mxaccessgw/docs/DesignDecisions.md

Design Decisions

This document records current v1 choices for the MXAccess gateway design. These decisions can change, but implementation should follow them until a later design update says otherwise.

Source References

Use these local analysis sources when answering MXAccess-specific design or implementation questions:

C:\Users\dohertj2\Desktop\mxaccess
C:\Users\dohertj2\Desktop\mxaccess\docs\MXAccess-Public-API.md
C:\Users\dohertj2\Desktop\mxaccess\docs\MXAccess-Reverse-Engineering.md

Use these local notes for Galaxy Repository SQL metadata:

C:\Users\dohertj2\Desktop\lmxopcua\gr

MXAccess COM Target

Decision: target the installed MXAccess COM interop surface directly from the x86 worker.

Concrete COM details from the MXAccess analysis:

• Interop assembly: C:\Program Files (x86)\ArchestrA\Framework\Bin\ArchestrA.MXAccess.dll
• Assembly identity: ArchestrA.MxAccess, Version=3.2.0.0, PublicKeyToken=23106a86e706d0ae
• COM class: ArchestrA.MxAccess.LMXProxyServerClass
• CLSID: {C30B52F5-2CB5-4760-AF0A-3A344A7EB5DC}
• ProgID: LMXProxy.LMXProxyServer.1
• Version-independent ProgID: LMXProxy.LMXProxyServer
• Registered server: C:\Program Files (x86)\ArchestrA\Framework\Bin\LmxProxy.dll
• Registry view: HKCR\Wow6432Node\CLSID\{C30B52F5-2CB5-4760-AF0A-3A344A7EB5DC}
• Threading model: Apartment

Rationale: LMXProxyServer is a 32-bit in-process COM server, so a .NET 10 x64 gateway cannot instantiate it directly. The x86 sidecar worker is the reliable parity path.

Implementation guidance:

• Worker should reference ArchestrA.MXAccess.dll.
• Worker should instantiate new LMXProxyServerClass() on the dedicated STA.
• Worker should expose the resolved class, ProgID, CLSID, interop assembly version, and LmxProxy.dll path through GetWorkerInfo / WorkerReady.
• Keep the ProgID/path configurable for diagnostics, but the default should be the installed MXAccess class above.

Session Reconnect

Decision: no reconnectable sessions for v1.

One OpenSession creates one gateway session and one worker process. The session ends on CloseSession, client disconnect policy, lease expiry, worker fault, or gateway shutdown.

Rationale: reconnectable sessions require event replay, orphan ownership, security checks, and more complicated worker lifetime rules. They are not needed for the first parity slice.

Event Subscribers

Decision: one active StreamEvents subscriber per session for v1.

A second subscriber should be rejected with a clear session error. Multi-client fan-out may be added later with explicit backpressure semantics.

Rationale: one subscriber preserves simple event ordering and failure behavior while parity is being proven.

Alarms — superseded for the alarm subsystem

The single-subscriber rule above no longer applies to alarms. The gateway runs an always-on central alarm monitor (GatewayAlarmMonitor) that owns one gateway-managed worker session, caches the active-alarm set, and fans it out to any number of clients through the session-less StreamAlarms RPC. Per-session alarm auto-subscribe is removed; AcknowledgeAlarm is session-less and routes through the monitor. Data-side StreamEvents remains one subscriber per session. Rationale: alarm state is gateway-wide, not session-scoped — every client wants the same current set plus updates, and forcing each to own a worker would multiply AVEVA polling load for no benefit.

Authentication

Decision: API key authentication for the public gateway.

API keys are stored in a gateway-owned SQLite database. Store hashed API key secrets only; never store raw key material.

Recommended client format:

authorization: Bearer mxgw_<key-id>_<secret>

Recommended SQLite tables:

CREATE TABLE api_keys (
    key_id TEXT PRIMARY KEY,
    key_prefix TEXT NOT NULL,
    secret_hash BLOB NOT NULL,
    display_name TEXT NOT NULL,
    scopes TEXT NOT NULL,
    created_utc TEXT NOT NULL,
    last_used_utc TEXT NULL,
    revoked_utc TEXT NULL
);

CREATE TABLE api_key_audit (
    audit_id INTEGER PRIMARY KEY AUTOINCREMENT,
    key_id TEXT NULL,
    event_type TEXT NOT NULL,
    remote_address TEXT NULL,
    created_utc TEXT NOT NULL,
    details TEXT NULL
);

Recommended scopes:

• session:open
• session:close
• invoke:read
• invoke:write
• invoke:secure
• events:read
• metadata:read
• admin

Hashing recommendation:

• Use HMAC-SHA256 with a gateway-local secret/pepper stored outside SQLite, or use Argon2id if a suitable dependency is already accepted.
• Compare hashes using constant-time comparison.
• Log only the key id or prefix, not the raw key.

Storage recommendation:

• Default SQLite path should be under ProgramData or another configured gateway data directory.
• Apply restrictive filesystem ACLs for the gateway service identity and administrators.
• Require TLS when the gateway is reachable off-machine.

Authorization

Decision: start with scope checks by command category.

Suggested mapping:

• OpenSession: session:open
• CloseSession: session:close
• Register, Unregister, AddItem, AddItem2, RemoveItem, Advise, UnAdvise, AdviseSupervisory, AddBufferedItem, SetBufferedUpdateInterval, Suspend, Activate: invoke:read
• Write, Write2: invoke:write
• WriteSecured, WriteSecured2, AuthenticateUser, ArchestrAUserToId: invoke:secure
• StreamEvents: events:read
• Galaxy SQL metadata endpoints if added: metadata:read
• worker shutdown diagnostics and key management: admin

Worker Process Identity

Decision: run workers as the gateway service identity for v1.

Rationale: this avoids early COM/DCOM permission failures and keeps the first implementation focused on MXAccess parity. The worker launcher should keep an extension point for a restricted service account later.

Event Backpressure

Decision: fail-fast bounded queues for v1 and parity testing.

If worker or gateway event queues fill, fault the session. Do not silently drop or coalesce events in parity mode.

Rationale: event drops would hide parity defects. Production coalescing by item handle can be added later as an explicit opt-in mode once event rates are measured.

Event-Rate Target

Decision: do not set a production event-rate target before measurement.

For v1, expose queue depth, event rate, stream send latency, and overflow metrics. Keep bounded queues and fail-fast behavior. Use observed load from live systems to set a later coalescing or scaling target.

Command Batching

Decision: no public command batching for v1.

Use one command per request so replies, HRESULTs, status arrays, event ordering, and failure behavior are easy to compare against direct MXAccess.

Batch tag registration can be added later if measured setup latency requires it.

Bulk Command Family

Decision: the gateway exposes a fixed set of bulk command kinds — AddItemBulk, AdviseItemBulk, RemoveItemBulk, UnAdviseItemBulk, SubscribeBulk, UnsubscribeBulk, WriteBulk, Write2Bulk, WriteSecuredBulk, WriteSecured2Bulk, ReadBulk — that carry a list of entries in one round-trip and return one per-entry result. Each command kind runs the corresponding single-item MXAccess COM call sequentially on the worker STA; per-entry failures populate was_successful = false with the underlying HRESULT and never throw. There is no transactional / fail-fast semantic — bulk here means "one round-trip, per-entry results", not "atomic".

Rationale: MXAccess COM itself has no native bulk API for any of these operations. Surfacing the per-entry result list keeps parity transparent — the caller sees the same per-item HRESULT they would see calling MXAccess N times directly — while the bulk shape collapses the gateway/IPC overhead to one round-trip per batch and lets the worker keep the STA hot.

ReadBulk is the only bulk command without a 1:1 MXAccess analogue. Two choices were considered:

1. Cache-then-snapshot (chosen): when a requested tag is already in the session's item registry AND advised, the worker returns the last cached OnDataChange value without touching the subscription (was_cached = true). Otherwise it takes the full AddItem + Advise + wait-for-first-OnDataChange + UnAdvise + RemoveItem lifecycle itself (was_cached = false) and leaves the session exactly as it was before the call. The cache lives on a per-session MxAccessValueCache, populated by MxAccessBaseEventSink on every OnDataChange after the event clears the outbound queue.

2. Always-snapshot: take the AddItem-through-RemoveItem lifecycle for every requested tag. Cleaner conceptually but pays the full lifecycle cost on every call and would interfere with existing subscriptions if MXAccess reuses item handles.

The chosen behavior matches what callers actually want from "current value" — a free read of an already-streaming tag, and a one-shot snapshot otherwise — and never disturbs subscriptions the caller did not create. The decision intentionally does NOT synthesize an OnDataChange event from the snapshot path: the snapshot value reaches the caller through ReadBulk's reply payload only, not through the event stream. This preserves the "Don't synthesize events" rule that scopes the rest of the worker.

ReadBulk's wait loop pumps Windows messages on the worker STA (StaRuntime.PumpPendingMessages) on every poll iteration so the inbound MXAccess COM event can dispatch while the bulk executor still holds the thread — without the pump the OnDataChange would never deliver.

Graceful Worker Shutdown

Decision: best-effort cleanup before COM release.

During graceful shutdown, the worker should attempt:

1. UnAdvise for advised items.
2. RemoveItem for active item handles.
3. Unregister for active server handles.
4. Event detach.
5. COM release.

Failures during cleanup should be logged and preserved diagnostically, but the gateway may still kill the worker after shutdown timeout.

OperationComplete

Decision: model and forward OperationComplete only when native MXAccess fires it. Do not synthesize OperationComplete from writes, command replies, ASB completion queues, or other status frames.

Rationale: the event signature is known, but the MXAccess analysis has not yet captured the runtime condition that triggers the public event. Synthesizing it would risk breaking parity.

Buffered Data Change

Decision: include OnBufferedDataChange in the protocol and worker event model, but treat multi-sample payload conversion as capture-validated work.

The event signature and native path are known. A live buffered sample batch has not yet been observed. Until then, preserve raw value, quality, timestamp, data type, and status metadata whenever conversion is incomplete.

Completion-Only Status Mapping

Decision: preserve completion-only operation-status bytes as raw diagnostic metadata unless native MXAccess raises a public event or the MXAccess analysis proves an exact MXSTATUS_PROXY[] mapping.

Do not guess status category/source/detail values for frames that MXAccess does not expose through its public COM events.

API Key Administration

Decision: v1 API key management is a local administrative CLI/tool, not a public admin API.

The tool should support:

• initialize auth database,
• create key,
• list keys without showing secrets,
• revoke key,
• rotate key,
• print the raw secret exactly once at creation.

Public gRPC key-management endpoints can be added later only behind admin scope and TLS.

SQLite Migrations

Decision: use simple startup migrations with a schema_version table.

Recommended table:

CREATE TABLE schema_version (
    id INTEGER PRIMARY KEY CHECK (id = 1),
    version INTEGER NOT NULL,
    applied_utc TEXT NOT NULL
);

Migrations should be idempotent, run inside transactions, and fail gateway startup if the database is newer than the running binary understands.

Web Dashboard

Decision: host a basic gateway dashboard with Blazor Server and Bootstrap CSS/JS.

The dashboard should show gateway health, active sessions, worker instances, basic metrics, queue depths, and recent faults. It should update in real time through Blazor Server component updates.

Allowed UI stack:

• Blazor Server,
• Bootstrap CSS,
• Bootstrap JavaScript,
• small local CSS.

Do not use MudBlazor or other Blazor UI component libraries for v1.

Dashboard access should require API-key-backed dashboard authentication with admin scope when enabled. For local development, anonymous localhost access is enabled by default through Dashboard:AllowAnonymousLocalhost; the bypass is limited to loopback requests.

Later Revisit Items

These are explicit post-v1 revisit items, not open blockers:

• reconnectable sessions,
• multiple event subscribers per session,
• restricted worker service account,
• production coalescing by item handle,
• command batching for high-volume tag setup.

Related Documentation

• Gateway Process Detailed Design
• MXAccess Worker Instance Detailed Design
• Authentication
• Authorization
• Galaxy Repository