histsdk/docs/reverse-engineering/event-session-reuse-spike-results.md

# Event-session reuse spike — live results

> **Question:** does the 2023 R2 historian honor REUSING one authenticated **v8 Event**
> session (ECDH `ExchangeKey` → RC4 token → `ConnectionType=Event`, then `RegisterCmEventTag`)
> across multiple `SendEvent` ops, instead of the per-op open+register the SDK does today?
> This is the precondition for amortizing the EVENT path (HistorianGateway `pending.md` A1
> broadening, Stage B0 / B1).
>
> **Verdict: GREEN — a v8 Event session reuses across sends, register-once is sufficient,
> and the amortization is ~10–16×. Event READS stay gated (C2) and are not a reuse signal.**

**Date:** 2026-06-25
**Branch:** `feat/amortization-broadening`
**Server:** live 2023 R2 (`wonder-sql-vd03`), RemoteGrpc transport.
**Sandbox identity:** `HISTORIAN_EVENT_SANDBOX_TAG=HistGW.LiveTest.EventSpike` — the CM_EVENT send
buffer has **no per-tag routing field** (it registers against a fixed system tag), so the sandbox
value is stamped into the event `Type`/`SourceName`/`Namespace` + a `SpikeMarker` property as an
**identity marker**; no real tag is written or overwritten.
**Harness:** `tests/AVEVA.Historian.Client.Tests/EventSessionReuseSpikeTests.cs` driving the B0a
seams `HistorianGrpcEventWriteOrchestrator.OpenAndRegisterEventSession` (open v8 Event session +
`RegisterCmEventTag` ONCE) and `SendEventOnSession` (send only — no open/register).

---

## 1. Send reuse — GREEN

`ReusedEventSession_SendsTwice_SecondSkipsHandshake` **passed** (both runs): one
`OpenAndRegisterEventSession` then **two `SendEventOnSession` on the same v8 Event session** — both
accepted (`AddStreamValues` `BSuccess=true`).

```
open+register (ECDH handshake + RegisterCmEventTag) = 242 ms   (run 1: 350 ms)
registration diag: RTag=True; EnsT=True
reused-send[0] = 23 ms, ok=True
reused-send[1] = 22 ms, ok=True
```

The server accepts the same v8 Event client handle across back-to-back sends. The session handle is
an immutable `readonly record struct (uint ClientHandle, Guid StorageSessionId)`; the send is
stateless on the client side (each call reserializes a fresh `"OS"` buffer), so nothing per-op is
baked into the handle.

## 2. Amortization — ~10–16×

The open+register (P-256 ECDH `ExchangeKey` → RC4 credential token → v8 `OpenConnection` →
`RegisterCmEventTag`) costs ~242–350 ms and is paid **once**; a reused send is ~22 ms. So over a
burst of N sends the per-send cost collapses from ~(265 ms open + 22 ms) to ~22 ms — a ~10–16× win
on the send path, same shape as the v6 read/write amortization (`handshake-reuse-spike-results.md`).

## 3. Register-once is sufficient — GREEN

`ReusedEventSession_RegisterOnce_ThenSendMany` **passed**: `RegisterCmEventTag` run **once** (inside
`OpenAndRegisterEventSession`), then **three** sends, all accepted.

```
register-once send[0] = 25 ms, ok=True
register-once send[1] = 22 ms, ok=True
register-once send[2] = 22 ms, ok=True
```

CM_EVENT registration is **session-scoped, not per-send** — the server holds the registration for the
session's lifetime. A reuse pool registers once per warm session, not per op.

## 4. Idle tolerance — survived ≥25 s (best-effort, single sample)

`ReusedEventSession_IdleSweep_BestEffort` (log-only): after a send, a **25 s idle gap**, then another
send — **the second send succeeded** (`session SURVIVED the idle gap`). Notable: the v6 read session
idle-expires at a ≥25 s gap (`handshake-reuse-spike-results.md` §3), but this v8 Event session
survived 25 s. This is a single-sample best-effort observation — a keepalive should still ping under
the ~20 s floor for safety margin until the v8 Event idle boundary is characterized more finely.

## 5. Read-after-send — GATED (C2), not a reuse signal

`ReusedEventSession_ServesReadAfterSend_BestEffort` (log-only, hard-bounded by a 5 s gRPC deadline +
an 8 s cancellation): the read-after-send on the same session **did not return data** — it cancels at
the bound:

```
read-after-send -> swallowed (RpcException Cancelled / OperationCanceled)
 => read gated/unverified over gRPC (expected)
```

This matches the pre-existing C2 gate: event **reads** over gRPC long-poll `GetNext` to a no-data
terminal and are unverified. So the spike did **not** prove a one-session-serves-both-kinds property
for reads — `SendEvent` is the only trustworthy reuse signal. (An unbounded read hung the first run;
the harness now bounds it so the spike is a clean, re-runnable record.)

---

## 6. Implications for Stage B1 (the event-pool build)

GREEN → a **separate event-session pool** (the approved B1 approach) is warranted and high-value:

1. **Amortize `SendEvent` through a bounded event-session pool.** Open+register a v8 Event session
   once per warm session; lease it per send op (exclusive, like the v6 pool); reuse across a burst.
   ~10–16× on the send path.
2. **Keep the event pool SEPARATE from the v6 pool** (B1, as approved) — different auth (ECDH/v8),
   heavier re-handshake on drop, and its own idle characteristics.
3. **`ReadEvents` stays PER-CALL / gated (C2).** Reads are unverified over gRPC regardless of reuse,
   so the event pool amortizes **sends only**; `ReadEvents` is unaffected by B1 and stays on the
   per-call path. (This refines the design's "route SendEvent + ReadEvents through the pool": only
   `SendEvent` is routed; `ReadEvents` remains per-call because it is gated, not because of reuse.)
4. **Keepalive:** ping the warm event session under the idle floor. The cheap keepalive op for the
   event channel is TBD in B1 (the v6 pool uses `GetSystemParameter`; the event session's equivalent
   warm-touch needs picking — likely a no-op send or a lightweight event-channel status op).
5. **Reactive re-auth:** on an expiry-looking failure, evict + full v8 re-handshake (heavier than the
   v6 re-auth — one ECDH + register penalty).

**Gate decision: GREEN → HistorianGateway A1 Stage B1 (a bounded `HistorianEventSessionPool` for
`SendEvent`, default-on, parallel to the v6 `HistorianSessionPool`) is warranted and earns its own
re-planned design + plan.**