ClusterClient→gRPC migration Phase 4 (docs/plans/2026-07-22-clusterclient-to-grpc-plan.md).
Phases 2/3 proved both directions on gRPC; this removes the Akka transport underneath.
Deleted:
- AkkaCentralTransport, AkkaSiteTransport (+ their dedicated tests)
- ISiteClientFactory + DefaultSiteClientFactory; CentralCommunicationActor legacy
ctor + SelectTransport (Host now builds GrpcSiteTransport and injects it)
- ClusterClient creation + both ClusterClientReceptionist.RegisterService calls in
AkkaHostedService; the RegisterCentralClient message + receive block
- CommunicationOptions.CentralContactPoints; the CentralTransport/SiteTransport
coexistence flags; the CentralTransportMode/SiteTransportKind enums
gRPC is now the only site↔central transport (site→central CentralControlService via
GrpcCentralTransport; central→site SiteCommandService via GrpcSiteTransport), both
built unconditionally by the Host. NoOpCentralTransport is the fail-loud null-default
so TestKit command-dispatch suites still construct the site actor without a wired
transport; production always injects GrpcCentralTransport.
Config: CentralGrpcEndpoints is now unconditional — CommunicationOptionsValidator
rejects blank entries (role-agnostic), and StartupValidator requires a Site node to
list >=1 endpoint (fail-fast, mirrors GrpcPsk). Rig configs moved
CentralContactPoints -> CentralGrpcEndpoints (docker x6, docker-env2 x2, Host default,
deploy/wonder-app-vd03). Kept Akka.Cluster.Tools (ClusterSingleton still used).
Tests: build 0/0; Communication.Tests 640, Host.Tests 421 green. Removed the
ClusterClient.Send per-site-routing tests (covered by the transport suites), swapped
the ISiteClientFactory-based ctors to a substitute ISiteCommandTransport, converted
the audit-push integration relay to an in-process bridge transport.
Docs: Component-Communication/Host/StoreAndForward, components/Communication,
topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired
amendment), and CLAUDE.md transport decisions.
Not included: the dead IntegrationCallRequest path (#32) is a separate user-owned
behavioral decision — SiteEnvelope routing is transport-agnostic so it still compiles.
Extract the central→site send path in CentralCommunicationActor behind a new
ISiteCommandTransport, selected by ScadaBridge:Communication:SiteTransport
(Akka | Grpc, default Akka — rollback = flip the flag). CommunicationService's
27 commands, SiteCallAuditActor's 2 parked relays and DebugStreamBridgeActor's
subscribe/unsubscribe are untouched; the seam sits below SiteEnvelope.
- AkkaSiteTransport: today's per-site ClusterClient path extracted verbatim
(the _siteClients lookup + ClusterClient.Send with the reply-to sender
preserved, and the "no client ⇒ warn + drop, caller's Ask times out" path).
- GrpcSiteTransport: dials the site SiteCommandService (T1B.1 proto client) via
SiteCommandDtoMapper, PSK + x-scadabridge-site on the channel through
ControlPlaneCredentials, per-command deadlines set EQUAL to today's
CommunicationService Ask timeouts (per-command, not per-group: DeploymentState
query and TriggerSiteFailover use QueryTimeout; the two parked relays map to
QueryTimeout so SiteCallAudit's inner RelayTimeout 10s < 30s ordering holds;
WaitForAttribute keeps its dynamic Timeout + IntegrationTimeout).
- SitePairChannelProvider: per-site A/B channel pair with sticky failover
(flip only on Unavailable — NEVER on DeadlineExceeded, a write/deploy/failover
may have run), background failback probe to the preferred node with 1s→60s
doubling backoff, PSK invalidation on site removal. Fed by the SAME DB refresh
loop (extended to carry GrpcNodeA/GrpcNodeBAddress) — no second poll.
Tests: actor-with-substitute-transport (routing, Ask-reply plumbing, per-site
lifecycle across refreshes), ResolveDeadline pinned to each command's current
Ask timeout, and GrpcSiteTransport/SitePairChannelProvider over dual in-process
TestServers (PSK+header+deadline attached, Unavailable failover + stickiness,
failback to preferred, no-retry-on-DeadlineExceeded). Proto csproj untouched
(no active <Protobuf> item). Full solution builds 0 warnings; Communication.Tests
607 green with Akka default.
Joseph Doherty