1. Writeback Link

When OPC UA client executes Write on a point Variable:

1. Server only allows writing AttributeId::Value
2. Reverse lookup point_id by NodeId
3. Validate if point is writable (access_mode = Write/ReadWrite)
4. Strict type matching (Variant type must correspond to point data_type)
5. Send NorthwardEvent::WritePoint to core (With timeout_ms)
6. Wait for NorthwardData::WritePointResponse
7. Update value of that variable in AddressSpace on success, and notify subscribers

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2. Type Matching

Current writeback is strict type matching, no automatic conversion like "String to Number".

Example:

• point.data_type = Float64 → Write must be OPC UA Double
• point.data_type = Boolean → Write must be OPC UA Boolean

WARNING

If client write type mismatch, returns BadTypeMismatch (or BadInvalidArgument).

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3. Error to StatusCode Mapping

Gateway Error	Typical Cause	StatusCode (Indicative)
NotFound(node_id/point)	NodeId not found or point deleted	BadNodeIdUnknown
NotFound(action)	Point exists but no corresponding write action	BadNotWritable
NotConnected	Device/Channel not connected	BadNotConnected
Timeout	Write timeout (Queue wait or driver timeout)	BadTimeout
ValidationFailed(type mismatch)	Type mismatch	BadTypeMismatch
ValidationFailed(out of range)	Out of range	BadOutOfRange
ValidationFailed(not writeable)	Not writable point	BadUserAccessDenied

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4. Write Timeout

Configuration item:

• writeTimeoutMs (Default 5000)

It constrains:

• Wait in core write serial queue
• Southward driver write timeout