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feat(sessions): multi-subscriber cap enforcement + mode-gated FailFast

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This commit is contained in:
Joseph Doherty
2026-06-15 15:32:08 -04:00
parent 7b12eebbd1
commit ac42783e36
12 changed files with 399 additions and 81 deletions
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src/ZB.MOM.WW.MxGateway.Server/Sessions/GatewaySession.cs
+38 -11
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@@ -419,7 +419,8 @@ public sealed class GatewaySession
eventOptions.ReplayRetentionSeconds,
_eventStreaming.DistributorLogger,
_eventStreaming.TimeProvider,
CreateOverflowHandler(eventOptions.BackpressurePolicy));
CreateOverflowHandler(eventOptions.BackpressurePolicy),
singleSubscriberMode: !_eventStreaming.AllowMultipleEventSubscribers);
}
startNow = false;
@@ -680,14 +681,36 @@ public sealed class GatewaySession
/// <summary>
/// Attaches an event subscriber and returns a lease whose
/// <see cref="IEventSubscriberLease.Reader"/> reads the fanned public
/// <see cref="MxEvent"/>s for this subscriber. The single-subscriber guard
/// (Tasks 7/8 relax it) is unchanged: with multi-subscriber disabled a second
/// attach is rejected. The returned lease, when disposed, unregisters the
/// distributor subscriber AND decrements the active-subscriber count.
/// <see cref="MxEvent"/>s for this subscriber. The returned lease, when disposed,
/// unregisters the distributor subscriber AND decrements the active-subscriber count.
/// </summary>
/// <param name="allowMultipleSubscribers">If true, allows multiple concurrent event subscribers.</param>
public IEventSubscriberLease AttachEventSubscriber(bool allowMultipleSubscribers)
/// <param name="allowMultipleSubscribers">
/// When <see langword="false"/>, single-subscriber mode: a second concurrent EXTERNAL
/// subscriber is rejected with <see cref="SessionManagerErrorCode.EventSubscriberAlreadyActive"/>.
/// When <see langword="true"/>, multi-subscriber mode: up to
/// <paramref name="maxSubscribers"/> concurrent EXTERNAL subscribers are allowed; the
/// next attach is rejected with
/// <see cref="SessionManagerErrorCode.EventSubscriberLimitReached"/>.
/// </param>
/// <param name="maxSubscribers">
/// Maximum concurrent external subscribers in multi-subscriber mode
/// (<c>MxGateway:Sessions:MaxEventSubscribersPerSession</c>). Ignored when
/// <paramref name="allowMultipleSubscribers"/> is <see langword="false"/> (the effective
/// cap is then 1). The gateway-owned internal dashboard subscriber is registered
/// directly on the distributor and is NOT counted here, so it never consumes cap budget.
/// </param>
/// <remarks>
/// The count-check-and-increment runs atomically under <c>_syncRoot</c>, so two
/// concurrent attaches racing toward the cap can never both succeed past it. On
/// distributor-register failure the count is rolled back (see the catch below).
/// </remarks>
public IEventSubscriberLease AttachEventSubscriber(bool allowMultipleSubscribers, int maxSubscribers)
{
// Effective cap: 1 in single-subscriber mode, otherwise the configured maximum
// (clamped to at least 1 so a misconfigured non-positive value can never deadlock
// attaches in multi-subscriber mode).
int effectiveCap = allowMultipleSubscribers ? Math.Max(1, maxSubscribers) : 1;
lock (_syncRoot)
{
if (_state != SessionState.Ready || _workerClient?.State != WorkerClientState.Ready)
@@ -697,11 +720,15 @@ public sealed class GatewaySession
$"Session {SessionId} is not ready for event streaming. Current state is {_state}.");
}
if (!allowMultipleSubscribers && _activeEventSubscriberCount > 0)
if (_activeEventSubscriberCount >= effectiveCap)
{
throw new SessionManagerException(
SessionManagerErrorCode.EventSubscriberAlreadyActive,
$"Session {SessionId} already has an active event stream subscriber.");
throw allowMultipleSubscribers
? new SessionManagerException(
SessionManagerErrorCode.EventSubscriberLimitReached,
$"Session {SessionId} has reached its maximum of {effectiveCap} concurrent event stream subscribers.")
: new SessionManagerException(
SessionManagerErrorCode.EventSubscriberAlreadyActive,
$"Session {SessionId} already has an active event stream subscriber.");
}
_activeEventSubscriberCount++;