitted from this subsystem; the code is carried in the `ErrorCode` property and is also surfaced to metrics tags via `SessionManagerErrorCode.ToString()`.

| Code | Meaning |
|------|---------|
| `SessionNotFound` | The session id is not in the registry. |
| `SessionNotReady` | The session or its `IWorkerClient` is not in `Ready` state. |
| `EventSubscriberAlreadyActive` | A second event subscriber attached when only one is allowed. |
| `EventQueueOverflow` | Reserved for the worker event channel overflow path. |
| `SessionLimitExceeded` | `MaxSessions` is in use. |
| `OpenFailed` | `OpenSessionAsync` failed; the inner exception carries the cause. |
| `CloseFailed` | A close started but did not complete cleanly; the session is removed and faulted. |

## Lifecycle

### Open

`SessionManager.OpenSessionAsync` allocates a session slot, builds the `GatewaySession`, registers it, and asks the factory to bring up the worker. Failures roll back every preceding step:

```csharp
catch (Exception exception)
{
    session?.MarkFaulted(exception.Message);
    if (session is not null)
    {
        _registry.TryRemove(session.SessionId, out _);
        await session.DisposeAsync().ConfigureAwait(false);
    }

    ReleaseSessionSlot();
    _metrics.Fault(SessionManagerErrorCode.OpenFailed.ToString());
    _logger.LogWarning(
        exception,
        "Failed to open gateway session {SessionId}.",
        session?.SessionId ?? "<not-created>");

    throw new SessionManagerException(
        SessionManagerErrorCode.OpenFailed,
        session is null ? "Failed to create session." : $"Failed to open session {session.SessionId}.",
        exception);
}
```

The order — fault, deregister, dispose, release slot, record metric, log, rethrow — matters because releasing the semaphore before disposal would let the next open race the worker process tear-down on the same machine.

### Run

While `Ready`, callers reach the worker through `SessionManager.InvokeAsync` or `ReadEventsAsync`. Both delegate to `GatewaySession`, which checks the state under lock and updates `LastClientActivityAt` on every invocation. `GatewaySession` also exposes typed bulk helpers (`AddItemBulkAsync`, `SubscribeBulkAsync`, etc.) that wrap `WorkerCommand` round-trips and translate non-`Ok` `ProtocolStatus` replies into `SessionManagerException` with `SessionNotReady`.

Event streaming uses `AttachEventSubscriber` which returns a disposable lease. When `allowMultipleSubscribers` is false the second attach throws `EventSubscriberAlreadyActive`; this prevents two gRPC streams from racing on the same worker event channel. Active event subscribers keep the session lease from expiring until the stream is disposed.

Sessions open with `MxGateway:Sessions:DefaultLeaseSeconds` (default 1800) added to the open timestamp. Unary client activity refreshes the lease by the same duration. `ExtendLease` and `IsLeaseExpired` cooperate with `SessionManager.CloseExpiredLeasesAsync`, which iterates a registry snapshot and closes any session whose lease has expired with `LeaseExpiredReason`. `SessionLeaseMonitorHostedService` runs that sweep every `MxGateway:Sessions:LeaseSweepIntervalSeconds` seconds (default 30).

### Close

`GatewaySession.CloseAsync` is serialized by a per-session `SemaphoreSlim` (`_closeLock`). It transitions to `Closing`, asks the worker client to shut down within `ShutdownTimeout`, and on success transitions to `Closed`. If `WorkerClient.ShutdownAsync` throws, the session falls back to `IWorkerClient.Kill` (forced close):

```csharp
if (_workerClient is not null)
{
    try
    {
        await _workerClient.ShutdownAsync(ShutdownTimeout, cancellationToken).ConfigureAwait(false);
    }
    catch (Exception exception)
    {
        try
        {
            _workerClient.Kill(reason);
        }
        catch (Exception killException)
        {
            throw new SessionCloseStartedException(
                $"Session {SessionId} close failed after worker shutdown started.",
                new AggregateException(exception, killException));
        }

        throw;
    }
}
```

If both graceful shutdown and the kill fall-back fail, the original and kill exceptions are bundled into an `AggregateException` and surfaced as `SessionCloseStartedException`. `SessionManager.CloseSessionCoreAsync` then translates that into a `SessionManagerException` with `CloseFailed` and removes the session.

`GatewaySession.KillWorker` is the unconditional forced-close path used by shutdown when graceful close itself throws.

## Shutdown Coordination

`SessionShutdownHostedService.StopAsync` calls `SessionManager.ShutdownAsync`, which closes every registered session with `GatewayShutdownReason`. The shutdown loop catches per-session exceptions, calls `KillWorker`, and removes the session so that one stuck worker cannot block the rest of the host:

```csharp
public async Task ShutdownAsync(CancellationToken cancellationToken)
{
    foreach (GatewaySession session in _registry.Snapshot())
    {
        try
        {
            await CloseSessionCoreAsync(session, GatewayShutdownReason, cancellationToken).ConfigureAwait(false);
        }
        catch (Exception exception)
        {
            _logger.LogWarning(
                exception,
                "Graceful shutdown failed for session {SessionId}; killing worker.",
                session.SessionId);
            if (_registry.TryGet(session.SessionId, out _))
            {
                session.KillWorker(GatewayShutdownReason);
                await RemoveSessionAsync(session).ConfigureAwait(false);
            }
        }
    }
}
```

Iterating over `Snapshot` rather than the live dictionary lets `RemoveSessionAsync` mutate the registry inside the loop without throwing.

## Dependency Injection

`SessionServiceCollectionExtensions.AddGatewaySessions` registers the four singletons and the hosted service:

```csharp
public static IServiceCollection AddGatewaySessions(this IServiceCollection services)
{
    services.AddSingleton<ISessionRegistry, SessionRegistry>();
    services.AddSingleton<ISessionWorkerClientFactory, SessionWorkerClientFactory>();
    services.AddSingleton<ISessionManager, SessionManager>();
    services.AddHostedService<SessionShutdownHostedService>();

    return services;
}
```

The registry must be a singleton because its `ConcurrentDictionary` is the source of truth for session state across the gRPC service, the lease sweeper, the dashboard, and the shutdown hosted service. Registering `SessionShutdownHostedService` last ensures it is constructed after `ISessionManager` and therefore drains sessions during host stop.

## Related Documentation

- [Gateway Process Design](./GatewayProcessDesign.md)
- [Gateway Configuration](./GatewayConfiguration.md)
- [Worker Process Launcher](./WorkerProcessLauncher.md)
