# \# AquaCom Integration Gateway – Architecture

# 

# \## 1. Purpose

# 

# This document defines the high-level software architecture of the \*\*AquaCom Integration Gateway\*\*.

# 

# It describes:

# 

# \- major system components;

# \- dependency direction;

# \- data flows;

# \- architectural boundaries;

# \- live-data handling;

# \- historical-data handling;

# \- OPC UA integration;

# \- storage integration;

# \- diagnostics;

# \- security boundaries;

# \- extension points.

# 

# Detailed functional requirements are defined in:

# 

# ```text

# docs/product/software-specification.md

# ```

# 

# Implementation requirements and Codex-specific development rules are defined in:

# 

# ```text

# docs/product/codex-implementation-specification.md

# AGENTS.md

# ```

# 

# Detailed subsystem architecture is documented under:

# 

# ```text

# docs/architecture/

# ```

# 

# AquaCom-specific protocol knowledge is documented under:

# 

# ```text

# docs/protocol/

# ```

# 

# \---

# 

# \# 2. System purpose

# 

# The AquaCom Integration Gateway provides a modern integration boundary between AquaCom-compatible field equipment and higher-level systems.

# 

# The gateway shall:

# 

# \- communicate with AquaCom field devices;

# \- acquire current process values;

# \- retrieve historical process values;

# \- normalize AquaCom-specific information into a protocol-independent model;

# \- expose current values using OPC UA;

# \- expose historical values using OPC UA Historical Access;

# \- store historical data in one or more configurable destinations;

# \- recover historical information following communication interruptions;

# \- provide operational and protocol diagnostics.

# 

# The gateway shall not make higher-level systems dependent on AquaCom-specific behavior.

# 

# \---

# 

# \# 3. Architectural principle

# 

# The central architecture is:

# 

# ```text

# AquaCom field device

#         │

#         ▼

# AquaCom Transport

#         │

#         ▼

# AquaCom Protocol Engine

#         │

#         ▼

# Station / Session Manager

#         │

#         ▼

# Canonical Data Model

#         │

#         ├─────────────────────► Current-State Service

#         │                              │

#         │                              ▼

#         │                        OPC UA Data Access

#         │

#         └─────────────────────► Historical Pipeline

#                                        │

#                            ┌───────────┼────────────┐

#                            │           │            │

#                            ▼           ▼            ▼

#                       Repository    Storage A    Storage B

#                            │

#                            ▼

#                    OPC UA Historical Access

# ```

# 

# The canonical data model forms the boundary between AquaCom-specific functionality and all northbound integrations.

# 

# \---

# 

# \# 4. Architectural goals

# 

# The architecture shall support:

# 

# \- protocol isolation;

# \- storage independence;

# \- stable internal identifiers;

# \- stable OPC UA NodeIds;

# \- correct handling of source timestamps;

# \- reliable history recovery;

# \- multiple historical storage destinations;

# \- store-and-forward;

# \- independent subsystem testing;

# \- restart recovery;

# \- future replacement of individual technologies;

# \- future addition of other northbound interfaces.

# 

# The architecture shall avoid tight coupling between:

# 

# ```text

# AquaCom

# OPC UA

# Database technology

# Historian technology

# SCADA platform

# ```

# 

# \---

# 

# \# 5. Architectural boundaries

# 

# The following boundaries are mandatory.

# 

# \## 5.1 AquaCom boundary

# 

# Only the AquaCom subsystem may:

# 

# \- encode AquaCom telegrams;

# \- decode AquaCom telegrams;

# \- calculate or verify AquaCom checksums;

# \- interpret AquaCom operation codes;

# \- interpret AquaCom addressing;

# \- interpret AquaCom-specific value formats.

# 

# No OPC UA or storage module shall perform AquaCom protocol processing.

# 

# \---

# 

# \## 5.2 Canonical model boundary

# 

# AquaCom data shall be converted into canonical gateway objects before being forwarded to:

# 

# \- current-state handling;

# \- history handling;

# \- OPC UA;

# \- storage systems;

# \- diagnostics where appropriate.

# 

# The canonical model shall remain independent of AquaCom and OPC UA.

# 

# \---

# 

# \## 5.3 OPC UA boundary

# 

# The OPC UA subsystem shall consume canonical gateway information.

# 

# It shall not:

# 

# \- communicate directly with AquaCom stations;

# \- decode telegrams;

# \- synchronously request historical data from an RTU;

# \- depend upon a particular storage technology.

# 

# \---

# 

# \## 5.4 Storage boundary

# 

# Storage destinations shall receive canonical historical records.

# 

# Storage implementations shall not depend upon:

# 

# \- AquaCom telegrams;

# \- AquaCom transport;

# \- OPC UA server internals.

# 

# \---

# 

# \# 6. Source tree

# 

# The logical source structure is:

# 

# ```text

# src/

# ├── core/

# ├── aquacom/

# ├── history/

# ├── storage/

# ├── opcua/

# ├── diagnostics/

# └── host/

# ```

# 

# The exact language-specific package or project structure may differ, but the architectural responsibilities shall remain equivalent.

# 

# \---

# 

# \# 7. Dependency direction

# 

# Allowed dependencies shall broadly follow:

# 

# ```text

#                  host

#                   │

#       ┌───────────┼─────────────┐

#       │           │             │

#       ▼           ▼             ▼

#    opcua       history      diagnostics

#       │           │

#       │        storage

#       │           │

#       └──────┬────┘

#              ▼

#             core

#              ▲

#              │

#           aquacom

# ```

# 

# The most important rule is:

# 

# ```text

# core

# ```

# 

# shall not depend upon:

# 

# ```text

# aquacom

# opcua

# storage technology

# ```

# 

# \---

# 

# \# 8. Core subsystem

# 

# The `core` subsystem contains protocol-independent domain concepts.

# 

# Examples include:

# 

# ```text

# StationId

# PointId

# CurrentValue

# HistoricalSample

# DataQuality

# AcquisitionSource

# CommunicationState

# StationState

# HistorySynchronizationState

# ```

# 

# The core subsystem defines shared contracts used between other modules.

# 

# It shall contain no AquaCom protocol logic.

# 

# It shall contain no OPC UA-specific types.

# 

# It shall contain no database-specific types.

# 

# \---

# 

# \# 9. AquaCom subsystem

# 

# The AquaCom subsystem contains:

# 

# ```text

# Transport

# Protocol codec

# Telegram validation

# Station/session handling

# Live acquisition

# Historical acquisition

# Protocol diagnostics

# ```

# 

# Logical structure:

# 

# ```text

# AquaCom Transport

#         │

#         ▼

# Protocol Codec

#         │

#         ▼

# Protocol Operations

#         │

#         ▼

# Station Session

#         │

#         ├── Live Acquisition

#         └── Historical Acquisition

# ```

# 

# Protocol implementation shall be evidence-based.

# 

# Undocumented AquaCom behavior shall not be guessed.

# 

# \---

# 

# \# 10. AquaCom transport

# 

# The transport layer is responsible for communication only.

# 

# It may support implementations such as:

# 

# ```text

# TCP

# Serial

# Serial-over-IP

# Other supported AquaCom transport

# ```

# 

# The transport layer shall not interpret process values.

# 

# Conceptual interface:

# 

# ```text

# open

# close

# send

# receive

# health

# ```

# 

# \---

# 

# \# 11. AquaCom protocol engine

# 

# The protocol engine is responsible for:

# 

# \- framing;

# \- telegram validation;

# \- checksum handling;

# \- request encoding;

# \- response decoding;

# \- protocol error interpretation.

# 

# Conceptual transformation:

# 

# ```text

# raw bytes

#     │

#     ▼

# AquaCom message

#     │

#     ▼

# logical protocol result

# ```

# 

# The codec shall be independently testable without a physical AquaCom device.

# 

# \---

# 

# \# 12. Station/session manager

# 

# Each AquaCom station has an independent logical session.

# 

# Conceptual station states:

# 

# ```text

# DISABLED

# INITIALIZING

# CONNECTING

# ONLINE

# DEGRADED

# OFFLINE

# RECOVERING

# ```

# 

# One failing station shall not stop communication with unrelated stations.

# 

# The Station Manager owns:

# 

# \- station lifecycle;

# \- communication status;

# \- retries;

# \- scheduling;

# \- station diagnostics;

# \- history recovery coordination.

# 

# \---

# 

# \# 13. Canonical data model

# 

# The gateway uses a protocol-independent canonical model.

# 

# \## 13.1 CurrentValue

# 

# Conceptually:

# 

# ```text

# CurrentValue

#     StationId

#     PointId

#     Value

#     DataType

#     Quality

#     SourceTimestamp

#     ReceivedTimestamp

#     UpdatedTimestamp

#     AcquisitionSource

# ```

# 

# \---

# 

# \## 13.2 HistoricalSample

# 

# Conceptually:

# 

# ```text

# HistoricalSample

#     StationId

#     PointId

#     Value

#     DataType

#     Quality

#     SourceTimestamp

#     ReceivedTimestamp

#     AcquisitionSource

#     AcquisitionId

# ```

# 

# Historical samples shall retain the timestamp associated with the process state.

# 

# \---

# 

# \# 14. Timestamp model

# 

# The gateway shall distinguish at least:

# 

# ```text

# SourceTimestamp

# ReceivedTimestamp

# ```

# 

# Where applicable also:

# 

# ```text

# UpdatedTimestamp

# ServerTimestamp

# ```

# 

# Their meanings are:

# 

# | Timestamp | Meaning |

# |---|---|

# | SourceTimestamp | Time associated with the value at the source |

# | ReceivedTimestamp | Time the gateway received the information |

# | UpdatedTimestamp | Time current gateway state was changed |

# | ServerTimestamp | Timestamp used by the OPC UA server |

# 

# The following must never be assumed:

# 

# ```text

# ReceivedTimestamp = SourceTimestamp

# ```

# 

# \---

# 

# \# 15. Fundamental historical rule

# 

# The fundamental historical-data rule is:

# 

# > A historical sample represents the process at its SourceTimestamp, not at the time the gateway receives the sample.

# 

# Example:

# 

# ```text

# Historical value:

#     SourceTimestamp = 14:05

#     ReceivedTimestamp = 18:30

# ```

# 

# The value belongs to history at:

# 

# ```text

# 14:05

# ```

# 

# It shall not become a new current process value at:

# 

# ```text

# 18:30

# ```

# 

# \---

# 

# \# 16. Live-data architecture

# 

# Live AquaCom values follow:

# 

# ```text

# AquaCom response

#        │

#        ▼

# Protocol decode

#        │

#        ▼

# Point mapping

#        │

#        ▼

# Canonical CurrentValue

#        │

#        ▼

# Current-State Service

#        │

#        ├────────► OPC UA

#        │

#        └────────► Historical pipeline

#                   if live historization is enabled

# ```

# 

# The Current-State Service maintains the latest accepted value for each configured point.

# 

# \---

# 

# \# 17. Current-State Service

# 

# Responsibilities:

# 

# \- maintain the latest current value;

# \- reject obsolete current-state updates where applicable;

# \- expose current values to OPC UA;

# \- notify subscribers of valid changes;

# \- preserve quality and timestamps.

# 

# Conceptual interface:

# 

# ```text

# update(CurrentValue)

# 

# get(PointId)

# 

# getAll(StationId)

# 

# subscribe(PointId)

# ```

# 

# Historical backfill shall not directly call `update(CurrentValue)`.

# 

# \---

# 

# \# 18. Historical-data architecture

# 

# Historical AquaCom data follows:

# 

# ```text

# AquaCom history

#        │

#        ▼

# Historical Acquisition Manager

#        │

#        ▼

# Canonical HistoricalSample

#        │

#        ▼

# Historical Pipeline

#        │

#        ├────────► History Repository

#        ├────────► Storage Sink A

#        ├────────► Storage Sink B

#        └────────► Other configured destinations

# ```

# 

# The historical pipeline is independent from OPC UA.

# 

# \---

# 

# \# 19. Historical Acquisition Manager

# 

# The Historical Acquisition Manager is responsible for:

# 

# \- determining missing intervals;

# \- requesting AquaCom history;

# \- processing historical responses;

# \- maintaining synchronization state;

# \- managing watermarks;

# \- performing reconnect/backfill;

# \- throttling history retrieval;

# \- avoiding interference with live polling.

# 

# Live acquisition shall normally have higher communication priority than bulk history recovery.

# 

# \---

# 

# \# 20. Historical synchronization

# 

# Each station may maintain persistent synchronization state.

# 

# Conceptually:

# 

# ```text

# StationId

# HistoryStreamId

# LastCompleteTimestamp

# LastAttemptTimestamp

# LastSuccessfulTimestamp

# SynchronizationState

# ```

# 

# Example:

# 

# ```text

# 12:00  history synchronized

# 

# 12:03  communication lost

# 

# 17:24  communication restored

# 

# 12:00 → 17:24

#        identified as recovery interval

# ```

# 

# The gateway then retrieves available AquaCom history for that interval.

# 

# \---

# 

# \# 21. Watermark rule

# 

# A history synchronization watermark shall only advance after the corresponding interval has been successfully handled according to the configured persistence policy.

# 

# The following is prohibited:

# 

# ```text

# receive historical packet

#         │

#         ▼

# advance watermark

#         │

#         ▼

# storage fails

# ```

# 

# Preferred behavior is:

# 

# ```text

# receive

#    ↓

# validate

#    ↓

# persist/durably queue

#    ↓

# advance synchronization state

# ```

# 

# \---

# 

# \# 22. Historical pipeline

# 

# The historical pipeline shall conceptually perform:

# 

# ```text

# HistoricalSample

#        │

#        ▼

# Validation

#        │

#        ▼

# Normalization

#        │

#        ▼

# Deduplication

#        │

#        ▼

# Durable Spool

#        │

#        ▼

# Distribution

# ```

# 

# This pipeline provides a stable boundary between acquisition and storage.

# 

# \---

# 

# \# 23. Deduplication

# 

# The default historical logical identity is:

# 

# ```text

# StationId

# \+

# PointId

# \+

# SourceTimestamp

# ```

# 

# A stronger AquaCom-provided identifier may be used where verified.

# 

# Identical samples shall not create unintended duplicate history.

# 

# Conflicting samples sharing the same logical identity shall generate diagnostics.

# 

# The default conflict strategy should favor explicit conflict detection rather than silent overwrite.

# 

# \---

# 

# \# 24. Durable spool

# 

# The historical spool provides restart-safe delivery.

# 

# Conceptually:

# 

# ```text

# HistoricalSample

#        │

#        ▼

# Persistent spool

#        │

#        ├────► Repository

#        ├────► Sink A

#        └────► Sink B

# ```

# 

# Each destination maintains independent acknowledgement.

# 

# Example:

# 

# ```text

# Record 4711

# 

# Repository         ACK

# CentralHistorian   ACK

# Archive            PENDING

# ```

# 

# The record remains until configured delivery requirements have been fulfilled.

# 

# \---

# 

# \# 25. History Repository

# 

# A History Repository is a queryable historical source.

# 

# It provides operations conceptually equivalent to:

# 

# ```text

# store

# storeBatch

# readRaw

# readAtTime

# getBounds

# health

# ```

# 

# The repository used by OPC UA Historical Access is selected by configuration.

# 

# The OPC UA implementation shall not know whether the repository is:

# 

# ```text

# relational database

# time-series database

# embedded historian

# industrial historian

# other queryable implementation

# ```

# 

# \---

# 

# \# 26. Storage Sink

# 

# A Storage Sink is a destination receiving historical records.

# 

# Conceptual interface:

# 

# ```text

# initialize

# write

# writeBatch

# flush

# health

# shutdown

# ```

# 

# A sink does not need to support historical queries.

# 

# Examples:

# 

# ```text

# SQL database

# time-series database

# enterprise historian

# file archive

# message broker

# data lake connector

# ```

# 

# \---

# 

# \# 27. Repository versus sink

# 

# These shall remain separate concepts:

# 

# ```text

# Repository

#     = query historical data

# 

# Sink

#     = receive historical data

# ```

# 

# One implementation may support both.

# 

# Example:

# 

# ```text

# Local historian

#     Repository: yes

#     Sink: yes

# 

# Archive export

#     Repository: no

#     Sink: yes

# ```

# 

# \---

# 

# \# 28. Multiple storage destinations

# 

# A historical record may be delivered to multiple independent destinations.

# 

# Example:

# 

# ```text

#                          Historical Pipeline

#                                 │

#                 ┌───────────────┼────────────────┐

#                 │               │                │

#                 ▼               ▼                ▼

#           Local Repository   Central DB       Archive

# ```

# 

# Failure of one destination shall not unnecessarily block successful destinations.

# 

# \---

# 

# \# 29. OPC UA architecture

# 

# The OPC UA subsystem provides northbound access to normalized gateway information.

# 

# Logical components:

# 

# ```text

# OPC UA Server

#     │

#     ├── Namespace Manager

#     ├── Current Value Provider

#     ├── Historical Provider

#     ├── Quality Mapper

#     ├── Diagnostics Provider

#     └── Security

# ```

# 

# \---

# 

# \# 30. OPC UA current data

# 

# Current values are exposed through normal OPC UA Data Access.

# 

# Mapping:

# 

# ```text

# CurrentValue.Value

#         ↓

# OPC UA Value

# 

# CurrentValue.SourceTimestamp

#         ↓

# OPC UA SourceTimestamp

# 

# Gateway server time

#         ↓

# OPC UA ServerTimestamp

# 

# CurrentValue.Quality

#         ↓

# OPC UA StatusCode

# ```

# 

# \---

# 

# \# 31. OPC UA Historical Access

# 

# Historical Access shall query the configured History Repository.

# 

# Correct flow:

# 

# ```text

# OPC UA Client

#        │

#        │ HistoryRead

#        ▼

# OPC UA Server

#        │

#        ▼

# Historical Provider

#        │

#        ▼

# History Repository

#        │

#        ▼

# HistoricalSample\[]

#        │

#        ▼

# OPC UA DataValue\[]

# ```

# 

# \---

# 

# \# 32. Prohibited HistoryRead architecture

# 

# The following architecture is prohibited:

# 

# ```text

# OPC UA HistoryRead

#        │

#        ▼

# AquaCom request

#        │

#        ▼

# wait for RTU

#        │

#        ▼

# OPC UA response

# ```

# 

# Historical synchronization and historical client access must be decoupled.

# 

# \---

# 

# \# 33. OPC UA namespace

# 

# The default logical namespace shall resemble:

# 

# ```text

# AquaCom

# │

# ├── Gateway

# │   ├── Status

# │   ├── Version

# │   ├── StartTime

# │   ├── ConfigurationVersion

# │   ├── HistoricalQueueDepth

# │   └── Storage

# │

# └── Stations

#     │

#     ├── PS001

#     │   ├── Process

#     │   │   ├── Level

#     │   │   ├── Pump01Running

#     │   │   └── Pump02Running

#     │   │

#     │   └── Diagnostics

#     │       ├── CommunicationState

#     │       ├── LastCommunication

#     │       ├── HistoryState

#     │       └── HistoryWatermark

#     │

#     └── PS002

# ```

# 

# Exact organization may be configurable.

# 

# \---

# 

# \# 34. Stable OPC UA NodeIds

# 

# OPC UA NodeIds shall derive from stable engineering identifiers.

# 

# Example:

# 

# ```text

# station/PS001/point/LEVEL

# ```

# 

# Changing:

# 

# ```text

# Wet well level

# ```

# 

# to:

# 

# ```text

# Inlet sump level

# ```

# 

# shall not normally change the NodeId.

# 

# Stable NodeIds are required to preserve client configuration.

# 

# \---

# 

# \# 35. Diagnostics architecture

# 

# Diagnostics are separate from process data.

# 

# Conceptually:

# 

# ```text

# Diagnostics

# │

# ├── Gateway

# │

# ├── Station

# │

# ├── AquaCom Protocol

# │

# ├── History

# │

# ├── Storage

# └── OPC UA

# ```

# 

# Diagnostics may be exposed through:

# 

# \- logs;

# \- metrics;

# \- OPC UA diagnostic nodes;

# \- future management interfaces.

# 

# \---

# 

# \# 36. Gateway diagnostics

# 

# Examples:

# 

# ```text

# GatewayState

# Version

# ConfigurationVersion

# StartTime

# Uptime

# StationCount

# OnlineStationCount

# HistoricalQueueDepth

# SpoolDiskUsage

# OPCUAState

# ```

# 

# \---

# 

# \# 37. Station diagnostics

# 

# Examples:

# 

# ```text

# CommunicationState

# LastRequest

# LastResponse

# LastSuccessfulCommunication

# ConsecutiveFailures

# ProtocolErrors

# HistorySynchronizationState

# HistoryWatermark

# HistoricalBacklog

# ```

# 

# \---

# 

# \# 38. Storage diagnostics

# 

# Per destination:

# 

# ```text

# State

# LastSuccessfulWrite

# LastFailure

# PendingRecords

# RetryCount

# Latency

# ```

# 

# A storage failure shall be identifiable independently from an AquaCom communication failure.

# 

# \---

# 

# \# 39. Logging

# 

# Structured logging is preferred.

# 

# Useful context includes:

# 

# ```text

# timestamp

# severity

# component

# stationId

# pointId

# operation

# correlationId

# message

# error

# ```

# 

# Raw AquaCom telegram logging shall be separately configurable.

# 

# \---

# 

# \# 40. Configuration architecture

# 

# Configuration shall be external and machine-readable.

# 

# Conceptual structure:

# 

# ```text

# Gateway Configuration

# │

# ├── Gateway

# ├── AquaCom

# ├── Stations

# ├── Points

# ├── History

# ├── Storage

# ├── OPC UA

# ├── Security

# ├── Diagnostics

# └── Logging

# ```

# 

# Configuration validation occurs before activation.

# 

# \---

# 

# \# 41. Station configuration

# 

# A station definition contains concepts equivalent to:

# 

# ```text

# StationId

# Enabled

# Transport

# AquaComAddress

# ProtocolProfile

# LiveAcquisition

# HistoricalAcquisition

# PointDefinitionSource

# Timezone

# ```

# 

# Station configuration shall not be embedded directly in protocol source code.

# 

# \---

# 

# \# 42. Point configuration

# 

# A point contains concepts equivalent to:

# 

# ```text

# PointId

# DisplayName

# Description

# SourceReference

# DataType

# EngineeringUnit

# Scaling

# LiveEnabled

# HistoryEnabled

# OPCUAEnabled

# Historizing

# StoragePolicy

# ```

# 

# `PointId` shall remain stable even if presentation metadata changes.

# 

# \---

# 

# \# 43. Security boundaries

# 

# The architecture assumes AquaCom is a legacy field protocol.

# 

# Security shall therefore be applied around the AquaCom communication environment.

# 

# Conceptually:

# 

# ```text

# SCADA / client network

#         │

#         ▼

# Firewall / segmentation

#         │

#         ▼

# AquaCom Gateway

#         │

#         ▼

# OT communication network

#         │

#         ▼

# Site firewall/router/VPN

#         │

#         ▼

# AquaCom RTU

# ```

# 

# The gateway shall not assume that AquaCom itself provides modern confidentiality, authentication or authorization.

# 

# \---

# 

# \# 44. OPC UA security

# 

# OPC UA security is handled within the OPC UA subsystem.

# 

# Production architecture shall support:

# 

# ```text

# application certificates

# trusted client management

# secure channels

# authentication

# authorization

# encrypted communication

# ```

# 

# Insecure endpoints shall not be the normal production default.

# 

# \---

# 

# \# 45. Secrets boundary

# 

# Secrets shall not form part of normal version-controlled configuration.

# 

# Examples:

# 

# ```text

# database passwords

# API tokens

# private keys

# certificate private keys

# ```

# 

# These shall be injected through an external secret mechanism.

# 

# \---

# 

# \# 46. Host subsystem

# 

# The `host` subsystem composes the application.

# 

# Responsibilities include:

# 

# \- configuration loading;

# \- dependency construction;

# \- subsystem startup;

# \- lifecycle management;

# \- shutdown;

# \- top-level health state.

# 

# The host shall not become a location for protocol or business logic.

# 

# \---

# 

# \# 47. Startup architecture

# 

# Conceptual startup sequence:

# 

# ```text

# Load configuration

#         │

#         ▼

# Validate configuration

#         │

#         ▼

# Load persistent state

#         │

#         ▼

# Initialize storage

#         │

#         ▼

# Initialize historical services

#         │

#         ▼

# Initialize OPC UA

#         │

#         ▼

# Initialize AquaCom stations

#         │

#         ▼

# Start live acquisition

#         │

#         ▼

# Start historical synchronization

# ```

# 

# A temporary failure of a non-critical secondary storage destination should not necessarily prevent the gateway from starting.

# 

# \---

# 

# \# 48. Shutdown architecture

# 

# Conceptual controlled shutdown:

# 

# ```text

# Stop accepting administrative changes

#         │

#         ▼

# Stop scheduling new acquisition

#         │

#         ▼

# Complete/cancel active transactions safely

#         │

#         ▼

# Persist synchronization state

#         │

#         ▼

# Flush durable state

#         │

#         ▼

# Stop OPC UA

#         │

#         ▼

# Close storage

#         │

#         ▼

# Terminate

# ```

# 

# \---

# 

# \# 49. Restart recovery

# 

# After an unclean restart:

# 

# ```text

# Load configuration

#         │

#         ▼

# Recover persistent spool

#         │

#         ▼

# Restore synchronization watermarks

#         │

#         ▼

# Reconnect AquaCom stations

#         │

#         ▼

# Resume live acquisition

#         │

#         ▼

# Resume historical recovery

# ```

# 

# Live acquisition should resume before large backlog processing.

# 

# \---

# 

# \# 50. Scheduling priorities

# 

# Where AquaCom communication capacity is limited, scheduling shall conceptually prioritize:

# 

# ```text

# 1\. communication/session health

# 2\. current/live acquisition

# 3\. alarms/events when implemented

# 4\. explicit configuration operations

# 5\. historical backfill

# 6\. low-priority maintenance

# ```

# 

# Historical synchronization shall be throttleable.

# 

# \---

# 

# \# 51. Resource boundaries

# 

# All queues and caches shall be bounded.

# 

# This applies to:

# 

# ```text

# AquaCom request queues

# history queues

# storage retry queues

# OPC UA historical result sets

# continuation state

# logging buffers

# spool storage

# ```

# 

# Overflow behavior shall be explicit and observable.

# 

# \---

# 

# \# 52. Testing architecture

# 

# The system shall support independent testing of each architectural boundary.

# 

# Conceptually:

# 

# ```text

# Protocol Fixtures

#       │

#       ▼

# AquaCom Codec Tests

# 

# Simulator

#       │

#       ▼

# Session / Acquisition Tests

# 

# Historical Fixtures

#       │

#       ▼

# History Pipeline Tests

# 

# Repository

#       │

#       ▼

# OPC UA HA Tests

# 

# Current-State Service

#       │

#       ▼

# OPC UA DA Tests

# ```

# 

# \---

# 

# \# 53. AquaCom simulator

# 

# The simulator shall permit development without operational AquaCom equipment.

# 

# Supported scenarios shall eventually include:

# 

# ```text

# normal live values

# normal history

# timeouts

# disconnects

# delayed response

# malformed telegram

# bad checksum

# duplicate history

# conflicting history

# clock error

# reconnect

# backfill

# ```

# 

# The simulator is part of the architecture, not merely a temporary development tool.

# 

# \---

# 

# \# 54. Protocol evidence architecture

# 

# AquaCom implementation is linked to documented evidence.

# 

# Relevant repository area:

# 

# ```text

# docs/protocol/

# ```

# 

# Recommended structure:

# 

# ```text

# aquacom.md

# known-operations.md

# protocol-gaps.md

# test-vectors.md

# ```

# 

# Every implemented AquaCom behavior should be traceable to:

# 

# ```text

# Specification

# Capture

# Known-good implementation

# Approved test vector

# ```

# 

# Unknown behavior remains explicitly unresolved.

# 

# \---

# 

# \# 55. Error isolation

# 

# Failures shall remain contained where possible.

# 

# Examples:

# 

# | Failure | Expected isolation |

# |---|---|

# | Station A offline | Station B continues |

# | Storage A offline | Storage B continues |

# | OPC UA client disconnect | Acquisition continues |

# | OPC UA server problem | AquaCom acquisition should continue where possible |

# | Historical backfill fails | Live acquisition continues |

# | Malformed AquaCom telegram | Telegram rejected, station diagnostics updated |

# | Secondary storage failure | Durable spool retains pending delivery |

# 

# \---

# 

# \# 56. Future extension points

# 

# The architecture shall permit future addition of:

# 

# ```text

# OPC UA Alarms \& Conditions

# AquaCom alarms/events

# AquaCom commands

# setpoint changes

# MQTT

# REST API

# external analytics

# central configuration

# redundant gateways

# high availability

# additional history repositories

# additional storage sinks

# ```

# 

# These shall be added behind explicit subsystem interfaces.

# 

# \---

# 

# \# 57. Future process-control architecture

# 

# If control functionality is introduced later, it shall use a separate command pipeline:

# 

# ```text

# Northbound request

#         │

#         ▼

# Authentication

#         │

#         ▼

# Authorization

#         │

#         ▼

# Command validation

#         │

#         ▼

# Command service

#         │

#         ▼

# AquaCom command

#         │

#         ▼

# Confirmation / readback

#         │

#         ▼

# Audit

# ```

# 

# A direct mapping from arbitrary OPC UA writes to raw AquaCom telegrams is not an acceptable control architecture.

# 

# \---

# 

# \# 58. Architectural invariants

# 

# The following invariants shall hold throughout implementation.

# 

# \## INV-001

# 

# AquaCom-specific logic exists only behind the AquaCom abstraction.

# 

# \## INV-002

# 

# The canonical model contains no AquaCom telegram representation.

# 

# \## INV-003

# 

# The canonical model contains no OPC UA-specific representation.

# 

# \## INV-004

# 

# Historical samples do not update current state solely because they were recently retrieved.

# 

# \## INV-005

# 

# Historical SourceTimestamp is preserved.

# 

# \## INV-006

# 

# OPC UA HistoryRead queries a repository, not an AquaCom device.

# 

# \## INV-007

# 

# A storage failure does not directly stop unrelated live AquaCom acquisition.

# 

# \## INV-008

# 

# Point identity remains stable across display-name changes.

# 

# \## INV-009

# 

# OPC UA NodeIds remain stable across restart and normal upgrades.

# 

# \## INV-010

# 

# Unverified AquaCom protocol details are not implemented as guesses.

# 

# \## INV-011

# 

# Synchronization state required for recovery is persistent.

# 

# \## INV-012

# 

# Historical delivery supports bounded persistent buffering.

# 

# \---

# 

# \# 59. Primary end-to-end data flows

# 

# \## 59.1 Live data

# 

# ```text

# AquaCom RTU

#     │

#     ▼

# Transport

#     │

#     ▼

# Protocol Engine

#     │

#     ▼

# Station Manager

#     │

#     ▼

# CurrentValue

#     │

#     ▼

# Current-State Service

#     │

#     ├────► OPC UA

#     │

#     └────► Historical Pipeline

#            when live historization is enabled

# ```

# 

# \---

# 

# \## 59.2 AquaCom historical recovery

# 

# ```text

# AquaCom RTU

#     │

#     ▼

# Historical Acquisition

#     │

#     ▼

# HistoricalSample

#     │

#     ▼

# Validation

#     │

#     ▼

# Deduplication

#     │

#     ▼

# Durable Spool

#     │

#     ├────► Repository

#     ├────► Storage A

#     └────► Storage B

# ```

# 

# \---

# 

# \## 59.3 OPC UA historical request

# 

# ```text

# OPC UA Client

#     │

#     ▼

# HistoryRead

#     │

#     ▼

# OPC UA Historical Provider

#     │

#     ▼

# History Repository

#     │

#     ▼

# HistoricalSample\[]

#     │

#     ▼

# OPC UA DataValue\[]

# ```

# 

# \---

# 

# \# 60. Reference architecture

# 

# The complete reference architecture is:

# 

# ```text

#                          FIELD / AQUACOM

#                                │

#                                ▼

#                     ┌─────────────────────┐

#                     │ AquaCom Transport   │

#                     └──────────┬──────────┘

#                                │

#                                ▼

#                     ┌─────────────────────┐

#                     │ Protocol Engine     │

#                     └──────────┬──────────┘

#                                │

#                                ▼

#                     ┌─────────────────────┐

#                     │ Station Manager     │

#                     └──────────┬──────────┘

#                                │

#                                ▼

#                     ┌─────────────────────┐

#                     │ Canonical Model     │

#                     └───────┬───────┬─────┘

#                             │       │

#                          LIVE       HISTORY

#                             │       │

#                             ▼       ▼

#                   ┌──────────────┐  ┌──────────────────┐

#                   │Current-State │  │Historical        │

#                   │Service       │  │Acquisition       │

#                   └──────┬───────┘  └────────┬─────────┘

#                          │                   │

#                          │                   ▼

#                          │          ┌──────────────────┐

#                          │          │Historical        │

#                          │          │Pipeline          │

#                          │          └────────┬─────────┘

#                          │                   │

#                          │        ┌──────────┼───────────┐

#                          │        │          │           │

#                          │        ▼          ▼           ▼

#                          │   Repository   Sink A      Sink B

#                          │        │

#                          │        │

#                          ▼        ▼

#                   ┌────────────────────────────┐

#                   │        OPC UA Server       │

#                   │                            │

#                   │ Data Access                │

#                   │ Historical Access          │

#                   │ Diagnostics                │

#                   └──────────────┬─────────────┘

#                                  │

#                                  ▼

#                          SCADA / Clients

# ```

# 

# \---

# 

# \# 61. Architecture decision summary

# 

# The gateway is intentionally designed as an:

# 

# ```text

# AquaCom Integration Gateway

# ```

# 

# rather than merely an:

# 

# ```text

# AquaCom OPC UA Server

# ```

# 

# This allows:

# 

# \- OPC UA to be replaced or supplemented;

# \- storage technology to change;

# \- multiple historians to coexist;

# \- AquaCom equipment to be gradually retired;

# \- other northbound interfaces to be added;

# \- historical acquisition to continue independently of OPC UA clients.

# 

# The long-term architectural asset is therefore the normalized integration layer and historical recovery mechanism, not any single external interface.

# 

# \---

# 

# \# 62. Final architectural rule

# 

# The most important data rule is:

# 

# > Historical data is stored and exposed according to when the process value was valid, not when the gateway happened to retrieve it.

# 

# The most important protocol rule is:

# 

# > AquaCom behavior must be based on verified protocol knowledge and must never be invented merely to complete an implementation.

# 

# The most important software rule is:

# 

# > AquaCom, canonical data handling, historical persistence and OPC UA shall remain separate architectural concerns.