histsdk/docs/plans/hcal-roadmap.md

HCAL modern-.NET client — implementation roadmap

Ordered, actionable plan to grow histsdk from "reads + basic config" into a broad HCAL replacement, built on the 2023 R2 gRPC transport. Derived from hcal-capability-matrix.md; event details in histevents.md.

Move to the repo's docs/plans/ when execution starts. Each work item lands as: a protocol serializer/parser + golden-byte unit test + an env-gated live integration test against the local Historian.

Progress (updated 2026-06-21)

• ✅ R0.6 version gate — HistorianServerVersionGate + HistorianClientOptions.VerifyServerInterfaceVersion; fail-closed on connect, wired into both WCF and gRPC paths. Supported versions are evidence-based (Hist=11/12, Retr=4, Trx=2; Status reachability-only), captured from the live server. History 12 (2023 R2 gRPC) accepted alongside 11 (buffer-compatible).
• ✅ CW-1 capture pipeline — ProtocolCaptureSanitizer + ProtocolFixtureWriter + capture-tag-info CLI command; produces sanitized fixtures/protocol/<op>/ golden files. 11 unit tests. First fixture: get-tag-info/analog-*.json.
• ✅ gRPC auth handshake (read chain) — LIVE-VERIFIED 2026-06-21 against a real 2023 R2 server: ReadRawAsync over RemoteGrpc returns rows. Token loop routes to StorageService.ValidateClientCredential. Shared handshake extracted to Grpc/HistorianGrpcHandshake for reuse by the status/browse/metadata paths.
• ✅ R0.4 Probe over gRPC — Grpc/HistorianGrpcProbe (History/Retrieval/Status GetInterfaceVersion); ProbeAsync routes over gRPC when Transport==RemoteGrpc. LIVE-VERIFIED 2026-06-21 (no credentials required — runs before the auth loop).
• ✅ R0.3 System parameter over gRPC — Grpc/HistorianGrpcStatusClient.GetSystemParameterAsync (StatusService.GetSystemParameter); routed in the dialect. Built + unit-tested + LIVE-VERIFIED 2026-06-21 against a real 2023 R2 server (returned HistorianVersion). Code path is the proven handshake + a single string-in/string-out RPC.
• ✅ R0.2 Tag metadata over gRPC — Grpc/HistorianGrpcTagClient.GetTagMetadataAsync (RetrievalService.GetTagInfosFromName, the plural string-handle op). GetTagMetadataAsync routes over gRPC when Transport==RemoteGrpc. Request btTagNames = uint count + per-name(uint charCount + UTF-16LE) (golden-byte unit-tested); response btTagInfos = uint count + CTagMetadata records (reuses ParseGetTagInfoResponse); string handle = uppercase Open2 storage GUID. The 2020 WCF string-handle wall does not apply on the gRPC front door (as predicted). LIVE-VERIFIED 2026-06-21 — GetTagMetadataAsync returned the requested tag + a valid data type.
• ✅ R0.1 Browse over gRPC — DONE, LIVE-VERIFIED 2026-06-21. HistorianClient.BrowseTagNamesAsync routes over gRPC via Grpc/HistorianGrpcTagClient.BrowseTagNamesAsync: StartTagQuery(OData filter) → paged QueryTag (btRequest = u16 0x6752 + u16 1 + u16 queryType + u32 startIndex + u32 count) → EndTagQuery; response = u32 count + per-name(u32 charCount + UTF-16LE) + trailer. The SDK glob filter is translated by GlobToODataFilter (Pre*→startswith, *suf→endswith, *mid*→ contains, exact→eq). The QueryTag packet-id 0x6752 was recovered from a .rdata packet-descriptor table ({0x6751,1}=StartTagQuery, {0x6752,1}=QueryTag) — no Ghidra needed. Golden-byte + glob unit tests + gated live test. Full finding: docs/reverse-engineering/grpc-tag-query-odata.md.

✅ Milestone 0 (gRPC parity) is COMPLETE — probe, system-param, metadata, and browse all run over RemoteGrpc and are live-verified against a real 2023 R2 server, alongside the read chain.

ℹ️ Auth note (2026-06-21, resolved): an apparent NTLM round-1 SEC_E_LOGON_DENIED blocker turned out to be a test-harness credential-parsing bug, not a server/account/SDK issue — the gitignored creds file stores quoted values ("nam\user", "pass"), and the env-setup must strip surrounding quotes before exporting HISTORIAN_USER/HISTORIAN_PASSWORD. With quotes stripped, the domain account authenticates and the full read + system-param + probe chain passes live. The round-failure diagnostic added during the hunt is kept (HistorianNativeHandshake.DescribeError decodes the native error + hex/ASCII preview).

⚠️ Live-verification constraint: the local Historian is 2020 (WCF, port 32568) — the 2023 R2 gRPC endpoint (32565) is absent. M0's gRPC routing (R0.1–R0.4) can be built and golden-byte/unit-tested here but cannot be live-verified without an actual 2023 R2 server. Treat gRPC ops as unverified until then; the byte payloads remain the proven 2020 protocol.

🔬 M1a re-classification (2026-06-20). Two "trivial" items were live-probed against the 2020 WCF server and found not deliverable here, both for evidence-backed reasons:

• R1.3 GetServerTimeZoneAsync — Status.GetSystemTimeZoneName is a client-side stub on 2020 (rc=0, empty value), same family as GetServerTime. gRPC/2023R2-only.
• R1.1 ExecuteSqlCommandAsync — ExeC returns native error 51 (InvalidParameter); the contract-3 string-handle ops require an unmapped native session/filter registration step (the StartTagQuery wall).

Takeaway: the M1a "cheap surface" is cheap only on the 2023 R2 gRPC front door. On 2020 WCF the boundary is the handle type (see the string-handle wall note under §1b and docs/reverse-engineering/wcf-string-handle-wall.md): uint-handle ops work, string-handle ops are blocked. GETHI/GetTepByNm were probed and confirmed blocked (not, as first guessed, reachable). The reachable uint-handle items are now DONE: R1.8/R1.9 StartQuery summary/state modes (resolved = existing ReadAggregateAsync) and R1.7 event filters (✅ 2026-06-20 — ReadEventsAsync(…, HistorianEventFilter), live-honored). M2 event send is also done (✅ WCF AddS2). R1.2 GetRuntimeParameterAsync is also done (✅ 2026-06-20, aa/Stat/GETRP, live-verified) — notably a string-handle op that punches through the wall using the Open2 storage-session GUID as an uppercase string handle, which proved the GETHI/ExeC failures were a handle-format issue rather than a missing native registration. Follow-up done: R1.1 ExecuteSqlCommandAsync shipped; R1.5 extended-property read shipped (R1.6 collapsed into it — no distinct localized op). R1.4 GetHistorianInfo bounded out on 2020 WCF — GETHI there is a named-value query (only HistorianVersion); EventStorageMode is 2023R2-gRPC-only (see wcf-historian-info.md). Net: the reachable 2020-WCF M1 read surface is complete; what remains is config writes (M1c — gated on an explicit user request) and the gRPC/2023R2-only items (R1.3 timezone, R1.4 EventStorageMode — need a live 2023 R2 server).

Update 2026-06-21 (live 2023 R2 gRPC probe — both closed): R1.3 SHIPPED on gRPC — GetServerTimeZoneAsync returns a real zone ("Eastern Daylight Time") via StatusService.GetSystemTimeZoneName; non-gRPC path fails closed (ProtocolEvidenceMissingException). R1.4 bounded out on gRPC too — GetHistorianInfo is named-value-only on the gRPC wire as well, EventStorageMode resolves under no name on either GetHistorianInfo or GetSystemParameter, and the 518-byte struct is C++-HCAL-internal (filled via native vtable+648, not the gRPC op). So no gRPC/2023R2-specific reads remain open — the entire M1 read surface (2020 WCF + 2023 R2 gRPC) is now closed.

Guiding principles

1. gRPC-first. New ops go on the RemoteGrpc transport (clean protobuf envelope); the inner bytes blob is the only thing to RE. Keep WCF as the legacy/Windows path.
2. Two tests per op, always. A golden-byte test (deterministic, no server) and a gated live test (HISTORIAN_GRPC_HOST / HISTORIAN_HOST). No op is "done" without both.
3. Version-pin, fail closed. Read server version at connect; gate every byte serializer on it; throw ProtocolEvidenceMissingException on mismatch — never best-effort parse.
4. Capture once, encode forever. For CAPTURE-tier items, instrument one native call, save a sanitized fixture under fixtures/protocol/, then implement against the fixture.
5. Ship per milestone. Each milestone is independently releasable.

Effort: S ≈ days · M ≈ ~1 week · L ≈ weeks. Estimates are incremental on histsdk's existing infra (auth chain, transport, frame primitives, test harness).

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Milestone 0 — Foundation: full gRPC parity for the DONE surface (M)

Goal: everything already working over WCF also works over RemoteGrpc, so the whole read/browse/status surface is Windows-free and the gRPC stack is the default path.

ID	Work	gRPC op	Files	Verify	Effort
R0.1	Route browse over gRPC	Retrieval.StartTagQuery/QueryTag or GetTagInfosFromName	Grpc/HistorianGrpcReadOrchestrator (+ new …GrpcBrowseClient), Historian2020ProtocolDialect	browse tags live over gRPC	S
R0.2	Route tag metadata over gRPC	Retrieval.GetTagInfosFromName	dialect + grpc client	metadata matches WCF result	S
R0.3	Route status/system-param over gRPC	Status.GetSystemParameter, Status.GetHistorianConsoleStatus	new Grpc/HistorianGrpcStatusClient	system param + conn status live	S
R0.4	Probe over gRPC	*.GetInterfaceVersion	grpc clients	ProbeAsync Windows-free	XS
R0.5	Capture harness for gRPC payloads	n/a	reuse instrument-wcf-* tooling (same byte blobs) + add a grpc-call-dump helper	dump any request/response bytes to a fixture	S
R0.6	Version gate	server version at connect	HistorianClientOptions, orchestrators	mismatched version → throws	S

Acceptance: the entire Phase-0 capability set runs end-to-end over RemoteGrpc (incl. Linux), no WCF on the path. 188+ unit tests green; live gRPC integration suite green.

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Milestone 1 — Cheap surface completion (TRIVIAL/BOUNDED) (M–L total)

Goal: knock out the remaining read/config surface. Order = ascending payload difficulty.

1a. Trivial (XS–S each, no new payload format)

ID	Capability	gRPC op	Notes
R1.1	ExecuteSqlCommandAsync	Retrieval.ExecuteSqlCommand (ExeC+GetR)	✅ DONE (2026-06-20), live-verified. ExecuteSqlCommandAsync(sql) → HistorianSqlResult (columns + typed rows). String-handle op via the uppercase storage GUID. Chain: Retr.GetV prime → ExeC(handle, sql, option=0, ref queryHandle) → GetR loop (note: GetR returns false even on success — the stream is in pResultBuff regardless; false = final page). GetR's pResultBuff is an NRBF-serialized DataTable (SerializationFormat.Xml: members XmlSchema + XmlDiffGram). BinaryFormatter is gone from .NET 10, so it's decoded read-only with System.Formats.Nrbf + XDocument (no BinaryFormatter). Shipped: HistorianSqlResult/HistorianSqlColumn/HistorianSqlExecuteOption, HistorianSqlResultProtocol, HistorianWcfSqlClient, golden WcfSqlResultProtocolTests, gated live tests. See docs/reverse-engineering/wcf-exec-sql.md.
R1.2	GetRuntimeParameterAsync	Status.GetRuntimeParameter (aa/Stat/GETRP)	✅ DONE (2026-06-20), live-verified. Captured (scripts/Capture-RuntimeParam.ps1): GETRP is a string-handle op (GETHI's shape), but reachable from the managed client using the Open2 storage-session GUID as an uppercase string handle (ToString("D").ToUpperInvariant()). Returns HistorianVersion = 20,0,000,000 live. pRequestBuff = 54 67 01 00 + uint nameCount + per-name(uint charCount + UTF-16); pResponseBuff = version + uint resultCount + CRetVariant(0x43 VT_BSTR + uint16 len + uint16 charCount + UTF-16). Single string-valued param only (multi-name framing inferred, not captured). Shipped: HistorianClient.GetRuntimeParameterAsync(name); golden WcfRuntimeParameterProtocolTests. Note: GETRP punching through the string-handle wall with the uppercase storage GUID is a strong lead that GETHI/ExeC may be a handle-format issue — see wcf-string-handle-wall.md §Update.
R1.3	GetServerTimeZoneAsync	Status.GetSystemTimeZoneName	✅ DONE on gRPC (2026-06-21), LIVE-VERIFIED against the real 2023 R2 server — returns "Eastern Daylight Time". HistorianClient.GetServerTimeZoneAsync routes over RemoteGrpc (HistorianGrpcStatusClient.GetSystemTimeZoneNameAsync, uiHandle-in/string-out, no buffer). The 2020 WCF op stays a client-side stub (rc=0, empty), so the non-gRPC path throws ProtocolEvidenceMissingException (fail-closed) rather than return an empty string. Golden message-shape + non-gRPC guardrail unit tests + gated live test. (2020-only routes — per-block HistoryBlock.TimeZoneOffset, SQL via R1.1 — remain DST-specific and are not this op.)

✅ String-handle "wall" RESOLVED (2026-06-20) — it was a handle-FORMAT bug. R1.4/R1.5/R1.6 (and R1.1) take a string GUID handle; the earlier "code 1/51 blocked" verdict came from passing the Open2 storage GUID in .NET's default lowercase. Sent uppercase (storageSessionId.ToString("D").ToUpperInvariant()) the same handle works: GETRP (R1.2, shipped), GETHI (R1.4) and ExeC (R1.1) are all live-verified reachable, and R1.5 GetTepByNm is now shipped + live-verified (GetTagExtendedPropertiesAsync). R1.6 has no distinct op (collapses into R1.5). Note: QTB (StartTagQuery) does not punch through — it fails server-side (CMdServer::StartActiveTagnamesQuery over the aahMetadataServer pipe), independent of handle format, so the index-based property/query paths stay blocked here. Full analysis: docs/reverse-engineering/wcf-string-handle-wall.md (RESOLVED banner) and docs/reverse-engineering/wcf-tag-extended-properties.md. R1.8/R1.9 (StartQuery summary/state modes) are uint-handle and were already reachable.

1b. Bounded (decode one bytes payload; S–M each)

ID	Capability	gRPC op	Payload to decode	Depends
R1.4	GetHistorianInfoAsync	Status.GetHistorianInfo (GETHI)	⛔ BOUNDED OUT — now confirmed on the 2023 R2 gRPC front door too (2026-06-21, live-probed). The motivating field EventStorageMode is not on the wire on either transport. Live gRPC probe against the real 2023 R2 server: GetHistorianInfo is a named-value query exactly like 2020 WCF — only HistorianVersion resolves (→ "23,1,000,000" + 02 00 01 00 trailer); EventStorageMode + 7 name variants fail (success=false) on both GetHistorianInfo and GetSystemParameter. The 518-byte HISTORIAN_INFO struct (mode@514) is the C++ HCAL in-memory model (managed HistorianAccess.GetHistorianInfo fills it via a native vtable+648 call, not the gRPC op — verified in the 2023 R2 decompile), derived outside the wire. The only wire-reachable field (version) is already shipped (ProbeAsync/GetSystemParameterAsync/GetRuntimeParameterAsync), so a struct API would be hollow + misleading. Closes the prior "build against a live 2023 R2 server" caveat — done, and there is nothing to ship. See docs/reverse-engineering/wcf-historian-info.md.	uppercase string handle
R1.5	Extended-property read	Retrieval.GetTagExtendedPropertiesFromName (GetTepByNm)	✅ DONE (2026-06-20), live-verified. GetTagExtendedPropertiesAsync(tag) → name/value pairs. String-handle op via the uppercase storage GUID; name-based path (GetTagExtendedPropertiesByName, not the QTB-gated TagQuery path). Request tagNames = uint count + per-name(uint charCount+UTF-16); response = uint tagCount + per-tag(marker + compact-ASCII name + uint propCount + per-prop(marker + compact-ASCII name + 0x43 VT_BSTR value) + trailer). Sequence-paged. Shipped: HistorianTagExtendedPropertyProtocol, golden WcfTagExtendedPropertyProtocolTests, gated live test. See docs/reverse-engineering/wcf-tag-extended-properties.md.	uppercase string handle
R1.6	Localized-property read	(no op)	⛔ No distinct op on 2020 — collapses into R1.5. There is no GetTagLocalizedPropertiesFromName/GetTlpByNm or GetTagLocalizedPropertiesByName in current/aahClientManaged.dll; the only "localized" surfaces are error-message/UI-text localization. Extended properties (R1.5) are the user-defined tag-property read surface. Closed, not throwing.	—
R1.7	Event filters	filter bytes in Retrieval.StartEventQuery	✅ DONE (2026-06-20), live-honored. ReadEventsAsync(start, end, HistorianEventFilter). The filter rides StartEventQuery's pRequestBuff (captured via EventQuery.AddEventFilter + instrument-wcf-writemessage; Equal vs Contains diffed to isolate the op). Filter block: ushort 0 + uint filterCount + uint condCount + uint nameLen + name(UTF-16) + uint 1 + ushort op + uint 1 + value(0x09-len-0x00 compact-ASCII) + byte 0. REAL, not inert (a non-matching predicate returns 0 events; matching returns the subset). Single string-valued predicate only; multi-filter (OR) / multi-condition (AND via AddEventFilterCondition) framing not yet fully captured. See HistorianEventFilter, golden WcfEventQueryProtocolTests.	—
R1.8	Analog-summary query	Retrieval.StartQuery (summary mode)	summary row layout — uint-handle, reachable. Scoped + decode targets located (CAnalogSummaryValue.UnpackFromValueBuffer, fields Min/Max/First/Last/ValueCount/Integral/…). Plan: r1.8-r1.9-summary-queries.md	—
R1.9	State-summary query	Retrieval.StartQuery (state mode)	state-summary row layout — uint-handle, reachable. Scoped (CStateSummaryStruct: MinContained/MaxContained/TotalContained/PartialStart/PartialEnd/StateEntryCount). Plan: r1.8-r1.9-summary-queries.md	—

1c. Bounded config writes (S–M each)

ID	Capability	gRPC op	Payload	Notes
R1.10	RenameTagsAsync	History rename op	rename request buffer	AllowRenameTags already probed
R1.11	Extended-property write	History.AddTagExtendedProperties (AddTEx)	✅ Add DONE (2026-06-21), live-verified. AddTagExtendedPropertiesAsync/AddTagExtendedPropertyAsync (write mode, uppercase handle). inBuff = exact inverse of the R1.5 read framing (uint32 groupCount + 0x01 + compact-ASCII tag + uint32 propCount + per prop[0x02 + compact-ASCII name + 0x43 VT_BSTR value] + 0x01 trailer + 0x00 terminator); the trailing 0x00 is required or the server throws. Golden WcfTagExtendedPropertyWriteProtocolTests + gated live write/read-back test. Delete (DelTep): wire format CAPTURED + serializer golden-proven (2026-06-21), but live delete is server-blocked and NOT shipped. Captured via a two-session trick (add in Run A → fresh-session read-sync → delete in Run B, past the native err-229 client gate); inBuff = same group framing as Add but property-name-only and a 0x00 group trailer. A decisive experiment shows SDK-added properties ARE deletable (the native client deletes one), so SDK-add is complete; the SDK's own DelTep is rejected (SErrorException in CHistStorage::DeleteTagExtendedProperties) despite matching mode/handle/inBuff + GetTgByNm/GetTepByNm prime + open channel + 60s retries. Root cause: the native multiplexes services over ONE connection (per-connection working set), which the SDK's per-service WCF channels don't reproduce — needs transport-level multiplexing. See docs/reverse-engineering/wcf-add-tag-extended-properties.md §Delete.
R1.12	Localized-property write	(no op)	⛔ No distinct op on 2020 — closed (mirror of R1.6). A symbol sweep of current/*.dll finds no AddTagLocalizedProperties / DeleteTagLocalizedProperties / any *LocalizedPropert* / TagLocalized*; only UI/error-text localization (GetLocalizedText/GetLocalizedMessage/LocalizedResourcesDir). Localized properties are a 2023 R2/gRPC concept. Closed, not throwing. See docs/reverse-engineering/wcf-tag-extended-properties.md §R1.12.	2026-06-21
R1.13	Non-analog tag create (string/discrete)	History.EnsureTags	distinct CTagMetadata variant	⛔ GATED — bounded out (2026-06-21, live-probed). Native AddTag rejects every non-analog type client-side (ErrorCode=ValidationFailed / "Transaction validation failed", before any WCF op): SingleByteString, DoubleByteString, and Int1 all fail; Float (control) succeeds. The native HistorianDataType enum has no Discrete/Boolean and no Int8/UInt8 (SDK-only extensions); HistorianTag has no TagType setter (type is data-type-derived). So no non-analog wire request is ever emitted → nothing to capture/implement. String/discrete create goes via a different subsystem (config editor / SQL), not this client's AddTag. EnsureTagAsync stays analog-only. See docs/reverse-engineering/wcf-non-analog-tag-create.md.

Acceptance: read + browse + metadata + system/status + property R/W + summaries + event-filtered reads + rename all live-verified over gRPC.

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Milestone 2 — Event sending (CAPTURE) (S–M) ← headline gap

Goal: SendEventAsync(HistorianEvent). Path fully mapped in histevents.md; one capture away.

✅ DONE (2026-06-20) — HistorianClient.SendEventAsync(HistorianEvent) shipped and live-accepted over 2020 WCF. The headline assumption — that event delivery would ride the non-WCF storage-engine pipe (and so be blocked like revision writes) — was disproved by capture: a native AddStreamedValue(HistorianEvent) leaves over WCF as AddS2 (IHistoryServiceContract2.AddStreamValues2). CM_EVENT is a built-in registered tag, so the 129 TagNotFoundInCache gate that blocks AddS2 for user tags does not apply to events. The full managed chain (Open2 event-mode 0x501 → CM_EVENT RTag2/EnsT2 → AddS2) is accepted by the server (AddS2 returns success, empty error buffer). See the event-send field map under §"Event-send wire format" in histevents.md and HistorianEventWriteProtocol.

⚠️ Persistence caveat (environment, not SDK): on the local dev Historian, accepted events are not persisted to the queryable store (v_AlarmEventHistory2 latest stays at the pre-test date; count only ages down). The native client exhibits the identical behaviour (its AddS2 also returns success but nothing lands), so this is the box's event-ingestion pipeline not being active — not an SDK protocol gap. The SDK emits byte-equivalent AddS2 (golden-tested). Full send→store→read-back round-trip awaits a Historian with an active event storage pipeline.

ID	Work	Status
R2.1	Capture the event value blob	✅ scripts/Capture-EventSend.ps1 (event-send harness scenario + instrument-wcf-{write,read}message); two captures diffed to separate constant framing from value fields. Decisive finding: event-send = WCF AddS2, not storage pipe.
R2.2	HistorianEventWriteProtocol	✅ Serializes the AddS2 pBuf (storage sample buffer wrapping the event VTQ): "OS" sig + sampleCount + length fields + CM_EVENT tag id + EventTime FILETIME + OpcQuality + opaque descriptor + event Id + ReceivedTime FILETIME + Namespace + EventType + version + typed property bag (string props reuse the read parser's 0x43 encoding). Golden-byte test pins capture A.
R2.3	Event write orchestrator	✅ HistorianWcfEventOrchestrator.SendEventAsync: Open2 (0x501) → reuse CM_EVENT RTag2/EnsT2 registration → AddStreamValues2(handle, pBuf, out err) on the same /Hist channel + storage-session handle.
R2.4	Public API	✅ HistorianClient.SendEventAsync(HistorianEvent). Original events only (RevisionVersion=0) with string-valued properties; other property types + revision/update/delete throw ProtocolEvidenceMissingException until captured.
R2.5	Round-trip test	✅ Golden-byte on R2.2 + gated live test SendEventAsync_AgainstLocalHistorian_AcceptedByServer (asserts server acceptance; SQL read-back best-effort given the persistence caveat).

Acceptance: an event sent from histsdk is accepted by the historian over WCF with a byte-correct AddS2 (✅). Appears-and-reads-back is environment-gated on event persistence (see caveat).

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Milestone 3 — Historical / non-streamed value writes (BOUNDED) (M)

Goal: insert original historical VTQs (backfill), the path that is NOT the gated cache push.

⛔ BLOCKED on 2020 WCF — re-confirmed by the D2 probe (2026-05-05), see revision-write-path.md. The premise above ("the path that is NOT the gated cache push") was disproved: R3.1's op (Transaction.AddNonStreamValuesBegin/AddNonStreamValues/End) is the same ITransactionServiceContract2.AddNonStreamValuesBegin2 D2 probed, and over WCF it returns 04 33 00 00 00 = UnknownClient (51) for every handle format and the full priming chain (Stat/Hist/Retr/Trx GetV + UpdC3 + 6× GetSystemParameter + RTag2). Root cause (IL-walk: CClient.TransactionBegin → CHistStorageConnection.StartTransaction → CStorageEngineConsoleClient.StartTransaction): the real transaction rides a shared-memory + named-pipe channel (STransactPipeClient2 + SCrtMemFile) to aaStorageEngine.exe, separate from WCF. The WCF Trx op is a server-side relay that requires a pre-existing storage-engine pipe session, which no WCF op can establish. So M3 over 2020 WCF is unimplementable as a pure-managed SDK — same architectural wall as R4.2 (revisions) and the AddS2 cache gate.

Only remaining lever: the 2023 R2 gRPC front door (HCAL-native, no legacy storage-engine pipe). Whether the gRPC services expose a non-streamed/revision write that bypasses the pipe is untested — it needs the live 2023 R2 server + a native gRPC capture of the write op, then decode/implement. Treat as on-demand (no current demand signal); the WCF path is closed.

ID	Work	gRPC op	Status
R3.1	Decode non-streamed VTQ packet	Transaction.AddNonStreamValuesBegin/AddNonStreamValues/End	⛔ WCF blocked (storage-engine pipe — D2). gRPC: untested
R3.2	AddHistoricalValuesAsync	batched begin→values→end	⛔ gated on R3.1
R3.3	Ingest-permission validation	confirm the target accepts original-data insert (distinct from AddS2 cache wall)	⛔ proven to share the same gate, not distinct

Acceptance: historical points inserted and read back. WCF path closed (D2); would require the gRPC write path (live 2023 R2 server + capture) to reopen.

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Milestone 4 — HARD subsystems (deferred / optional) (L each)

Only if the use case demands them. Each is a real subsystem, not an op.

ID	Capability	Approach	Risk
R4.1	Store-and-forward	Pragmatic local queue (durable outbox + replay on reconnect) rather than bit-faithful SF cache + Forward*Snapshot. Faithful SF = decode SF cache format + snapshot framing + recovery log	high; consider "good enough"
R4.2	Revision / edit writes	AddRevisionValue(s) go via the non-WCF storage-engine pipe (STransactPipeClient2) — separate transport RE	high
R4.3	Real store-forward status	duplex push (SetStoreForwardEvent) or a decoded pull endpoint — see store-forward plan	medium
R4.4	Multi-historian / redundancy	client-side orchestration over N single-historian sessions (failover, ReSyncTags, partner watchdog) — build last	medium

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Won't-do from the client (GATED)

• Streaming process-sample writes (AddStreamedValue(HistorianDataValue) / AddS2): runtime cache only ingests from configured IOServer/AppServer pipelines. Confirm your ingestion architecture instead of pursuing this.

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Cross-cutting workstreams (run alongside all milestones)

• CW-1 Capture tooling (enables R0.5, R1.x, R2.1): one reusable "call op → dump request/response bytes → sanitized fixture" path. Highest leverage — do first.
• CW-2 Version compatibility: matrix of tested Historian versions; serializers keyed by version; CI gate.
• CW-3 Cross-platform CI: run the gRPC suite on Linux/macOS (transport is portable; explicit-cred auth path).
• CW-4 Fixtures discipline: every new op ships a fixtures/protocol/<op>/ golden file; sanitize hostnames/tags/GUIDs before commit.
• CW-5 Public API shape: keep the modern surface (async, IAsyncEnumerable, cancellation, options record, DI-friendly) consistent as the surface grows.

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Sequencing (critical path)

CW-1 capture tooling ─┐
M0 gRPC parity ───────┼─→ M1 cheap surface ─→ M2 event send ─→ M3 historical writes ─→ (M4 optional)
R0.6 version gate ────┘

Recommended first sprint: CW-1 + M0 (R0.1–R0.6) → a fully Windows-free, version-safe gRPC client at today's capability. Second sprint: M1a + M2 (cheap wins + the headline event-send). M3/M4 as demand dictates.

Status 2026-06-21: sprints 1 + 2 are complete (M0 gRPC parity, the reachable M1 surface, and M2 event-send all shipped + live-verified; remaining M1 items are evidence-bounded-out). The reachable surface on the available 2020 WCF infrastructure is exhausted — every remaining roadmap item is now either (a) blocked by the storage-engine-pipe architecture (M3-WCF, R4.2), (b) gRPC/2023R2-only and needs the live 2023 R2 server for a native capture (R1.3 timezone, R1.4 EventStorageMode, M3/revisions over gRPC), or (c) a HARD deferred subsystem (M4). No further work lands without one of: a live-2023R2 capture session, or a customer-demand trigger.

One-glance status

Milestone	Tier	Effort	Value	When
M0 gRPC parity + capture tooling	foundation	M	unblocks everything, Windows-free	✅ done
M1 cheap surface	TRIVIAL/BOUNDED	M–L	most remaining read/config	✅ done (reachable surface; rest bounded out)
M2 event send	CAPTURE	S–M	headline write capability	✅ done
M3 historical writes	BOUNDED	M	backfill	⛔ WCF blocked (D2); gRPC = on-demand + live 2023R2
M4 SF / revisions / redundancy	HARD	L×N	parity completeness	defer (R4.2 = same pipe wall)