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`](hcal-capability-matrix.md); event details in
[`histevents.md`](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).

## 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).

---

## 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.

---

## 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` | ⛔ **gRPC/2023R2-only — re-confirmed 2026-06-21 from 3 angles.** (1) native `GetSystemTimeZoneName` is a stub (rc=0, empty) in the `GetServerTime` family; (2) Runtime DB has no timezone `SystemParameter` — the zone exists only as per-block `HistoryBlock.TimeZoneOffset`/`wwTimeZone` (DST-specific, SQL-only) + a `TimeZone` lookup table, `StorageShard.TimeZoneId`=NULL; (3) `GetSystemParameter` + `GETRP` return null/throw for every timezone candidate (only `HistorianVersion` works). The sole 2020 route is a SQL read via `ExecuteSqlCommand` (R1.1) — DST-specific, different mechanism. Build the real op only against a live 2023 R2 server. See `docs/reverse-engineering/wcf-status-localhost.md`. |

> ✅ **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 on 2020 (re-confirmed 2026-06-21).** GETHI is a **named-value** query reachable via the uppercase storage GUID, but **only `HistorianVersion` resolves** — the full 518-byte `HISTORIAN_INFO` struct (incl. `EventStorageMode`@514) is the 2023R2 HCAL-native/gRPC model. `EventStorageMode` has **no 2020 representation at all**: not a `SystemParameter` (only `EventStorageDuration`/`EventStorageLogPath`), not a DB column, and `GETRP`/`GetSystemParameter` return null/throw for it. The only 2020-reachable field (version) is already shipped via `GetSystemParameterAsync`/`GetRuntimeParameterAsync`, so a struct API would be hollow + misleading. Build the real op only against a live 2023 R2 server. See `docs/reverse-engineering/wcf-status-localhost.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`](r1.8-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`](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.

---

## 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).

---

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

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

| ID | Work | gRPC op |
|---|---|---|
| R3.1 | Decode non-streamed VTQ packet | `Transaction.AddNonStreamValuesBegin/AddNonStreamValues/End` |
| R3.2 | `AddHistoricalValuesAsync` | batched begin→values→end |
| R3.3 | Ingest-permission validation | confirm the target accepts original-data insert (distinct from `AddS2` cache wall) |

**Acceptance:** historical points inserted and read back. Document clearly where this
differs from (gated) streaming sample writes.

---

## 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 |

---

## 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.

---

## 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.

---

## 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.

## One-glance status

| Milestone | Tier | Effort | Value | When |
|---|---|---|---|---|
| M0 gRPC parity + capture tooling | foundation | M | unblocks everything, Windows-free | **now** |
| M1 cheap surface | TRIVIAL/BOUNDED | M–L | most remaining read/config | next |
| M2 event send | CAPTURE | S–M | headline write capability | next |
| M3 historical writes | BOUNDED | M | backfill | on demand |
| M4 SF / revisions / redundancy | HARD | L×N | parity completeness | defer |