UA-.NETStandard/samples/GeneratorServer/Generators.md

Generators (generating sets)

The draft Generators companion specification (http://opcfoundation.org/UA/Generators/) realised end to end by GeneratorServer, and composed with the pump sample by SiteCompositionServer.

The model

A generating set is two things at once: an asset with a nameplate and a health, and a machine whose core is an engine coupled to an alternator. The specification layers on existing building blocks rather than reinventing them:

Layer	Role
DI (OPC 10000-100)	GeneratorSetType derives from DeviceType, inheriting the nameplate and DeviceHealth; each subsystem derives from ComponentType
Machinery (OPC 40001-1)	Identification add-in and the MachineryItemState / MachineryOperationMode building blocks
Part 9	GeneratorProtectionAlarmType, a subtype of OffNormalAlarmType
Part 16	GeneratorStateMachineType, a twelve-state FiniteStateMachineType

Composition over deep inheritance: subsystems are separate ComponentType subtypes referenced by HasComponent, so a set is assembled from exactly the components it has.

Source generation

The sample generates the model locally from vendored NodeSet2 files rather than referencing a library:

<AdditionalFiles Include="Model\Opc.Ua.Generators.NodeSet2.xml">
  <ModelSourceGeneratorPrefix>Opc.Ua.Generators</ModelSourceGeneratorPrefix>
</AdditionalFiles>
<AdditionalFiles Include="Model\Opc.Ua.Generators.NodeSet2.csv" />

The CSV alongside each XML is the stable NodeId table. Without it the generator assigns synthetic NodeIds at compile time, which breaks byte-stable wire formats for clients that cache NodeIds across builds.

The Machinery dependency

The specification composes three Machinery types — MachineryItemState_StateMachineType, MachineryOperationModeStateMachineType and MachineIdentificationType — that the reduced Machinery nodeset carried by the pump sample does not define. The full official nodeset defines them but does not survive the model source generator (it fails with MODELGEN003), and it declares a dependency on the IA namespace through a single optional Stacklight member that a generating set does not have.

Model/prepare_machinery_nodeset.py therefore derives a reduced-but-sufficient set from the official nodeset by whitelist, strips the references left dangling and drops the IA dependency. Deriving it mechanically keeps the provenance checkable, and the whitelist is the only thing to edit when more types are needed.

Removing the IA URI is index-safe because it is the last entry in NamespaceUris, so the Machinery (1) and DI (2) indices used throughout the file do not move.

Simulation design

The sample's organising idea is that load fraction is the only independent variable. Everything else is a function of it:

V̇_f(x) = 3.67 + 100·x                      fuel rate       [L/h]
η(x)   = P(x) / (V̇_f(x) · ρ · LHV)         efficiency
S      = P / PF                            apparent power  [VA]
I      = S / (√3 · V_LL)                   line current    [A]
f      = N · p / 120                       frequency       [Hz]

This is not a stylistic choice. When each measurement is an independent oscillator — as the pump sample's simulation once was — a server happily publishes a duty point that no real machine could occupy, and no test can catch it because there is nothing for the values to be inconsistent with. Deriving them from one variable means P = √3·V·I·PF and η = P/(V̇·ρ·LHV) hold at every tick by construction, and GeneratorDatasheetConformanceTests asserts exactly that.

The published datasheet, the engineering ranges, the trip points and the simulation all read the same constants, so they cannot drift apart.

Control surfaces

The specification defines three ways to observe and command a set, and all three are driven from the same decider: the simulation's operating state. This is the design point worth copying. A state machine advanced independently of the physics that produced it will eventually report a machine as running while the simulation says it is stopped — and because both halves are internally consistent, nothing at run time notices.

GeneratorStateMachineType

Twelve states and twenty-two declared transitions, mandatory on the type. Because the node already exists, the sample attaches behaviour to it in lifecycle mode (StateMachineBuilder.For(machine, context)) rather than defining states — see State machines.

The simulation raises a transition callback; the address space follows it. Both CurrentState and its Id property are written, because a client that receives a state name it cannot resolve to a state node is no better off than one that received nothing.

The state and transition number tables are deliberately kept apart from the node manager, in GeneratorStateMap, and a test holds them against the simulation's own legality function in both directions: a transition the physics permits but the map lacks moves a machine without telling a client; one the map holds but the physics refuses is dead weight that looks supported.

GeneratorProtectionAlarmType

One instance per protection function rather than a single instance whose ProtectionFunction changes. A set can trip on low oil pressure and overspeed in the same moment, and collapsing them loses the second; it is also how a real control panel annunciates.

Because OffNormalAlarmType takes healthy as the normal state, each instance carries InputNode pointing at the variable it supervises. Without it a client can see that something tripped but not what was being watched.

Optional members must be opted into. ProtectionFunction is mandatory on the type and is materialised by the generated factory; IsShutdown and SubsystemName are optional and are not. Writing to an optional member with CreateOrReplace alone produces a child that exists, appears in GetChildren and holds the value — but carries no ReferenceTypeId, so no browse can reach it and the property is absent from a client's view. Call AddXxx(context) before writing. The failure is silent in both directions: the code looks right, and the server answers reads on everything it does publish, so only a client comparing against the type definition notices.

A simulation that cannot leave its healthy band cannot exercise its protections. The datasheet curves are bounded inside every trip point by construction, so a faithful simulation of a healthy machine leaves the entire protection path — alarms, shutdown class, ResetFaults, the Fault branch of the state machine — as dead code, while the documentation describes it as working. The sample therefore injects a deviation into the measurement rather than the curve, which is what a real fault is, and rotates it so a long run exercises every protection. The lesson generalises: when the model says a thing cannot happen, the code that handles it needs a deliberate way to be reached, or it is untested.

Trip after evaluating, not during. Stopping the set inside the evaluation loop makes its remaining conditions read healthy, so simultaneous trips collapse to whichever protection came first in the table — defeating the reason for having one alarm instance per function.

A shutdown protection must latch. The trip removes the condition that caused it, because the supervised quantities are only meaningful while the machine runs. An alarm that tracks its input therefore annunciates for one tick and clears, which tells an operator that something stopped the set but not what. Latch until the set leaves the shutdown state. This one is easy to miss in review and in unit tests — the condition logic is correct in isolation; the defect only appears when the trip and the supervision interact over time.

Clearing is an event, not just a state change. A client learns of condition state changes only through events. Clearing an alarm node without reporting leaves an alarm-list client displaying it as active and retained until a ConditionRefresh.

Trip conditions read the hysteresis the simulation already applies, so alarms latch and clear cleanly rather than chattering on the threshold, and events are reported only on change. A shutdown-class trip stops the machine; a warning does not — reporting a trip without stopping the set would publish a generator that is on fire and still loaded.

One trap worth naming: supervising low oil pressure during cranking trips every set the moment it tries to start, because pressure has not built yet. Real sets bypass that trip for exactly this reason.

Methods

Legality is decided in exactly one place, so no caller can drive a machine from Off straight to Loaded by picking the right method. A refused request answers BadInvalidState rather than silently doing nothing — a method that appears to succeed without acting is indistinguishable from a real success, which is worse than an honest refusal.

The method semantics are expressed against the simulation rather than against address-space nodes, which keeps each handler down to one line and lets the behaviour be tested without standing up a server.

Concurrency

The simulation tick runs on a thread-pool thread while client requests are served on their own threads, and both drive state transitions and write the same address-space nodes. They are serialised by a single gate held across the whole tick and across every method handler.

This matters more than it looks. Without it, a tick moving a set to Cooldown and a concurrent EmergencyStop both fire the state-change callback, and the paired CurrentState / CurrentState.Id writes interleave — leaving a client with a state name from one transition and a state node from the other. That is the precise failure the paired write exists to prevent, reintroduced by the threading model rather than by the write itself. One gate for the whole plant rather than one per set: a tick is microseconds of arithmetic, so the contention is irrelevant, and a single gate cannot be acquired in the wrong order.

Cross-server composition

SiteCompositionServer demonstrates composing several servers at a supervisory level. It owns no devices; it publishes a site stage and one cross-server component binding per subordinate, carrying that server's ComponentServerUri and ComponentEndpointUrl:

Opc.Ua.OpenUsd.Connector --server <site> --federate --view

With --federate the connector opens a session to each named server, discovers its representations and drives its bindings into the same stage — one scene, live machines, three servers.

Nothing is mirrored. The site server never proxies a subordinate's address space, so there is no cache to invalidate and no second copy of the truth.

--federate is opt-in because the endpoint the connector dials comes from the server being rendered rather than from the operator, which makes honouring it a trust decision. Federation is also best-effort per component: a subordinate that is down is logged and skipped, and the rest of the scene still renders.

See also

• GeneratorServer and its datasheet
• SiteCompositionServer
• OpenUSD
• Device Integration
• State machines