Phase 3 PR 22 — Modbus ISubscribable via polling overlay. Modbus has no push model at the wire protocol (unlike MXAccess's OnDataChange callback or OPC UA's own Subscription service), so the driver layers a per-subscription polling loop on top of the existing IReadable path: SubscribeAsync returns an opaque ModbusSubscriptionHandle, starts a background Task.Run that sleeps for the requested publishing interval (floored to 100ms so a misconfigured sub-10ms request doesn't hammer the PLC), reads every subscribed tag through the same FC01/03/04 path the one-shot ReadAsync uses, diffs the returned DataValueSnapshot against the last known value per tag, and raises OnDataChange exactly when (a) it's the first poll (initial-data push per OPC UA Part 4 convention) or (b) boxed value changed or (c) StatusCode changed — stable values don't generate event traffic after the initial push, matching the v1 MXAccess OnDataChange shape. SubscriptionState record holds the handle + tag list + interval + per-subscription CancellationTokenSource + ConcurrentDictionary<string,DataValueSnapshot> LastValues; UnsubscribeAsync removes the state from _subscriptions and cancels the CTS, stopping the polling loop cleanly. Multiple overlapping subscriptions each get their own polling Task so a slow PLC on one subscription can't stall the others. ShutdownAsync cancels every active subscription CTS before tearing down the transport so the driver doesn't leave orphaned polling tasks pumping requests against a disposed socket. Transient poll errors are swallowed inside the loop (the loop continues to the next tick) — the driver's health surface reflects the last-known Degraded state from the underlying ReadAsync path. OperationCanceledException is caught separately to exit the loop silently on unsubscribe/shutdown. Tests (6 new ModbusSubscriptionTests): Initial_poll_raises_OnDataChange_for_every_subscribed_tag asserts the initial-data push fires once per tag in the subscribe call (2 tags → 2 events with FullReference='Level' and FullReference='Temp'); Unchanged_values_do_not_raise_after_initial_poll lets the loop run ~5 cycles at 100ms with a stable register value, asserts only the initial push fires (no event spam on stable tags); Value_change_between_polls_raises_OnDataChange mutates the fake register bank between poll ticks and asserts a second event fires with the new value (verified via e.Snapshot.Value.ShouldBe((short)200)); Unsubscribe_stops_the_polling_loop captures the event count right after UnsubscribeAsync, mutates a register that would have triggered a change if polling continued, asserts the count stays the same after 400ms; SubscribeAsync_floors_intervals_below_100ms passes a 10ms interval + asserts only 1 event fires across 300ms (if the floor weren't enforced we'd see 30 — the test asserts the floor semantically by counting events on stable data); Multiple_subscriptions_fire_independently creates two subs on different tags, unsubscribes only one, mutates the other's tag, asserts only the surviving sub emits while the unsubscribed one stays at its pre-unsubscribe count. FakeTransport in this test file is scoped to FC03 only since that's all the subscription path exercises — keeps the test doubles minimal and the failure modes obvious. WaitForCountAsync helper polls a ConcurrentQueue up to a deadline, makes the tests tolerant of scheduler jitter on slow CI runners. Full solution: 0 errors, 195 tests pass (6 new subscription + 9 existing Modbus + 180 pre-existing). ModbusDriver now implements IDriver + ITagDiscovery + IReadable + IWritable + ISubscribable — five of the eight capability interfaces. IAlarmSource + IHistoryProvider remain unimplemented because Modbus has no wire-level alarm or history semantics; IHostConnectivityProbe is a plausible future addition (treat transport disconnect as a Stopped signal) but needs the socket-level connection-state tracking plumbed through IModbusTransport which is its own PR.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Joseph Doherty
@@ -17,8 +17,15 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
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/// </remarks>
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public sealed class ModbusDriver(ModbusDriverOptions options, string driverInstanceId,
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Func<ModbusDriverOptions, IModbusTransport>? transportFactory = null)
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: IDriver, ITagDiscovery, IReadable, IWritable, IDisposable, IAsyncDisposable
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: IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IDisposable, IAsyncDisposable
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{
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// Active polling subscriptions. Each subscription owns a background Task that polls the
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// tags at its configured interval, diffs against _lastKnownValues, and fires OnDataChange
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// per changed tag. UnsubscribeAsync cancels the task via the CTS stored on the handle.
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private readonly System.Collections.Concurrent.ConcurrentDictionary<long, SubscriptionState> _subscriptions = new();
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private long _nextSubscriptionId;
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public event EventHandler<DataChangeEventArgs>? OnDataChange;
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private readonly ModbusDriverOptions _options = options;
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private readonly Func<ModbusDriverOptions, IModbusTransport> _transportFactory =
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transportFactory ?? (o => new ModbusTcpTransport(o.Host, o.Port, o.Timeout));
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@@ -55,6 +62,13 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
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public async Task ShutdownAsync(CancellationToken cancellationToken)
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{
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foreach (var state in _subscriptions.Values)
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{
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try { state.Cts.Cancel(); } catch { }
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state.Cts.Dispose();
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}
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_subscriptions.Clear();
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if (_transport is not null) await _transport.DisposeAsync().ConfigureAwait(false);
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_transport = null;
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_health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
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@@ -220,6 +234,85 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
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}
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}
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// ---- ISubscribable (polling overlay) ----
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public Task<ISubscriptionHandle> SubscribeAsync(
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IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
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{
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var id = Interlocked.Increment(ref _nextSubscriptionId);
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var cts = new CancellationTokenSource();
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var interval = publishingInterval < TimeSpan.FromMilliseconds(100)
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? TimeSpan.FromMilliseconds(100) // floor — Modbus can't sustain < 100ms polling reliably
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: publishingInterval;
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var handle = new ModbusSubscriptionHandle(id);
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var state = new SubscriptionState(handle, [.. fullReferences], interval, cts);
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_subscriptions[id] = state;
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_ = Task.Run(() => PollLoopAsync(state, cts.Token), cts.Token);
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return Task.FromResult<ISubscriptionHandle>(handle);
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}
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public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
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{
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if (handle is ModbusSubscriptionHandle h && _subscriptions.TryRemove(h.Id, out var state))
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{
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state.Cts.Cancel();
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state.Cts.Dispose();
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}
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return Task.CompletedTask;
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}
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private async Task PollLoopAsync(SubscriptionState state, CancellationToken ct)
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{
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// Initial-data push: read every tag once at subscribe time so OPC UA clients see the
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// current value per Part 4 convention, even if the value never changes thereafter.
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try { await PollOnceAsync(state, forceRaise: true, ct).ConfigureAwait(false); }
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catch (OperationCanceledException) { return; }
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catch { /* first-read error — polling continues */ }
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while (!ct.IsCancellationRequested)
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{
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try { await Task.Delay(state.Interval, ct).ConfigureAwait(false); }
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catch (OperationCanceledException) { return; }
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try { await PollOnceAsync(state, forceRaise: false, ct).ConfigureAwait(false); }
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catch (OperationCanceledException) { return; }
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catch { /* transient polling error — loop continues, health surface reflects it */ }
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}
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}
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private async Task PollOnceAsync(SubscriptionState state, bool forceRaise, CancellationToken ct)
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{
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var snapshots = await ReadAsync(state.TagReferences, ct).ConfigureAwait(false);
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for (var i = 0; i < state.TagReferences.Count; i++)
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{
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var tagRef = state.TagReferences[i];
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var current = snapshots[i];
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var lastSeen = state.LastValues.TryGetValue(tagRef, out var prev) ? prev : default;
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// Raise on first read (forceRaise) OR when the boxed value differs from last-known.
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if (forceRaise || !Equals(lastSeen?.Value, current.Value) || lastSeen?.StatusCode != current.StatusCode)
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{
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state.LastValues[tagRef] = current;
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OnDataChange?.Invoke(this, new DataChangeEventArgs(state.Handle, tagRef, current));
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}
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}
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}
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private sealed record SubscriptionState(
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ModbusSubscriptionHandle Handle,
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IReadOnlyList<string> TagReferences,
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TimeSpan Interval,
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CancellationTokenSource Cts)
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{
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public System.Collections.Concurrent.ConcurrentDictionary<string, DataValueSnapshot> LastValues { get; }
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= new(StringComparer.OrdinalIgnoreCase);
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}
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private sealed record ModbusSubscriptionHandle(long Id) : ISubscriptionHandle
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{
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public string DiagnosticId => $"modbus-sub-{Id}";
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}
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// ---- codec ----
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internal static ushort RegisterCount(ModbusDataType t) => t switch
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@@ -0,0 +1,180 @@
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using System.Collections.Concurrent;
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using Shouldly;
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using Xunit;
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using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
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using ZB.MOM.WW.OtOpcUa.Driver.Modbus;
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namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
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[Trait("Category", "Unit")]
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public sealed class ModbusSubscriptionTests
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{
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/// <summary>
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/// Lightweight fake transport the subscription tests drive through — only the FC03
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/// (Read Holding Registers) path is used. Mutating <see cref="HoldingRegisters"/>
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/// between polls is how each test simulates a PLC value change.
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/// </summary>
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private sealed class FakeTransport : IModbusTransport
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{
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public readonly ushort[] HoldingRegisters = new ushort[256];
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public Task ConnectAsync(CancellationToken ct) => Task.CompletedTask;
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public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
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{
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if (pdu[0] != 0x03) return Task.FromException<byte[]>(new NotSupportedException("FC not supported"));
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var addr = (ushort)((pdu[1] << 8) | pdu[2]);
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var qty = (ushort)((pdu[3] << 8) | pdu[4]);
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var resp = new byte[2 + qty * 2];
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resp[0] = 0x03;
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resp[1] = (byte)(qty * 2);
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for (var i = 0; i < qty; i++)
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{
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resp[2 + i * 2] = (byte)(HoldingRegisters[addr + i] >> 8);
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resp[3 + i * 2] = (byte)(HoldingRegisters[addr + i] & 0xFF);
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}
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return Task.FromResult(resp);
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}
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public ValueTask DisposeAsync() => ValueTask.CompletedTask;
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}
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private static (ModbusDriver drv, FakeTransport fake) NewDriver(params ModbusTagDefinition[] tags)
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{
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var fake = new FakeTransport();
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var opts = new ModbusDriverOptions { Host = "fake", Tags = tags };
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return (new ModbusDriver(opts, "modbus-1", _ => fake), fake);
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}
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[Fact]
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public async Task Initial_poll_raises_OnDataChange_for_every_subscribed_tag()
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{
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var (drv, fake) = NewDriver(
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new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16),
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new ModbusTagDefinition("Temp", ModbusRegion.HoldingRegisters, 1, ModbusDataType.Int16));
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await drv.InitializeAsync("{}", CancellationToken.None);
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fake.HoldingRegisters[0] = 100;
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fake.HoldingRegisters[1] = 200;
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var events = new ConcurrentQueue<DataChangeEventArgs>();
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drv.OnDataChange += (_, e) => events.Enqueue(e);
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var handle = await drv.SubscribeAsync(["Level", "Temp"], TimeSpan.FromMilliseconds(200), CancellationToken.None);
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await WaitForCountAsync(events, 2, TimeSpan.FromSeconds(2));
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events.Select(e => e.FullReference).ShouldContain("Level");
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events.Select(e => e.FullReference).ShouldContain("Temp");
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await drv.UnsubscribeAsync(handle, CancellationToken.None);
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}
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[Fact]
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public async Task Unchanged_values_do_not_raise_after_initial_poll()
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{
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var (drv, fake) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
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await drv.InitializeAsync("{}", CancellationToken.None);
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fake.HoldingRegisters[0] = 100;
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var events = new ConcurrentQueue<DataChangeEventArgs>();
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drv.OnDataChange += (_, e) => events.Enqueue(e);
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var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
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await Task.Delay(500); // ~5 poll cycles at 100ms, value stable the whole time
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await drv.UnsubscribeAsync(handle, CancellationToken.None);
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events.Count.ShouldBe(1); // only the initial-data push, no change events after
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}
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[Fact]
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public async Task Value_change_between_polls_raises_OnDataChange()
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{
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var (drv, fake) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
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await drv.InitializeAsync("{}", CancellationToken.None);
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fake.HoldingRegisters[0] = 100;
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var events = new ConcurrentQueue<DataChangeEventArgs>();
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drv.OnDataChange += (_, e) => events.Enqueue(e);
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var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
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await WaitForCountAsync(events, 1, TimeSpan.FromSeconds(1));
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fake.HoldingRegisters[0] = 200; // simulate PLC update
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await WaitForCountAsync(events, 2, TimeSpan.FromSeconds(2));
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await drv.UnsubscribeAsync(handle, CancellationToken.None);
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events.Count.ShouldBeGreaterThanOrEqualTo(2);
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events.Last().Snapshot.Value.ShouldBe((short)200);
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}
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[Fact]
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public async Task Unsubscribe_stops_the_polling_loop()
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{
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var (drv, fake) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
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await drv.InitializeAsync("{}", CancellationToken.None);
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var events = new ConcurrentQueue<DataChangeEventArgs>();
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drv.OnDataChange += (_, e) => events.Enqueue(e);
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var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
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await WaitForCountAsync(events, 1, TimeSpan.FromSeconds(1));
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await drv.UnsubscribeAsync(handle, CancellationToken.None);
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var countAfterUnsub = events.Count;
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fake.HoldingRegisters[0] = 999; // would trigger a change if still polling
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await Task.Delay(400);
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events.Count.ShouldBe(countAfterUnsub);
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}
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[Fact]
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public async Task SubscribeAsync_floors_intervals_below_100ms()
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{
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var (drv, _) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
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await drv.InitializeAsync("{}", CancellationToken.None);
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// 10ms requested — implementation floors to 100ms. We verify indirectly: over 300ms, a
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// 10ms interval would produce many more events than a 100ms interval would on a stable
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// value. Since the value is unchanged, we only expect the initial-data push (1 event).
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var events = new ConcurrentQueue<DataChangeEventArgs>();
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drv.OnDataChange += (_, e) => events.Enqueue(e);
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var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(10), CancellationToken.None);
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await Task.Delay(300);
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await drv.UnsubscribeAsync(handle, CancellationToken.None);
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events.Count.ShouldBe(1);
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}
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[Fact]
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public async Task Multiple_subscriptions_fire_independently()
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{
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var (drv, fake) = NewDriver(
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new ModbusTagDefinition("A", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16),
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new ModbusTagDefinition("B", ModbusRegion.HoldingRegisters, 1, ModbusDataType.Int16));
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await drv.InitializeAsync("{}", CancellationToken.None);
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var eventsA = new ConcurrentQueue<DataChangeEventArgs>();
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var eventsB = new ConcurrentQueue<DataChangeEventArgs>();
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drv.OnDataChange += (_, e) =>
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{
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if (e.FullReference == "A") eventsA.Enqueue(e);
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else if (e.FullReference == "B") eventsB.Enqueue(e);
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};
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var ha = await drv.SubscribeAsync(["A"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
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var hb = await drv.SubscribeAsync(["B"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
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await WaitForCountAsync(eventsA, 1, TimeSpan.FromSeconds(1));
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await WaitForCountAsync(eventsB, 1, TimeSpan.FromSeconds(1));
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await drv.UnsubscribeAsync(ha, CancellationToken.None);
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var aCount = eventsA.Count;
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fake.HoldingRegisters[1] = 77; // only B should pick this up
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await WaitForCountAsync(eventsB, 2, TimeSpan.FromSeconds(2));
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eventsA.Count.ShouldBe(aCount); // unchanged since unsubscribe
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eventsB.Count.ShouldBeGreaterThanOrEqualTo(2);
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await drv.UnsubscribeAsync(hb, CancellationToken.None);
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}
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private static async Task WaitForCountAsync<T>(ConcurrentQueue<T> q, int target, TimeSpan timeout)
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{
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var deadline = DateTime.UtcNow + timeout;
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while (q.Count < target && DateTime.UtcNow < deadline)
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await Task.Delay(25);
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}
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}
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