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feat(sessions): detach-grace retention window for reconnect

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Joseph Doherty
2026-06-16 06:15:46 -04:00
parent 85e4334bb7
commit db95f8644f
10 changed files with 346 additions and 6 deletions
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docs/Sessions.md
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@@ -72,7 +72,7 @@ private void EnsureSessionCapacity()
}
```
`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.
`SessionManager` also defines four close-reason constants — `DefaultCloseReason` (`"client-close"`), `GatewayShutdownReason` (`"gateway-shutdown"`), `LeaseExpiredReason` (`"lease-expired"`), and `DetachGraceExpiredReason` (`"detach-grace-expired"`) — so that the metrics and worker shutdown paths agree on a fixed vocabulary.
### SessionRegistry (ISessionRegistry)
@@ -199,6 +199,14 @@ Event streaming uses `AttachEventSubscriber` which returns a disposable lease. W
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).
#### Detach-grace retention
`MxGateway:Sessions:DetachGraceSeconds` (default 30) is a bounded retention window kept after a session's *last external (gRPC) event-stream subscriber* drops, so a client can reconnect to the same session instead of having it torn down on the first stream disconnect. While the window is open the session stays `Ready` and fully usable — worker commands continue to work and a reconnecting subscriber re-attaches normally. Because retention is keyed on the *external* subscriber count (`_activeEventSubscriberCount`), and the gateway-owned internal dashboard mirror registers directly on the distributor with `isInternal: true` and is therefore *not* counted, a session whose only remaining subscriber is the dashboard mirror still enters detach-grace.
Mechanically: when the last external subscriber detaches and `DetachGraceSeconds > 0`, `DetachEventSubscriber` stamps `DetachedAtUtc` from the session's `TimeProvider` under `_syncRoot` (the detach→grace-start transition). `AttachEventSubscriber` clears `DetachedAtUtc` under the same lock when a subscriber re-attaches (the reattach→grace-cancel transition), so the two races and the sweeper's read all serialize on `_syncRoot`. `SessionManager.CloseExpiredLeasesAsync` checks `IsDetachGraceExpired(now)` alongside `IsLeaseExpired(now)`: a session detached for at least `DetachGraceSeconds` with no active external subscriber is closed by the same lease sweep, with the distinct `DetachGraceExpiredReason` (`"detach-grace-expired"`) so operators can tell a short reconnect-window expiry from a long idle-lease expiry. Setting `DetachGraceSeconds` to `0` disables retention and reverts to the original behavior: a detached session is retained only until its normal lease expires.
The reconnect/replay path that re-attaches a dropped client to a retained session is implemented separately (Task 12); `DetachGraceSeconds` controls retention and expiry only.
### Close
`GatewaySession.CloseAsync` is serialized by a per-session `SemaphoreSlim` (`_closeLock`) so only one close runs at a time, but every read/write of `_state` still passes through `_syncRoot` (via `TryBeginClose` and `MarkClosed`). The close path therefore obeys the same lock discipline as `TransitionTo` / `MarkFaulted`: it transitions to `Closing`, asks the worker client to shut down within `ShutdownTimeout`, and on success transitions to `Closed`. `DisposeAsync` waits on `_closeLock` once before disposing the semaphore so an in-flight close's `Release()` cannot race against the dispose. If `WorkerClient.ShutdownAsync` throws, the session falls back to `IWorkerClient.Kill` (forced close):