Phase 1B's contract slice: the wire shape and the canonical translation for the
28 central→site commands that leave ClusterClient. No behaviour changes yet —
SiteCommunicationActor and CentralCommunicationActor are untouched; the
dispatcher refactor (T1B.2) and the central transport seam (T1B.3) consume this.
Protos/site_command.proto (package scadabridge.sitecommand.v1, service
SiteCommandService): six domain RPCs, each with a `oneof` request/reply
envelope. The grouping is what carries deadline policy — every command inside a
group shares a CommunicationOptions timeout class today, so one RPC per group
keeps the deadline choice in one place on the client and one dispatch switch on
the server, while the oneof keeps each command individually typed:
ExecuteLifecycle(6) · ExecuteOpcUa(8) · ExecuteQuery(4) · ExecuteParked(5) ·
ExecuteRoute(4) · TriggerFailover(1). IntegrationCallRequest — the 29th entry on
SiteCommunicationActor's receive table — is deliberately excluded as dead code
(2026-07-22-integration-call-routing-is-dead-code.md).
Contract decisions worth knowing:
- Nullable COLLECTIONS ride in per-collection wrapper messages
(DeployArtifactsCommand's six artifact lists, CertTrustResult.Certs,
RouteToCallRequest.Parameters). proto3 repeated/map collapses null into empty,
and that distinction is live at the site — the same silent-data-loss class the
transport round-trip guard exposed in PLAN-05 T8. Goldens cover null, empty
and populated for each.
- Nullable strings use the empty-string-means-null convention already set by
AuditEventDtoMapper, with ONE exception: RouteToWaitForAttributeRequest's
TargetValueEncoded, where "wait for the empty string" is a real target, so it
carries a StringValue wrapper. Both behaviours are asserted, not assumed.
- Nullable enums ride in one-field messages (proto3 enums have no presence and
no stock wrapper). Enum translation is an explicit switch in both directions —
never by ordinal — so reordering a C# enum cannot re-map the wire; every wire
enum reserves 0 for _UNSPECIFIED and decodes to a documented safe default
rather than faulting a command from a version-skewed peer.
- New LooseValueCodec carries the surviving `object?` members (script params and
return values, attribute values, tag read/write values) as a type-tagged union
so a boxed value keeps its runtime CLR type, as it does today under Akka's
type-preserving JSON serializer. Dates ride as invariant round-trip strings,
not Timestamp, which would silently normalise away DateTime.Kind and
DateTimeOffset.Offset. Lists/maps recurse; anything outside the tagged set
falls back to JSON and is documented as CLR-type-lossy.
- DebugViewSnapshot gets its own full-fidelity alarm/attribute messages rather
than reusing sitestream's AlarmStateUpdate, which flattens values to display
strings — right for a live stream, lossy for a snapshot the UI treats as
authoritative. The encoder omits an AlarmStateChanged.Condition that already
equals the record's derived default, so computed alarms round-trip exactly
(record equality compares the nullable backing field, not the property).
Tests are reflection-driven so the coverage cannot drift: the round-trip theory
enumerates the mapper's own ToProto overloads, the envelope guards enumerate the
generated oneof descriptors, and a missing golden fails the build. 216 new tests
green (Communication 532 total, Commons 684 total, solution build 0/0).
Codegen is checked in under SiteCommandGrpc/ per the sitestream recipe; the
<Protobuf> ItemGroup stays commented out (an active one segfaults protoc in the
linux_arm64 Docker image). docker/regen-proto.sh now handles every proto in that
ItemGroup instead of just sitestream, and re-comments idempotently.
Joseph Doherty