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Part 16 State Machines

This guide describes the generic, extensible OPC UA Part 16 state- machine support layered on top of the source-generated ObjectType proxies and the existing FiniteStateMachineState server base.

For the formal model, see OPC UA Part 16 — State Machines.

Two-mode unified builder

The server side ships one fluent builder — StateMachineBuilder — with two complementary modes that can be mixed in a single chain:

Mode Entry point Use when …
Definition StateMachineBuilder.Create(parent, ctx, nodeId, name) You need a state machine that isn't already declared as a generator-emitted *StateMachineState subclass. Declare states, transitions, and cause mappings declaratively.
Lifecycle StateMachineBuilder.For<TState>(stateMachine, ctx) or INodeBuilder<TState>.AsStateMachine() You already have a FiniteStateMachineState subclass (stack-shipped, generator-emitted, or vendor) and want to attach behavior (enter/exit/transition hooks, method-to-cause bindings, auto-transitions).

Both modes share the same lifecycle surface (WithInitialState, OnEnterState, OnExitState, OnTransition, OnBeforeTransition, WithCause, WithTimedTransition, ConfigureStateMachine). Async overloads — OnEnterStateAsync, OnExitStateAsync, OnTransitionAsync — accept Func<ISystemContext, TState, CancellationToken, ValueTask> (or the four-arg form for transition observers) and are invoked fire-and-forget on the thread pool from the synchronous transition path. The handler therefore runs on a fully-async path (no GetAwaiter().GetResult() / Wait() / .Result anywhere) without blocking the transition or the calling client. Exceptions thrown from an async handler are captured and logged via Debug.WriteLine. Pre-transition guards (OnBeforeTransition, When*) intentionally remain sync-only because they must veto the transition synchronously before the state-machine state mutation completes. The definition methods (AddState, AddTransition, OnCause, UseElementNamespace) are only available in definition mode.

Quick reference

Concern Client entry point Server entry point
Read the current state StateMachineTypeClient.GetCurrentStateAsync FiniteStateMachineState.CurrentState
Read current state + last transition (finite) FiniteStateMachineTypeClient.GetCurrentFiniteStateAsync FiniteStateMachineState.LastState/LastTransition
Observe transitions (stream) (Finite)StateMachineTypeClient.ObserveFiniteTransitionsAsync n/a
Wait for a target state (Finite)StateMachineTypeClient.WaitForStateAsync n/a
Enumerate states / transitions FiniteStateMachineTypeClient.GetAvailableStatesAsync / GetAvailableTransitionsAsync n/a
Build a state machine declaratively n/a StateMachineBuilder.Create (definition mode)
Attach behavior to an existing FSM n/a StateMachineBuilder.For / INodeBuilder.AsStateMachine (lifecycle mode)

Client side

The source-generated *TypeClient proxies for state-machine types (StateMachineTypeClient, FiniteStateMachineTypeClient, ShelvedStateMachineTypeClient, ExclusiveLimitStateMachineTypeClient, ProgramStateMachineTypeClient, and any vendor subtypes) all inherit the generic API automatically — implemented as extension methods that hang off the proxy base.

Read the current state

var proxy = new ShelvedStateMachineTypeClient(session, conditionId, telemetry);
FiniteStateSnapshot snap = await proxy.GetCurrentFiniteStateAsync(ct);
Console.WriteLine($"State: {snap.CurrentState} ({snap.CurrentStateId})");
Console.WriteLine($"Last transition: {snap.LastTransition} ({snap.LastTransitionId})");

Stream transitions

Pair the proxy with the session's streaming subscription:

ManagedSession session = ...;
IStreamingSubscription streaming = session.DefaultStreaming;

await foreach (FiniteStateSnapshot snap in proxy
    .ObserveFiniteTransitionsAsync(streaming, ct: ct))
{
    Console.WriteLine($"{snap.Timestamp:O}  -> {snap.CurrentState}");
}

Each yielded snapshot is refreshed by reading the four state + transition variables in one round-trip, so consumers see consistent typed data per transition.

Wait for a target state

FiniteStateSnapshot reached = await proxy.WaitForStateAsync(
    streaming,
    targetStateId: new NodeId(Objects.ShelvedStateMachineType_TimedShelved),
    timeout: TimeSpan.FromSeconds(30),
    ct: ct);

The wait composes ObserveFiniteTransitionsAsync with timeout and cancellation; an immediate match against the current state is short-circuited.

Enumerate states + transitions

IReadOnlyList<FiniteStateInfo> states =
    await proxy.GetAvailableStatesAsync(ct);
IReadOnlyList<FiniteTransitionInfo> transitions =
    await proxy.GetAvailableTransitionsAsync(ct);

Browses the state machine instance's HasComponent children and filters by StateType / TransitionType. Useful for runtime introspection of vendor state machines.

Alarm shelving alignment

AlarmClient exposes the same API for the ShelvingState child of every Part 9 alarm condition:

AlarmClient alarms = session.GetAlarmClient(telemetry);

FiniteStateSnapshot snap = await alarms.GetShelvingStateAsync(conditionId, ct);

await foreach (FiniteStateSnapshot s in alarms
    .ObserveShelvingTransitionsAsync(conditionId, streaming, ct: ct))
{
    // …
}

Both methods delegate to ShelvedStateMachineTypeClient internally — same proxy-delegation pattern used by the rest of AlarmClient.

Device Integration (DI) software-update alignment

SoftwareUpdateClient (in Opc.Ua.Di.Client) wraps the same generic API for each of the four child state machines of OPC 10000-100 §10.3 (PrepareForUpdate, Installation, Confirmation, PowerCycle):

var su = new SoftwareUpdateClient(session, suNodeId, telemetry);

FiniteStateSnapshot? state = await su.GetInstallationStateAsync(ct);

await foreach (FiniteStateSnapshot snap in su
    .ObserveInstallationTransitionsAsync(streaming, options: null, ct))
{
    // …
}

await su.InstallSoftwarePackageAsync("urn:acme", "2.0.0",
    ArrayOf.Empty<string>(), default, ct);

See SoftwareUpdate.md for the full SU facet — including the server-side OnInstallationStateChanged / OnPrepareStateChanged / OnConfirmStateChanged instrumentation hooks that the DI server attaches to its built-in FSM lifecycle.

Vendor extensibility

Vendor concrete state machines declared in a NodeSet (e.g. MyVendor:FoodPreparationStateMachineType : FiniteStateMachineType) automatically get the generic API. The proxy generator emits a FoodPreparationStateMachineTypeClient that inherits from FiniteStateMachineTypeClient, and every extension method (GetCurrentFiniteStateAsync, ObserveFiniteTransitionsAsync, WaitForStateAsync, …) applies through the inheritance chain transparently. Vendor-declared methods (e.g. BeginPreparation) are emitted as instance methods on the vendor client by the generator.

Server side — definition mode

Use StateMachineBuilder.Create(...) when you need a state machine that isn't already a generator-emitted subclass. The builder owns a FluentFiniteStateMachineState instance, populates its state / transition / cause tables from your declarations, and freezes the definition the moment you read StateMachine or attach the first lifecycle hook.

using Opc.Ua.Server.StateMachines;

FluentFiniteStateMachineState sm = StateMachineBuilder
    .Create(parent, systemContext,
        nodeId: new NodeId(/*your numeric id*/, namespaceIndex),
        browseName: new QualifiedName("PowerSwitch", namespaceIndex))
    .AddState(id: 1, "Off", isInitial: true)
    .AddState(id: 2, "On")
    .AddTransition(id: 10, "OffToOn", from: 1, to: 2)
    .AddTransition(id: 20, "OnToOff", from: 2, to: 1)
    .OnCause(causeId: 100, from: 1, transition: 10)
    .OnCause(causeId: 200, from: 2, transition: 20)
    .WithInitialState(1)
    .StateMachine;

The builder validates structural integrity at the freeze step (the first lifecycle method or StateMachine access): every transition's from/to must reference declared states, every cause mapping must reference a declared transition, and state / transition / cause ids must be unique within their tables.

Server side — lifecycle mode

Use StateMachineBuilder.For(...) (or INodeBuilder<TState>.AsStateMachine() inside a fluent node-manager build pipeline) when you already have a state-machine instance — stack-shipped (ShelvedStateMachineState, ExclusiveLimitStateMachineState, ProgramStateMachineState), generator-emitted (MyVendor.FoodPreparationStateMachineState), or the result of an earlier StateMachineBuilder.Create(...) chain.

Stack-shipped state machine

// Existing ShelvedStateMachineState that lives under an alarm condition:
ShelvedStateMachineState shelving = alarm.ShelvingState;

StateMachineBuilder.For(shelving, systemContext)
    .OnEnterState(
        Objects.ShelvedStateMachineType_TimedShelved,
        (ctx, sm) => logger.LogInformation("Alarm timed-shelved"))
    .OnExitState(
        Objects.ShelvedStateMachineType_TimedShelved,
        (ctx, sm) => logger.LogInformation("Alarm un-timed-shelved"))
    .OnTransition((ctx, sm, from, to) =>
        logger.LogDebug("Shelving {From} -> {To}", from, to))
    .WithTimedTransition(
        fromStateId: Objects.ShelvedStateMachineType_OneShotShelved,
        timeout: TimeSpan.FromMinutes(15),
        transitionId: Objects.ShelvedStateMachineType_OneShotShelvedToUnshelved);

The lifecycle builder layers on top of any pre-existing OnBeforeTransition / OnAfterTransition delegates the state machine had — the stack-shipped behavior continues to run, with your handlers wrapping it.

Inside a fluent node-manager build pipeline

public sealed class MyNodeManager : FluentNodeManagerBase
{
    protected override void OnConfigure(INodeManagerBuilder builder)
    {
        builder
            .Node(BrowsePaths.SoftwareUpdate.Installation)
            .As<DI.InstallationStateMachineState>()
            .AsStateMachine()
            .OnEnterState(StateNumbers.Installing,
                (ctx, sm) => StartInstall(ctx))
            .OnTransition((ctx, sm, from, to) =>
                _logger.LogInformation("Install: {From} -> {To}", from, to))
            .WithTimedTransition(
                fromStateId: StateNumbers.Installing,
                timeout: TimeSpan.FromMinutes(10),
                transitionId: StateNumbers.InstallingToFailed,
                causeId: StateNumbers.TimeoutCause);
    }
}

AsStateMachine() pulls the ISystemContext from nodeBuilder.Builder.Context. The INodeBuilder<TState> typed view ensures the underlying node is a FiniteStateMachineState subclass at compile time — no runtime casts.

Lifecycle ordering

For every transition the dispatcher fires handlers in this order:

DispatchBefore:
  builder guards (in registration order)
   ↓ veto on any failure
  [original OnBeforeTransition, if any]
   ↓ framework state update (LastState ← CurrentState, CurrentState ← newState)
DispatchAfter:
  [original OnAfterTransition, if any]
   ↓
  OnExitState(from)   (each registered handler, in order)
   ↓
  OnTransition(from, to)   (each observer, in order)
   ↓
  OnEnterState(to)   (each registered handler, in order)

Builder guards run before any pre-existing OnBeforeTransition — so they can veto without the original side-effectful pre-handler running. Builder observers run after the original OnAfterTransition so stack-shipped audit / change notifications complete first.

If multiple handlers are registered for the same state or transition, they fire in the order they were added. Exceptions thrown by an individual handler do not interrupt the dispatch — they are caught and logged via Debug.WriteLine.

How the source-generator integrates

  • Client: every state-machine *TypeClient proxy (generated from the standard NodeSet and every vendor NodeSet you feed through the generator) inherits the streaming + read + browse API automatically. No hand-written client per state-machine type.
  • Server: when a NodeSet declares a concrete subtype of FiniteStateMachineType (vendor or standard), the generator emits a *StateMachineState server class with the hardcoded StateTable / TransitionTable / TransitionMappings / CauseMappings overrides. Hook behavior onto that instance via StateMachineBuilder.For(...) — no need to redeclare the tables.

The unified builder gives you a clean migration path:

  • Have a generator-emitted FSM type? Lifecycle mode only. Build the instance the usual way, then attach behavior with StateMachineBuilder.For(...).
  • Need an ad-hoc state machine with no generated subclass? Definition mode. StateMachineBuilder.Create(...) constructs a FluentFiniteStateMachineState and lets you chain definitions and lifecycle hooks in one expression.

Guard clauses (fluent sugar)

The lifecycle builder offers a coarse OnBeforeTransition that fires for every transition; the When* family provides per-trigger sugar that compiles down to the same pipeline but scopes the predicate automatically.

StateMachineBuilder.Create(parent, ctx, nodeId, browseName)
    .AddState(1, "Off", isInitial: true)
    .AddState(2, "On")
    .AddTransition(10, "OffToOn", from: 1, to: 2)
    .AddTransition(20, "OnToOff", from: 2, to: 1)
    .WhenTransition(10, (ctx, sm) => HasPermission(ctx))
    .WhenCause(causeId: 100, (ctx, sm) => sm.SafetyInterlockClear)
    .WhenEnter(toStateId: 2, (ctx, sm) => sm.PreflightOk,
        denyStatus: StatusCodes.BadInvalidState)
    .WhenExit(fromStateId: 2, (ctx, sm) => sm.ShutdownComplete);
Method Scope Available in
OnBeforeTransition(Func<ctx, sm, transitionId, causeId, ServiceResult>) Every transition (caller filters) Both modes
WhenTransition(transitionId, predicate[, denyStatus]) Transition matches transitionId Both modes
WhenCause(causeId, predicate[, denyStatus]) Cause matches causeId Both modes
WhenEnter(toStateId, predicate[, denyStatus]) Transition would enter toStateId Definition mode only
WhenExit(fromStateId, predicate[, denyStatus]) Machine is leaving fromStateId Both modes

Returning false from a predicate vetoes the transition with BadUserAccessDenied (or the caller-supplied denyStatus). Multiple guards on the same trigger AND together — the first predicate returning false wins, and its denyStatus flows through to DoTransition's return.

All guards (On* + When*) share one pipeline in registration order — the FIRST failing guard wins. This means a When* guard declared before an OnBeforeTransition guard runs first.

WhenEnter is definition-mode-only because the builder needs the transition table to map toStateId to the matching transition ids. In lifecycle mode the table is protected on the FSM; use WhenTransition with the explicit transition id instead.

Sub-state machines (hierarchical state)

OPC UA Part 16 §5.2.3 supports nested state machines via the HasSubStateMachine reference. Standard alarms use this pattern (ExclusiveLimitAlarmType.LimitState is itself an ExclusiveLimitStateMachineType distinguishing HighHigh / High / Low / LowLow while the parent alarm is Active). Vendor process-control servers compose hierarchical state machines extensively.

Server side — WithSubStateMachine

The unified builder attaches a sub-state-machine to a parent state. The dispatcher auto-manages the lifecycle: parent enters → child activates (and resets to its initial state unless preserveOnReentry: true is supplied); parent exits → child is suspended and rejects subsequent transitions until the next parent re-entry.

FluentFiniteStateMachineState parent = StateMachineBuilder
    .Create(parentNode, ctx, parentNodeId, browseName)
    .AddState(1, "Active", isInitial: true)
    .AddState(2, "Inactive")
    .AddTransition(12, "ActiveToInactive", from: 1, to: 2)
    .AddTransition(21, "InactiveToActive", from: 2, to: 1)
    // Attach a sub-state-machine to the Active parent state.
    .WithSubStateMachine(
        parentStateId: 1,
        browseName: new QualifiedName("LimitState", ns),
        configure: child => child
            .AddState(10, "HighHigh", isInitial: true)
            .AddState(11, "High")
            .AddState(12, "Low")
            .AddState(13, "LowLow")
            .AddTransition(110, "HighHighToHigh", from: 10, to: 11),
        preserveOnReentry: false)
    .StateMachine;

Available in definition mode only — in lifecycle mode the parent FSM already declares its sub-state machines as part of the type definition. Observe them through the client-side sub-SM accessors below.

While suspended, the child FSM's DoTransition and DoCause return BadInvalidState. The flag is exposed publicly as FluentFiniteStateMachineState.IsSuspended for diagnostics.

Client side — sub-SM observation

FiniteStateMachineTypeClient parent =
    new FiniteStateMachineTypeClient(session, alarmId, telemetry);

// Resolve the sub-SM attached to a parent state:
FiniteStateMachineTypeClient? limitSm =
    await parent.GetSubStateMachineAsync(
        parentStateNodeId: alarmStateNodeId, telemetry, ct);

if (limitSm != null)
{
    FiniteStateSnapshot snap = await limitSm.GetCurrentFiniteStateAsync(ct);
    Console.WriteLine($"Limit state: {snap.CurrentState}");
}

// Observe combined parent + sub-SM transitions:
await foreach (FiniteStateSnapshot snap in parent
    .ObserveEffectiveStateAsync(streaming, telemetry, ct: ct))
{
    Console.WriteLine($"Parent: {snap.CurrentState}");
    if (snap.SubMachine != null)
    {
        Console.WriteLine($"  Sub: {snap.SubMachine.CurrentState}");
    }
}

FiniteStateSnapshot gains an optional SubMachine: FiniteStateSnapshot? field carrying the snapshot of the parent's currently-active sub-SM (or null if none). ObserveEffectiveStateAsync yields a combined snapshot each time either the parent transitions or the parent's currently-active sub-SM transitions:

  • On every parent transition the yield carries the latest known snapshot of the sub-SM attached to the new state (or null if none).
  • On every sub-SM transition that occurs while the parent is in the state that owns the sub-SM, the yield carries the new sub-SM snapshot. Sub-SM transitions occurring while the parent is in a different state are silently discarded.

Under the hood all discovered sub-SMs are subscribed once up-front and their notifications are multiplexed through a Channel<T>; sub-SM events are filtered against the parent's currently-active state.

Client side — typed sub-SM accessors (generated)

For every <opc:Object> child declared on a generator-emitted ObjectType, the source generator now emits a typed, lazily-resolved accessor on the parent type's client. For state-machine types, this gives ergonomic access to known sub-SMs without needing to know the state NodeId:

// Generated on AlarmConditionTypeClient:
ShelvedStateMachineTypeClient? shelving = await alarm
    .GetShelvingStateAsync(telemetry, ct);

// Generated on ExclusiveLimitAlarmTypeClient:
ExclusiveLimitStateMachineTypeClient? limit = await alarm
    .GetLimitStateAsync(telemetry, ct);

Properties of the generated accessor:

  • Typed — returns the concrete sub-SM proxy type, not the generic FiniteStateMachineTypeClient.
  • Lazy — first call resolves the child NodeId via TranslateBrowsePathsToNodeIds against the parent's ObjectId. Optional children (ModellingRule="Optional") that aren't exposed by the server yield null.
  • Cached — subsequent calls return the same instance without another browse round-trip. The cache is per-parent-instance.

Vendor models that add their own <opc:Object> children on a custom ObjectType benefit automatically; the generator emits the same shape for every Object child it encounters.

Per-spec HasSubStateMachine placement (deferred)

Part 16 §B.3 places the HasSubStateMachine reference on the parent state node, not the FSM root. The fluent builder currently attaches the reference from the FSM root because FluentFiniteStateMachineState does not materialize per-state instance NodeStates — state nodes are shared across all instances of the type. Browsing HasSubStateMachine from the FSM root via GetSubStateMachineAsync and the typed accessors above works against this wiring; clients that strictly browse from the state node will not discover the sub-SM. A future iteration may materialize per-state instance nodes to align with the spec; this is tracked but deliberately deferred.

Extensibility recipes

  • Add convenience properties. Subclass FluentFiniteStateMachineState, override CreateChildren if you need vendor-specific child variables, and call StateMachineBuilder.For<MyVendorFsm>(instance, ctx) to attach behavior.
  • Bind a method node to a cause. WithCause(methodNodeId) installs an OnCallMethod2 handler that calls DoCause(...). The cause id is derived from the method NodeId's numeric identifier (OPC UA convention); the cause→transition mapping is whatever the underlying FSM declared (OnCause(...) in definition mode or hardcoded in a stack/vendor subclass).
  • Drive auto-transitions. WithTimedTransition(fromStateId, timeout, transitionId, causeId) arms a System.Threading.Timer on every entry into fromStateId (including the initial state) and cancels it on exit. The timer fires DoTransition(...) on a thread-pool thread, so the standard transition machinery (events, audit, observers) runs as expected.
  • Escape hatch. ConfigureStateMachine(Action<TState>) is invoked synchronously with the underlying state machine. Use it for properties or methods the builder doesn't surface directly.

Tests

  • Tests/Opc.Ua.Server.Tests/StateMachines/StateMachineBuilderTests.cs validates definition-mode chaining, validation, and freeze semantics.
  • Tests/Opc.Ua.Server.Tests/StateMachines/StateMachineBuilderLifecycleTests.cs exercises every lifecycle method, including timed transitions and the layering behavior on top of pre-existing delegates.
  • Tests/Opc.Ua.Server.Tests/StateMachines/FluentFiniteStateMachineStateTests.cs covers the table projections.
  • The Part 9 conformance tests in Tests/Opc.Ua.History.Tests/AlarmsAndConditions*.cs exercise AlarmClient.GetShelvingStateAsync and ObserveShelvingTransitionsAsync end-to-end against the reference server.

See also