feat(comm): batch site→central stream events over gRPC (R2)

Every AttributeValueChanged/AlarmStateChanged rode its own gRPC message on SiteStreamService; at target scale that is ~37.5k messages/s/site of pure framing overhead. Coalesce them, additively, with no new RPC. Wire (sitestream.proto, regenerated via docker/regen-proto.sh sitestream): InstanceStreamRequest.batching_supported = 3 SiteStreamRequest.batching_supported = 2 SiteStreamEvent.batch = 4 (new oneof case) SiteStreamEventBatch { repeated SiteStreamEvent events = 1 } The proto3 default of batching_supported IS the negotiation, and it is load-bearing: a batch frame reaches a pre-R2 central as EventOneofCase.None, whose ConvertToDomainEvent returns null — the whole batch would vanish with no error anywhere. An old central cannot set the flag so it never receives one; an old site ignores the unknown request field and keeps sending per-event frames, which the new client's ForEachEvent handles as the single-event case. Both skew directions are covered by tests that go through a real proto serialize/parse round-trip. Server: SiteStreamEventBatcher, a per-subscriber pump replacing the handler's await-foreach/WriteAsync loop and byte-identical to it at a cap of 1. It never delays a lone event — it drains the already-queued backlog for free and lingers only once a backlog is proven — emits a single-event buffer as a plain frame, preserves order and per-event Timestamps exactly, and flushes what is buffered when the send channel's writer completes. It sits DOWNSTREAM of StreamRelayActor's bounded DropOldest channel, so it changes framing only and does not move the burst ceiling (deferred register row 31, which lives in the shared publish stage upstream of the BroadcastHub). Client: sets the flag on both subscriptions and unpacks in order into the existing per-event pipeline, so SiteAlarmAggregatorActor, DebugStreamBridgeActor, consumer-keepalive/orphan logic, reconnect-on-graceful-completion, generation fencing, the (siteId, endpoint) factory key and IsLive semantics are untouched. Options (validated): GrpcStreamBatchMaxEvents 100 (1 disables), GrpcStreamBatchWindow 25 ms — validated strictly under 250 ms, the load test's end-to-end P99 threshold. Measured worst case (trickle-with-backlog, window-bound rather than cap-bound): P50 13.8 ms, P99 25.4 ms, max 25.8 ms; cap-bound case sent 600 queued events in 6 frames. Telemetry: histogram scadabridge.site.stream.batch_size tagged by stream kind, recorded only on negotiated streams (per-event otherwise). It rides ScadaBridgeTelemetry.MeterName, already in the ObservedMeters allowlist. Docs: Component-Communication.md gains an Event Batching section; CLAUDE.md's gRPC streaming bullet records the wire shape and the negotiation rationale.
2026-08-15 03:39:59 -04:00
parent 2b74851f96
commit 9b5cb3dd9d
16 changed files with 2150 additions and 131 deletions
@@ -20,6 +20,17 @@ public class ProtoContractTests
        SiteStreamEvent.EventOneofCase.AlarmChanged
    ];

+    /// <summary>
+    /// Oneof variants that are NOT per-event payloads and so are deliberately absent from
+    /// <see cref="HandledCases"/>. <c>Batch</c> (R2) is a framing envelope: it is unpacked
+    /// by <see cref="SiteStreamGrpcClient.ForEachEvent"/> into the per-event cases above
+    /// BEFORE conversion, and never reaches <c>ConvertToDomainEvent</c> as a whole frame.
+    /// </summary>
+    private static readonly SiteStreamEvent.EventOneofCase[] FramingCases =
+    [
+        SiteStreamEvent.EventOneofCase.Batch
+    ];
+
    [Fact]
    public void AllOneofVariants_HaveConversionHandlers()
    {
@@ -27,9 +38,37 @@ public class ProtoContractTests
            .Where(c => c != SiteStreamEvent.EventOneofCase.None)
            .ToArray();

-        Assert.Equal(allCases.Length, HandledCases.Length);
+        var accountedFor = HandledCases.Concat(FramingCases).ToArray();
+        Assert.Equal(allCases.Length, accountedFor.Length);
        foreach (var c in allCases)
-            Assert.Contains(c, HandledCases);
+            Assert.Contains(c, accountedFor);
+    }
+
+    [Fact]
+    public void BatchFrame_IsUnpackedIntoPerEventCases_NotConvertedWhole()
+    {
+        // The framing case's contract: ForEachEvent hands the per-event cases to the
+        // handler in order, and ConvertToDomainEvent is never asked to make sense of the
+        // envelope itself (it would return null, silently dropping the whole batch).
+        var inner = new[]
+        {
+            CreateTestEvent(SiteStreamEvent.EventOneofCase.AttributeChanged),
+            CreateTestEvent(SiteStreamEvent.EventOneofCase.AlarmChanged)
+        };
+        var frame = new SiteStreamEvent
+        {
+            CorrelationId = "test",
+            Batch = new SiteStreamEventBatch { Events = { inner } }
+        };
+
+        Assert.Null(SiteStreamGrpcClient.ConvertToDomainEvent(frame));
+
+        var seen = new List<SiteStreamEvent.EventOneofCase>();
+        SiteStreamGrpcClient.ForEachEvent(frame, e => seen.Add(e.EventCase));
+
+        Assert.Equal(
+            [SiteStreamEvent.EventOneofCase.AttributeChanged, SiteStreamEvent.EventOneofCase.AlarmChanged],
+            seen);
    }

    [Theory]
@@ -0,0 +1,316 @@
+using System.Diagnostics;
+using System.Threading.Channels;
+using Google.Protobuf.WellKnownTypes;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.Tests.Grpc;
+
+/// <summary>
+/// Unit tests for the per-subscriber coalescing pump behind R2 (gRPC event batching).
+/// The pump is the only new behaviour on the site→central hot path, so its contract is
+/// pinned directly rather than only through the server: the size cap, the time cap, the
+/// flush when the channel writer completes, the "never reorder" guarantee, and the
+/// un-negotiated (maxBatchEvents == 1) shape that keeps an older central working.
+/// </summary>
+public class SiteStreamEventBatcherTests
+{
+    private const string Corr = "corr-batch";
+
+    private static SiteStreamEvent Event(int seq) => new()
+    {
+        CorrelationId = Corr,
+        AttributeChanged = new AttributeValueUpdate
+        {
+            InstanceUniqueName = "SiteA.Pump01",
+            AttributePath = "Modules.IO",
+            AttributeName = "Seq",
+            Value = seq.ToString(),
+            Quality = Quality.Good,
+            Timestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UnixEpoch.AddSeconds(seq))
+        }
+    };
+
+    /// <summary>Flattens an emitted frame into the sequence numbers it carried, in order.</summary>
+    private static IEnumerable<int> Seqs(SiteStreamEvent frame)
+    {
+        if (frame.EventCase == SiteStreamEvent.EventOneofCase.Batch)
+        {
+            foreach (var inner in frame.Batch.Events)
+                yield return int.Parse(inner.AttributeChanged.Value);
+            yield break;
+        }
+
+        yield return int.Parse(frame.AttributeChanged.Value);
+    }
+
+    private sealed record PumpRun(List<SiteStreamEvent> Frames, List<int> FrameSizes, Task Completion);
+
+    private static PumpRun StartPump(
+        ChannelReader<SiteStreamEvent> reader,
+        int maxBatchEvents,
+        TimeSpan window,
+        CancellationToken ct = default)
+    {
+        var frames = new List<SiteStreamEvent>();
+        var sizes = new List<int>();
+        var task = SiteStreamEventBatcher.PumpAsync(
+            reader,
+            Corr,
+            maxBatchEvents,
+            window,
+            (evt, _) =>
+            {
+                lock (frames) { frames.Add(evt); }
+                return Task.CompletedTask;
+            },
+            size => { lock (frames) { sizes.Add(size); } },
+            ct);
+        return new PumpRun(frames, sizes, task);
+    }
+
+    // ── Size cap ────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task SizeCap_SplitsABacklogIntoFramesOfAtMostMaxEvents()
+    {
+        // A backlog already sitting in the channel is drained without waiting, but never
+        // beyond the size cap — 250 queued events at a cap of 100 must come out as
+        // 100 + 100 + 50, in order, with nothing lost or duplicated.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        for (var i = 0; i < 250; i++)
+            Assert.True(channel.Writer.TryWrite(Event(i)));
+        channel.Writer.Complete();
+
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window: TimeSpan.FromMilliseconds(25));
+        await run.Completion;
+
+        Assert.All(run.FrameSizes, s => Assert.True(s <= 100, $"frame carried {s} events (cap 100)"));
+        Assert.Equal([100, 100, 50], run.FrameSizes);
+        Assert.Equal(Enumerable.Range(0, 250), run.Frames.SelectMany(Seqs));
+    }
+
+    [Fact]
+    public async Task SizeCapOfOne_EmitsPlainPerEventFrames_TheUnnegotiatedShape()
+    {
+        // maxBatchEvents == 1 is what an un-negotiated subscription (an older central)
+        // gets. Every event must ride its own plain frame — never a Batch case, which
+        // that central's generated code cannot parse.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        for (var i = 0; i < 5; i++)
+            channel.Writer.TryWrite(Event(i));
+        channel.Writer.Complete();
+
+        var run = StartPump(channel.Reader, maxBatchEvents: 1, window: TimeSpan.FromMilliseconds(25));
+        await run.Completion;
+
+        Assert.Equal(5, run.Frames.Count);
+        Assert.All(run.Frames, f =>
+            Assert.Equal(SiteStreamEvent.EventOneofCase.AttributeChanged, f.EventCase));
+        Assert.All(run.Frames, f => Assert.Equal(Corr, f.CorrelationId));
+        Assert.Equal(Enumerable.Range(0, 5), run.Frames.SelectMany(Seqs));
+    }
+
+    // ── Time cap ────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task TimeCap_ClosesAnUnderfullBatchWhenTheWindowElapses()
+    {
+        // Two events arrive (a backlog, so the pump lingers), then the source goes quiet
+        // well short of the size cap. The window — not the cap — must close the batch,
+        // and it must do so within a bounded time rather than waiting for a 100th event
+        // that never comes.
+        var window = TimeSpan.FromMilliseconds(120);
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        channel.Writer.TryWrite(Event(0));
+        channel.Writer.TryWrite(Event(1));
+
+        var started = Stopwatch.GetTimestamp();
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window);
+
+        SiteStreamEvent frame;
+        while (true)
+        {
+            lock (run.Frames)
+            {
+                if (run.Frames.Count > 0) { frame = run.Frames[0]; break; }
+            }
+            Assert.True(Stopwatch.GetElapsedTime(started) < TimeSpan.FromSeconds(5),
+                "the window never closed the underfull batch");
+            await Task.Delay(5);
+        }
+
+        var elapsed = Stopwatch.GetElapsedTime(started);
+
+        channel.Writer.Complete();
+        await run.Completion;
+
+        Assert.Equal(SiteStreamEvent.EventOneofCase.Batch, frame.EventCase);
+        Assert.Equal([0, 1], Seqs(frame));
+        // The batch waited (it did not close instantly on the two queued events) and it
+        // closed on the window, not on a cap it never reached.
+        Assert.True(elapsed >= window - TimeSpan.FromMilliseconds(20),
+            $"batch closed after {elapsed.TotalMilliseconds:0.0} ms, before the {window.TotalMilliseconds:0} ms window");
+    }
+
+    [Fact]
+    public async Task LoneEventOnAQuietStream_IsNeverDelayedByTheWindow()
+    {
+        // The latency contract: the window applies only AFTER a backlog has been observed.
+        // A single event on an idle stream must be emitted immediately as a plain frame,
+        // so per-event latency on a quiet site is unchanged by batching.
+        var window = TimeSpan.FromSeconds(5);
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window);
+
+        var started = Stopwatch.GetTimestamp();
+        channel.Writer.TryWrite(Event(7));
+
+        while (true)
+        {
+            lock (run.Frames)
+            {
+                if (run.Frames.Count > 0) break;
+            }
+            Assert.True(Stopwatch.GetElapsedTime(started) < TimeSpan.FromSeconds(3),
+                "a lone event was held by the coalescing window");
+            await Task.Delay(2);
+        }
+
+        var elapsed = Stopwatch.GetElapsedTime(started);
+        channel.Writer.Complete();
+        await run.Completion;
+
+        Assert.Equal(SiteStreamEvent.EventOneofCase.AttributeChanged, run.Frames[0].EventCase);
+        Assert.True(elapsed < TimeSpan.FromSeconds(1),
+            $"lone event took {elapsed.TotalMilliseconds:0.0} ms against a {window.TotalSeconds:0} s window");
+    }
+
+    // ── Flush on stream close ───────────────────────────────────────────────────
+
+    [Fact]
+    public async Task WriterCompletion_FlushesTheInFlightBatchBeforeReturning()
+    {
+        // The channel writer completing mid-window (the site stopping the relay actor and
+        // calling channel.Writer.TryComplete()) must flush what is already buffered rather
+        // than silently discarding it while waiting out the window.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        channel.Writer.TryWrite(Event(0));
+        channel.Writer.TryWrite(Event(1));
+
+        // A long window guarantees the pump is lingering, not already past the emit.
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window: TimeSpan.FromSeconds(30));
+
+        await Task.Delay(100);
+        lock (run.Frames)
+        {
+            Assert.Empty(run.Frames); // still lingering
+        }
+
+        channel.Writer.Complete();
+        await run.Completion.WaitAsync(TimeSpan.FromSeconds(5));
+
+        Assert.Single(run.Frames);
+        Assert.Equal([0, 1], Seqs(run.Frames[0]));
+    }
+
+    [Fact]
+    public async Task WriterCompletionWithNothingBuffered_ReturnsWithoutEmitting()
+    {
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window: TimeSpan.FromMilliseconds(25));
+
+        channel.Writer.Complete();
+        await run.Completion.WaitAsync(TimeSpan.FromSeconds(5));
+
+        Assert.Empty(run.Frames);
+    }
+
+    [Fact]
+    public async Task Cancellation_EndsThePumpWithOperationCanceled()
+    {
+        // Client disconnect / duplicate replacement / site shutdown. The pump must
+        // surface OperationCanceledException exactly as the pre-batching await-foreach
+        // loop did, so SiteStreamGrpcServer's existing catch and finally are unchanged.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        using var cts = new CancellationTokenSource();
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window: TimeSpan.FromMilliseconds(25), cts.Token);
+
+        await cts.CancelAsync();
+
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(
+            () => run.Completion.WaitAsync(TimeSpan.FromSeconds(5)));
+    }
+
+    // ── Ordering ────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task Ordering_IsPreservedAcrossManyBatchesUnderAProducerRace()
+    {
+        // Batching is a framing change and nothing else: with a producer writing
+        // concurrently with the pump, the flattened output must be the exact input
+        // sequence — no reordering, no loss, no duplication, across many frames.
+        const int total = 5_000;
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        var run = StartPump(channel.Reader, maxBatchEvents: 32, window: TimeSpan.FromMilliseconds(5));
+
+        var producer = Task.Run(async () =>
+        {
+            for (var i = 0; i < total; i++)
+            {
+                channel.Writer.TryWrite(Event(i));
+                if (i % 250 == 0) await Task.Yield();
+            }
+            channel.Writer.Complete();
+        });
+
+        await producer;
+        await run.Completion.WaitAsync(TimeSpan.FromSeconds(30));
+
+        Assert.Equal(Enumerable.Range(0, total), run.Frames.SelectMany(Seqs));
+        Assert.All(run.FrameSizes, s => Assert.InRange(s, 1, 32));
+        Assert.Equal(total, run.FrameSizes.Sum());
+    }
+
+    // ── Frame shape ─────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task BatchFrame_CarriesTheCorrelationIdOnceAndBlanksItOnInnerEvents()
+    {
+        // The byte saving batching exists for: the correlation id is stamped once on the
+        // enclosing frame, not repeated on every packed event. No consumer reads the
+        // inner value (SiteStreamGrpcClient.ForEachEvent ignores it).
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        for (var i = 0; i < 4; i++) channel.Writer.TryWrite(Event(i));
+        channel.Writer.Complete();
+
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window: TimeSpan.FromMilliseconds(25));
+        await run.Completion;
+
+        var frame = Assert.Single(run.Frames);
+        Assert.Equal(SiteStreamEvent.EventOneofCase.Batch, frame.EventCase);
+        Assert.Equal(Corr, frame.CorrelationId);
+        Assert.All(frame.Batch.Events, e => Assert.Equal(string.Empty, e.CorrelationId));
+    }
+
+    [Fact]
+    public async Task PerEventTimestampsSurviveBatching()
+    {
+        // End-to-end latency measurement rides the per-event Timestamp; coalescing must
+        // not rewrite it to a single frame-level stamp.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        for (var i = 0; i < 3; i++) channel.Writer.TryWrite(Event(i));
+        channel.Writer.Complete();
+
+        var run = StartPump(channel.Reader, maxBatchEvents: 100, window: TimeSpan.FromMilliseconds(25));
+        await run.Completion;
+
+        var frame = Assert.Single(run.Frames);
+        Assert.Equal(
+            [
+                DateTimeOffset.UnixEpoch,
+                DateTimeOffset.UnixEpoch.AddSeconds(1),
+                DateTimeOffset.UnixEpoch.AddSeconds(2)
+            ],
+            frame.Batch.Events.Select(e => e.AttributeChanged.Timestamp.ToDateTimeOffset()));
+    }
+}
@@ -508,6 +508,125 @@ public class SiteStreamGrpcClientTests
        }
    }

+    // ── R2: batch unpacking on the client ───────────────────────────────────────
+
+    private static SiteStreamEvent Attr(string value, DateTimeOffset ts) => new()
+    {
+        AttributeChanged = new AttributeValueUpdate
+        {
+            InstanceUniqueName = "SiteA.Pump01",
+            AttributePath = "Modules.IO",
+            AttributeName = "Seq",
+            Value = value,
+            Quality = Quality.Good,
+            Timestamp = Timestamp.FromDateTimeOffset(ts)
+        }
+    };
+
+    [Fact]
+    public void ForEachEvent_PlainFrame_IsDeliveredAsIs()
+    {
+        // An OLD SITE (or any un-negotiated stream) sends one event per frame. The new
+        // client's unpack path must pass it straight through — this is the new-central ↔
+        // old-site skew direction.
+        var frame = Attr("1", DateTimeOffset.UnixEpoch);
+        var seen = new List<SiteStreamEvent>();
+
+        SiteStreamGrpcClient.ForEachEvent(frame, seen.Add);
+
+        Assert.Same(frame, Assert.Single(seen));
+    }
+
+    [Fact]
+    public void ForEachEvent_BatchFrame_UnpacksInOrderPreservingPerEventTimestamps()
+    {
+        // Order and per-event Timestamp fidelity are the two properties the downstream
+        // consumers (SiteAlarmAggregatorActor, DebugStreamBridgeActor) and the end-to-end
+        // latency measurement depend on.
+        var t0 = new DateTimeOffset(2026, 8, 15, 9, 0, 0, TimeSpan.Zero);
+        var frame = new SiteStreamEvent
+        {
+            CorrelationId = "corr-batch",
+            Batch = new SiteStreamEventBatch
+            {
+                Events =
+                {
+                    Attr("0", t0),
+                    Attr("1", t0.AddMilliseconds(3)),
+                    Attr("2", t0.AddMilliseconds(11))
+                }
+            }
+        };
+
+        var seen = new List<SiteStreamEvent>();
+        SiteStreamGrpcClient.ForEachEvent(frame, seen.Add);
+
+        Assert.Equal(["0", "1", "2"], seen.Select(e => e.AttributeChanged.Value));
+        Assert.Equal(
+            [t0, t0.AddMilliseconds(3), t0.AddMilliseconds(11)],
+            seen.Select(e => e.AttributeChanged.Timestamp.ToDateTimeOffset()));
+    }
+
+    [Fact]
+    public void ForEachEvent_IgnoresNestedAndUnknownInnerCases()
+    {
+        // The server never nests a batch inside a batch. A nested (or empty) inner frame
+        // from a malformed or hostile peer must be skipped, not followed — unpacking is
+        // deliberately non-recursive so a crafted frame cannot drive unbounded recursion.
+        var frame = new SiteStreamEvent
+        {
+            CorrelationId = "corr-nested",
+            Batch = new SiteStreamEventBatch
+            {
+                Events =
+                {
+                    Attr("0", DateTimeOffset.UnixEpoch),
+                    new SiteStreamEvent { Batch = new SiteStreamEventBatch { Events = { Attr("hidden", DateTimeOffset.UnixEpoch) } } },
+                    new SiteStreamEvent(),
+                    Attr("1", DateTimeOffset.UnixEpoch)
+                }
+            }
+        };
+
+        var seen = new List<SiteStreamEvent>();
+        SiteStreamGrpcClient.ForEachEvent(frame, seen.Add);
+
+        Assert.Equal(["0", "1"], seen.Select(e => e.AttributeChanged.Value));
+    }
+
+    [Fact]
+    public async Task ConsumeStream_MixedBatchedAndPlainFrames_DeliverEveryEventInOrder()
+    {
+        // A reconnect can straddle a site upgrade, so one stream may legitimately carry
+        // both frame shapes. Driving the real ConsumeStreamAsync with the real unpack
+        // proves the combination is flat and ordered from the consumer's point of view.
+        var client = SiteStreamGrpcClient.CreateForTesting();
+        var cts = new CancellationTokenSource();
+        var delivered = new List<string>();
+
+        void Deliver(SiteStreamEvent e) => delivered.Add(e.AttributeChanged.Value);
+
+        await client.ConsumeStreamAsync(
+            "corr-mixed",
+            cts,
+            () => FakeCall(new StubStreamReader(
+                Attr("0", DateTimeOffset.UnixEpoch),
+                new SiteStreamEvent
+                {
+                    CorrelationId = "corr-mixed",
+                    Batch = new SiteStreamEventBatch
+                    {
+                        Events = { Attr("1", DateTimeOffset.UnixEpoch), Attr("2", DateTimeOffset.UnixEpoch) }
+                    }
+                },
+                Attr("3", DateTimeOffset.UnixEpoch))),
+            frame => SiteStreamGrpcClient.ForEachEvent(frame, Deliver),
+            _ => { },
+            () => { });
+
+        Assert.Equal(["0", "1", "2", "3"], delivered);
+    }
+
    private static AsyncServerStreamingCall<SiteStreamEvent> FakeCall(StubStreamReader reader) =>
        FakeCall(reader, Task.FromResult(new Metadata()));

@@ -580,4 +580,216 @@ public class SiteStreamGrpcServerTests : TestKit
        var server = CreateServer();
        Assert.Equal(0, server.DroppedStreamEventCount);
    }
+
+    // ── R2: gRPC event batching, and its negotiation ────────────────────────────
+
+    [Fact]
+    public void BatchOptions_AreBoundFromOptions_AndClampDegenerateValues()
+    {
+        var options = Microsoft.Extensions.Options.Options.Create(new CommunicationOptions());
+        var server = new SiteStreamGrpcServer(_subscriber, _logger, options);
+
+        Assert.Equal(100, server.StreamBatchMaxEvents);
+        Assert.Equal(TimeSpan.FromMilliseconds(25), server.StreamBatchWindow);
+
+        // CommunicationOptionsValidator fails the boot on these, but a host composed
+        // without validation must not blow up deep inside a live RPC.
+        var degenerate = new SiteStreamGrpcServer(_subscriber, _logger,
+            Microsoft.Extensions.Options.Options.Create(new CommunicationOptions
+            {
+                GrpcStreamBatchMaxEvents = 0,
+                GrpcStreamBatchWindow = TimeSpan.FromMilliseconds(-5),
+            }));
+        Assert.Equal(1, degenerate.StreamBatchMaxEvents);
+        Assert.Equal(TimeSpan.Zero, degenerate.StreamBatchWindow);
+    }
+
+    [Fact]
+    public async Task UnnegotiatedSubscription_NeverEmitsABatchFrame()
+    {
+        // OLD-CENTRAL ↔ NEW-SITE skew. proto3 defaults batching_supported to false, which
+        // is exactly what a central built before R2 sends. The site must then keep to one
+        // event per frame — a Batch frame would arrive at that central as
+        // EventOneofCase.None and be silently dropped by its ConvertToDomainEvent.
+        var (server, capture, cts, streamTask, relay) =
+            await StartCapturingStreamAsync(batchingSupported: false);
+
+        for (var i = 0; i < 50; i++)
+        {
+            relay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
+                "Site1.Pump01", "Path", "Attr", i, "Good", DateTimeOffset.UtcNow));
+        }
+
+        await WaitForConditionAsync(() => CountEvents(capture) >= 50, 10_000);
+
+        cts.Cancel();
+        await streamTask;
+
+        lock (capture)
+        {
+            Assert.All(capture, f => Assert.NotEqual(SiteStreamEvent.EventOneofCase.Batch, f.EventCase));
+            Assert.Equal(50, capture.Count);
+        }
+
+        GC.KeepAlive(server);
+    }
+
+    [Fact]
+    public async Task NegotiatedSubscription_CoalescesABacklogIntoFewerFramesThanEvents()
+    {
+        // NEW-CENTRAL ↔ NEW-SITE. A burst pushed at the relay faster than the pump drains
+        // it must come out in strictly fewer frames than events, with every event
+        // preserved in order.
+        const int burst = 400;
+        var (server, capture, cts, streamTask, relay) =
+            await StartCapturingStreamAsync(batchingSupported: true);
+
+        for (var i = 0; i < burst; i++)
+        {
+            relay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
+                "Site1.Pump01", "Path", "Attr", i, "Good", DateTimeOffset.UtcNow));
+        }
+
+        await WaitForConditionAsync(() => CountEvents(capture) >= burst, 15_000);
+
+        cts.Cancel();
+        await streamTask;
+
+        List<SiteStreamEvent> frames;
+        lock (capture) { frames = [.. capture]; }
+
+        Assert.Equal(burst, frames.Sum(CountFrameEvents));
+        Assert.True(frames.Count < burst,
+            $"batching produced {frames.Count} frames for {burst} events — no coalescing happened");
+        Assert.Contains(frames, f => f.EventCase == SiteStreamEvent.EventOneofCase.Batch);
+
+        // Order is preserved end to end: the values arrive 0..burst-1 exactly once each.
+        var values = frames.SelectMany(FlattenAttributeValues).ToArray();
+        Assert.Equal(Enumerable.Range(0, burst).Select(i => i.ToString()), values);
+
+        GC.KeepAlive(server);
+    }
+
+    [Fact]
+    public async Task BatchSizeHistogram_IsRecordedOnlyForNegotiatedStreams()
+    {
+        // scadabridge.site.stream.batch_size rides ScadaBridgeTelemetry.MeterName, which is
+        // already in SiteServiceRegistration.ObservedMeters — an unlisted meter exports
+        // nothing, silently. Assert the instrument actually fires, and that it does NOT
+        // fire on an un-negotiated stream (where it would degenerate into a per-event
+        // instrument on the hottest path in the product).
+        var measurements = new List<int>();
+        using var listener = new MeterListener();
+        listener.InstrumentPublished = (instrument, l) =>
+        {
+            if (instrument.Meter.Name == ScadaBridgeTelemetry.MeterName &&
+                instrument.Name == "scadabridge.site.stream.batch_size")
+            {
+                l.EnableMeasurementEvents(instrument);
+            }
+        };
+        listener.SetMeasurementEventCallback<int>((_, m, _, _) =>
+        {
+            lock (measurements) { measurements.Add(m); }
+        });
+        listener.Start();
+
+        // Un-negotiated: no measurements at all.
+        var (_, plainCapture, plainCts, plainTask, plainRelay) =
+            await StartCapturingStreamAsync(batchingSupported: false, correlationId: "corr-hist-off");
+        plainRelay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
+            "Site1.Pump01", "Path", "Attr", 1, "Good", DateTimeOffset.UtcNow));
+        await WaitForConditionAsync(() => CountEvents(plainCapture) >= 1);
+        plainCts.Cancel();
+        await plainTask;
+
+        lock (measurements) { Assert.Empty(measurements); }
+
+        // Negotiated: one measurement per emitted frame, each within the size cap.
+        var (_, capture, cts, streamTask, relay) =
+            await StartCapturingStreamAsync(batchingSupported: true, correlationId: "corr-hist-on");
+        for (var i = 0; i < 20; i++)
+        {
+            relay.Tell(new Commons.Messages.Streaming.AttributeValueChanged(
+                "Site1.Pump01", "Path", "Attr", i, "Good", DateTimeOffset.UtcNow));
+        }
+        await WaitForConditionAsync(() => CountEvents(capture) >= 20, 10_000);
+        cts.Cancel();
+        await streamTask;
+
+        lock (measurements)
+        {
+            Assert.NotEmpty(measurements);
+            Assert.Equal(20, measurements.Sum());
+            Assert.All(measurements, m => Assert.InRange(m, 1, SiteStreamGrpcServer.DefaultStreamBatchMaxEvents));
+        }
+    }
+
+    /// <summary>Total events carried across all captured frames (unpacking batch frames).</summary>
+    private static int CountEvents(List<SiteStreamEvent> capture)
+    {
+        lock (capture) { return capture.Sum(CountFrameEvents); }
+    }
+
+    private static int CountFrameEvents(SiteStreamEvent frame) =>
+        frame.EventCase == SiteStreamEvent.EventOneofCase.Batch ? frame.Batch.Events.Count : 1;
+
+    private static IEnumerable<string> FlattenAttributeValues(SiteStreamEvent frame)
+    {
+        if (frame.EventCase == SiteStreamEvent.EventOneofCase.Batch)
+        {
+            foreach (var inner in frame.Batch.Events)
+                yield return inner.AttributeChanged.Value;
+            yield break;
+        }
+
+        yield return frame.AttributeChanged.Value;
+    }
+
+    /// <summary>
+    /// Starts a SubscribeInstance stream with the given batch negotiation, capturing every
+    /// written frame and handing back the relay actor so the test can drive domain events.
+    /// </summary>
+    private async Task<(SiteStreamGrpcServer Server, List<SiteStreamEvent> Capture,
+        CancellationTokenSource Cts, Task StreamTask, IActorRef Relay)>
+        StartCapturingStreamAsync(bool batchingSupported, string correlationId = "corr-batch")
+    {
+        IActorRef? capturedActor = null;
+        var subscriber = Substitute.For<ISiteStreamSubscriber>();
+        subscriber.Subscribe(Arg.Any<string>(), Arg.Any<IActorRef>())
+            .Returns(ci =>
+            {
+                capturedActor = ci.Arg<IActorRef>();
+                return "sub-batch";
+            });
+
+        var server = new SiteStreamGrpcServer(subscriber, _logger,
+            Microsoft.Extensions.Options.Options.Create(new CommunicationOptions()));
+        server.SetReady(Sys);
+
+        var capture = new List<SiteStreamEvent>();
+        var writer = Substitute.For<IServerStreamWriter<SiteStreamEvent>>();
+        writer.WriteAsync(Arg.Any<SiteStreamEvent>(), Arg.Any<CancellationToken>())
+            .Returns(Task.CompletedTask)
+            .AndDoes(ci =>
+            {
+                var frame = ci.Arg<SiteStreamEvent>();
+                lock (capture) { capture.Add(frame); }
+            });
+
+        var cts = new CancellationTokenSource();
+        var context = CreateMockContext(cts.Token);
+
+        var request = new InstanceStreamRequest
+        {
+            CorrelationId = correlationId,
+            InstanceUniqueName = "Site1.Pump01",
+            BatchingSupported = batchingSupported
+        };
+
+        var streamTask = Task.Run(() => server.SubscribeInstance(request, writer, context));
+        await WaitForConditionAsync(() => capturedActor != null);
+
+        return (server, capture, cts, streamTask, capturedActor!);
+    }
 }