mxaccessgw/docs/Metrics.md

Gateway Metrics

The metrics subsystem exposes counters, histograms, and observable gauges that describe gateway throughput, queue health, and worker lifecycle. Both the System.Diagnostics.Metrics pipeline and the in-memory GatewayMetricsSnapshot consume the same underlying state, so external collectors and the dashboard see consistent numbers.

Overview

GatewayMetrics is a singleton (registered in GatewayApplication.cs) that owns a single Meter named ZB.MOM.WW.MxGateway and a set of synchronised counters, histograms, and observable gauges. Subsystems call typed mutator methods (SessionOpened, CommandFailed, EventReceived, etc.) rather than touching the Meter directly, which keeps the OpenTelemetry instrument names and tag conventions in one place. A lock (_syncRoot) block guards the scalar fields used by GetSnapshot, while per-event maps use ConcurrentDictionary<string, long> so the hot event path avoids the lock.

Meter and OpenTelemetry Compatibility

The meter name is exposed as a constant so that hosting code can register it with an OpenTelemetry exporter:

public sealed class GatewayMetrics : IDisposable
{
    public const string MeterName = "ZB.MOM.WW.MxGateway";

    public GatewayMetrics()
    {
        _meter = new Meter(MeterName, typeof(GatewayMetrics).Assembly.GetName().Version?.ToString());
        _sessionsOpenedCounter = _meter.CreateCounter<long>("mxgateway.sessions.opened");
        ...
    }
}

The meter version is the gateway assembly version, which gives exporters a stable identifier per build. All instrument names use the dotted mxgateway.<area>.<event> convention so they group cleanly under a single namespace in tools such as Prometheus, OTLP collectors, or dotnet-counters.

Instrument Inventory

Counters

All counters are Counter<long>. Tag values come from the call sites listed under Recording Sites.

Instrument	Tags	What it measures
mxgateway.sessions.opened	none	Successful SessionManager.OpenSession completions.
mxgateway.sessions.closed	none	Sessions closed cleanly via SessionManager.
mxgateway.commands.started	method	Command dispatches initiated by WorkerClient.
mxgateway.commands.succeeded	method	Commands acknowledged with success by the worker.
mxgateway.commands.failed	method, category	Command failures, where category is the WorkerClientErrorCode or exception type name.
mxgateway.events.received	family	Worker events accepted into the event pipeline.
mxgateway.queues.overflows	queue	Drops when a bounded queue rejects a message (e.g. grpc-event-stream).
mxgateway.faults	category	Faults reported by session, event, or worker code paths. The category is a SessionManagerErrorCode or WorkerClientErrorCode name.
mxgateway.workers.killed	reason	Forced terminations of worker processes.
mxgateway.workers.exited	reason	Clean or fault-driven worker exits.
mxgateway.heartbeats.failed	(none)	Aggregate worker heartbeat misses. The exported counter carries no session_id tag — per-session attribution is unbounded cardinality (every session mints a new time series), so it is kept out of the exporter; use the dashboard snapshot or the log scope for per-session detail.
mxgateway.grpc.streams.disconnected	reason	Detachments of the dashboard or client gRPC event stream.
mxgateway.retries.attempted	area	Resilience retries executed by gateway components.

Histograms

Histograms record durations in seconds (the unit argument on CreateHistogram):

_workerStartupLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.workers.startup.duration", "s");
_commandLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.commands.duration", "s");
_eventStreamSendLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.events.stream_send.duration", "s");

Instrument	Tags	What it measures
mxgateway.workers.startup.duration	none	Time from WorkerClient launch to worker-ready.
mxgateway.commands.duration	method, optional category	Command round-trip time. The category tag is added on failure so success and failure latencies stay distinguishable.
mxgateway.events.stream_send.duration	family	Time spent writing each public event to the gRPC response stream in MxAccessGatewayService.StreamEvents.

Observable gauges

Observable gauges are pull-based; the Meter invokes the supplied callback whenever a listener samples it. Each callback re-acquires _syncRoot so the gauge value matches the snapshot taken at the same instant.

Instrument	Source field	Description
mxgateway.sessions.open	_openSessions	Currently open sessions tracked by SessionManager.
mxgateway.workers.running	_workersRunning	Worker clients in a running state.
mxgateway.events.worker_queue.depth	_workerEventQueueDepth	Last reported depth of the worker-side event queue.
mxgateway.events.grpc_stream_queue.depth	_eventStreamBacklogSources (summed on demand)	Live backlog buffered across every active EventStreamService subscriber, summed from the subscribers' channel Count at collection time.

Snapshot Shape

GatewayMetricsSnapshot is the immutable view of the same state, returned by GatewayMetrics.GetSnapshot() while holding _syncRoot. The dictionaries are copied so the caller can iterate without further synchronisation. The dashboard service is the primary consumer.

public sealed record GatewayMetricsSnapshot(
    int OpenSessions,
    int WorkersRunning,
    int WorkerEventQueueDepth,
    int GrpcEventStreamQueueDepth,
    long SessionsOpened,
    long SessionsClosed,
    long CommandsStarted,
    long CommandsSucceeded,
    long CommandsFailed,
    long EventsReceived,
    long QueueOverflows,
    long Faults,
    long WorkerKills,
    long WorkerExits,
    long HeartbeatFailures,
    long StreamDisconnects,
    long RetryAttempts,
    IReadOnlyDictionary<string, long> CommandFailuresByMethod,
    IReadOnlyDictionary<string, long> EventsByFamily,
    IReadOnlyDictionary<string, long> EventsBySession,
    IReadOnlyDictionary<string, long> RetryAttemptsByArea);

The scalar fields mirror the counters and gauges. The four dictionaries provide the breakdowns that counter tags would otherwise require an exporter to aggregate:

• CommandFailuresByMethod keys by gRPC method name.
• EventsByFamily keys by event family (the Family enum on a worker event).
• EventsBySession keys by sessionId; entries are removed via RemoveSessionEvents when a session closes so the map does not grow without bound.
• RetryAttemptsByArea keys by the resilience area tag, e.g. worker_startup.

EventsReceived is read with Interlocked.Read(ref _eventsReceived) because EventReceived increments it via Interlocked.Increment outside the lock to keep the event-ingestion path non-blocking.

Recording Sites

The recording call sites describe the code paths that write into each instrument. This mapping makes it easier to trace an unexpected counter reading back to a subsystem.

Session manager

Sessions/SessionManager.cs emits session lifecycle and fault counters:

_metrics.SessionOpened();
...
_metrics.Fault(SessionManagerErrorCode.OpenFailed.ToString());
...
_metrics.SessionClosed();
...
_metrics.SessionRemoved();
...
_metrics.Fault(SessionManagerErrorCode.CloseFailed.ToString());
...
_metrics.RemoveSessionEvents(session.SessionId);

SessionRemoved decrements the open-session gauge without incrementing the closed counter, which covers cases where a session is evicted rather than closed by the client.

Worker client

Workers/WorkerClient.cs records command throughput, worker lifecycle, heartbeat failures, and the worker-side event queue depth:

• CommandStarted(method) and CommandSucceeded(method, duration) / CommandFailed(method, category, duration) around the worker request/response pair.
• WorkerStarted(startupDuration) once the worker reports ready.
• RecordWorkerStoppedOnce calls WorkerStopped(reason) exactly once per worker, guarding against double-counting on simultaneous fault and exit signals.
• WorkerKilled(reason) when the client forcibly terminates the worker.
• HeartbeatFailed(SessionId) per missed heartbeat.
• SetWorkerEventQueueDepth(queueDepth) after each event ingest.
• EventReceived(SessionId, workerEvent.Event.Family.ToString()) for each worker event.

Worker process launcher

Workers/WorkerProcessLauncher.cs records process-level kills and startup retries:

_metrics.WorkerKilled(reason);
...
_metrics.RetryAttempted("worker_startup");

The worker_startup tag is hard-coded so the RetryAttemptsByArea snapshot reports launcher retries distinctly from other resilience areas.

Session worker client factory

Sessions/SessionWorkerClientFactory.cs records the worker kill that follows a failed OpenSession handshake:

_metrics.WorkerKilled("OpenSessionFailed");

This is the only fault path where the factory itself owns the kill decision; once the worker is bound to a session, the WorkerClient becomes responsible for lifecycle metrics.

gRPC event stream service

Grpc/EventStreamService.cs reports the event-stream backlog through a live backlog source and records the disconnect/overflow counters:

IDisposable backlogRegistration = metrics.RegisterEventStreamBacklogSource(
    () => subscriber.Reader.CanCount ? subscriber.Reader.Count : 0);
...
backlogRegistration.Dispose();
metrics.StreamDisconnected("Detached");
...
metrics.QueueOverflow("grpc-event-stream");
metrics.Fault(SessionManagerErrorCode.EventQueueOverflow.ToString());
...
metrics.Fault(WorkerClientErrorCode.WorkerFaulted.ToString());

Rather than pushing a running total on every streamed event (which took the subscriber channel's Count lock and the metric lock per event), each subscriber registers its channel as a backlog source for the duration of the stream. The gauge callback and GetSnapshot sum those sources' Count on demand — only when the metric is scraped or projected — so the streaming hot path does no per-event gauge bookkeeping (GWC-15). Disposing the registration in the finally removes the subscriber's contribution, so the gauge reflects only the remaining subscribers and returns to zero once the last stream detaches. Negative source returns are clamped to zero when summed.

Grpc/MxAccessGatewayService.cs records gRPC event send latency around each response-stream write using the allocation-free timestamp API (no per-event Stopwatch object):

long sendStartTimestamp = Stopwatch.GetTimestamp();
await responseStream.WriteAsync(publicEvent).ConfigureAwait(false);
metrics.RecordEventStreamSend(
    publicEvent.Family.ToString(),
    Stopwatch.GetElapsedTime(sendStartTimestamp));

Dashboard Consumption

Dashboard/DashboardSnapshotService.cs calls _metrics.GetSnapshot() once per GetSnapshot invocation and projects it into the dashboard transport types together with the session registry view. The dashboard receives a single, internally consistent snapshot per tick rather than reading individual counters at separate times. See Gateway Dashboard Design and Dashboard Interface Design for the projection rules and wire format.

Auth Store Write Churn

The per-RPC authentication path is a hidden write source: the shared verifier refreshes last_used_utc on every authenticated call. To keep that off the auth-store throughput ceiling, the gateway caches successful verifications for a short TTL and coalesces the last_used_utc write to at most one per key per window (MxGateway:Security:ApiKeyVerificationCacheSeconds / ApiKeyLastUsedCoalesceSeconds, defaults 15 s / 60 s). This bounds WAL churn on the bulk-read workload without dropping any metric — auth activity is still observable via command-throughput counters and audit rows. See Authentication.

Related Documentation

• Gateway Dashboard Design
• Dashboard Interface Design
• Sessions