Array Datatypes Design
Status
Approved
Context
The current data model does not directly provide for devices which provide array data. Small fixed-length arrays may be handled by defining multiple device resources - one for each element - and aggregating them via a resource command. Other array data may be passed using the Binary type. Neither of these approaches is ideal: the binary data is opaque and any service processing it would need specific knowledge to do so, and aggregation presents the device service implementation with a multiple-read request that could in many cases be better handled by a single request.
This design adds arrays of primitives to the range of supported types in EdgeX. It comprises an extension of the DeviceProfile model, and an update to the definition of Reading.
Decision
DeviceProfile extension
The permitted values of the Type field in PropertyValue are extended to include:
"BoolArray", "Uint8Array", "Uint16Array", "Uint32Array", "Uint64Array", "Int8Array", Int16Array", "Int32Array", "Int64Array", "Float32Array", "Float64Array"
Readings
Implementation in v2 API
The value field of SimpleReading becomes an array of strings. For non-array types, an array of length 1 is created.
Fallback position for v1 API
In the v1 API, Reading.Value is a string representation of the data. If this is maintained, the representation for Array types will follow the JSON array syntax, ie ["value1", "value2", ...]
Consequences
Any service which processes Readings will need to be reworked to account for the new Reading type.
Device Service considerations
The API used for interfacing between device SDKs and devices service implementations contains a local representation of reading values. This will need to be updated in line with the changes outlined here. For C, this will involve an extension of the existing union type. For Go, additional fields may be added to the CommandValue structure.
Processing of numeric data in the device service, ie offset, scale etc will not be applied to the values in an array.