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Convert data by using dataflow conversions

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Important

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You can use dataflow conversions to transform data in Azure IoT Operations. The conversion element in a dataflow is used to compute values for output fields. You can use input fields, available operations, data types, and type conversions in dataflow conversions.

The dataflow conversion element is used to compute values for output fields:

inputs: [
  '*.Max' // - $1
  '*.Min' // - $2
]
output: 'ColorProperties.*'
expression: '($1 + $2) / 2'

There are several aspects to understand about conversions:

  • Reference to input fields: How to reference values from input fields in the conversion formula.
  • Available operations: Operations that can be utilized in conversions. For example, addition, subtraction, multiplication, and division.
  • Data types: Types of data that a formula can process and manipulate. For example, integer, floating point, and string.
  • Type conversions: How data types are converted between the input field values, the formula evaluation, and the output fields.

Input fields

In conversions, formulas can operate on static values like a number such as 25 or parameters derived from input fields. A mapping defines these input fields that the formula can access. Each field is referenced according to its order in the input list:

inputs: [
  '*.Max'      // - $1
  '*.Min'      // - $2
  '*.Mid.Avg'  // - $3
  '*.Mid.Mean' // - $4
]
output: 'ColorProperties.*'
expression: '($1, $2, $3, $4)'

In this example, the conversion results in an array containing the values of [Max, Min, Mid.Avg, Mid.Mean]. The comments in the YAML file (# - $1, # - $2) are optional, but they help to clarify the connection between each field property and its role in the conversion formula.

Data types

Different serialization formats support various data types. For instance, JSON offers a few primitive types: string, number, Boolean, and null. It also includes arrays of these primitive types.

When the mapper reads an input property, it converts it into an internal type. This conversion is necessary for holding the data in memory until it's written out into an output field. The conversion to an internal type happens regardless of whether the input and output serialization formats are the same.

The internal representation utilizes the following data types:

Type Description
bool Logical true/false.
integer Stored as 128-bit signed integer.
float Stored as 64-bit floating point number.
string A UTF-8 string.
bytes Binary data, a string of 8-bit unsigned values.
datetime UTC or local time with nanosecond resolution.
time Time of day with nanosecond resolution.
duration A duration with nanosecond resolution.
array An array of any types listed previously.
map A vector of (key, value) pairs of any types listed previously.

Input record fields

When an input record field is read, its underlying type is converted into one of these internal type variants. The internal representation is versatile enough to handle most input types with minimal or no conversion.

For some formats, surrogate types are used. For example, JSON doesn't have a datetime type and instead stores datetime values as strings formatted according to ISO8601. When the mapper reads such a field, the internal representation remains a string.

Output record fields

The mapper is designed to be flexible by converting internal types into output types to accommodate scenarios where data comes from a serialization format with a limited type system. The following examples show how conversions are handled:

  • Numeric types: These types can be converted to other representations, even if it means losing precision. For example, a 64-bit floating-point number (f64) can be converted into a 32-bit integer (i32).
  • Strings to numbers: If the incoming record contains a string like 123 and the output field is a 32-bit integer, the mapper converts and writes the value as a number.
  • Strings to other types:
    • If the output field is datetime, the mapper attempts to parse the string as an ISO8601 formatted datetime.
    • If the output field is binary/bytes, the mapper tries to deserialize the string from a base64-encoded string.
  • Boolean values:
    • Converted to 0/1 if the output field is numerical.
    • Converted to true/false if the output field is string.

Use a conversion formula with types

In mappings, an optional formula can specify how data from the input is processed before being written to the output field. If no formula is specified, the mapper copies the input field to the output by using the internal type and conversion rules.

If a formula is specified, the data types available for use in formulas are limited to:

  • Integers
  • Floating-point numbers
  • Strings
  • Booleans
  • Arrays of the preceding types
  • Missing value

Map and byte can't participate in formulas.

Types related to time (datetime, time, and duration) are converted into integer values that represent time in seconds. After formula evaluation, results are stored in the internal representation and not converted back. For example, datetime converted to seconds remains an integer. If the value will be used in datetime fields, an explicit conversion method must be applied. An example is converting the value into an ISO8601 string that's automatically converted to the datetime type of the output serialization format.

Use irregular types

Special considerations apply to types like arrays and missing value.

Arrays

Arrays can be processed by using aggregation functions to compute a single value from multiple elements. For example, by using the input record:

{
  "Measurements": [2.34, 12.3, 32.4]
}

With the mapping:

inputs: [
  'Measurements' // - $1
]
output: 'Measurement'
expression: 'min($1)'

This configuration selects the smallest value from the Measurements array for the output field.

Arrays can also be created from multiple single values:

inputs: [
  'minimum' // - - $1
  'maximum' // - - $2
  'average' // - - $3
  'mean'    // - - $4
]
output: 'stats'
expression: '($1, $2, $3, $4)'

This mapping creates an array that contains the minimum, maximum, average, and mean.

Missing value

Missing value is a special type used in scenarios, such as:

  • Handling missing fields in the input by providing an alternative value.
  • Conditionally removing a field based on its presence.

Example mapping that uses a missing value:

{
  "Employment": {      
    "Position": "Analyst",
    "BaseSalary": 75000,
    "WorkingHours": "Regular"
  }
}

The input record contains the BaseSalary field, but possibly that's optional. Let's say that if the field is missing, a value must be added from a contextualization dataset:

{
  "Position": "Analyst",
  "BaseSalary": 70000,
  "WorkingHours": "Regular"
}

A mapping can check if the field is present in the input record. If the field is found, the output receives that existing value. Otherwise, the output receives the value from the context dataset. For example:

inputs: [
  'BaseSalary' // - - - - - - - - - - - $1
  '$context(position).BaseSalary' //  - $2
]
output: 'BaseSalary'
expression: 'if($1 == (), $2, $1)'

The conversion uses the if function that has three parameters:

  • The first parameter is a condition. In the example, it checks if the BaseSalary field of the input field (aliased as $1) is the missing value.
  • The second parameter is the result of the function if the condition in the first parameter is true. In this example, it's the BaseSalary field of the contextualization dataset (aliased as $2).
  • The third parameter is the value for the condition if the first parameter is false.

Available functions

Dataflows provide a set of built-in functions that can be used in conversion formulas. These functions can be used to perform common operations like arithmetic, comparison, and string manipulation. The available functions are:

Function Description Examples
min Return the minimum value from an array. min(2, 3, 1) returns 1, min($1) returns the minimum value from the array $1
max Return the maximum value from an array. max(2, 3, 1) returns 3, max($1) returns the maximum value from the array $1
if Return between values based on a condition. if($1 > 10, 'High', 'Low') returns 'High' if $1 is greater than 10, otherwise 'Low'
len Return the character length of a string or the number of elements in a tuple. len("Azure") returns 5, len(1, 2, 3) returns 3, len($1) returns the number of elements in the array $1
floor Return the largest integer less than or equal to a number. floor(2.9) returns 2
round Return the nearest integer to a number, rounding half-way cases away from 0.0. round(2.5) returns 3
ceil Return the smallest integer greater than or equal to a number. ceil(2.1) returns 3
scale Scale a value from one range to another. scale($1, 0, 10, 0, 100) scales the input value from the range 0 to 10 to the range 0 to 100

Conversion functions

Dataflows provide several built-in conversion functions for common unit conversions like temperature, pressure, length, weight, and volume. Here are some examples:

Conversion Formula Function name
Celsius to Fahrenheit F = (C * 9/5) + 32 cToF
PSI to bar Bar = PSI * 0.0689476 psiToBar
Inch to cm Cm = inch * 2.54 inToCm
Foot to meter Meter = foot * 0.3048 ftToM
Lbs to kg Kg = lbs * 0.453592 lbToKg
Gallons to liters Liters = gallons * 3.78541 galToL

Reverse conversions are also supported:

Conversion Formula Function name
Fahrenheit to Celsius C = (F - 32) * 5/9 fToC
Bar to PSI PSI = bar / 0.0689476 barToPsi
Cm to inch Inch = cm / 2.54 cmToIn
Meter to foot Foot = meter / 0.3048 mToFt
Kg to lbs Lbs = kg / 0.453592 kgToLb
Liters to gallons Gallons = liters / 3.78541 lToGal

Additionally, you can define your own conversion functions using basic mathematical formulas. The system supports operators like addition (+), subtraction (-), multiplication (*), and division (/). These operators follow standard rules of precedence, which can be adjusted using parentheses to ensure the correct order of operations. This allows you to customize unit conversions to meet specific needs.

Available operators by precedence

Operator Description
^ Exponentiation: $1 ^ 3

Because Exponentiation has the highest precedence, it's executed first unless parentheses override this order:

  • $1 * 2 ^ 3 is interpreted as $1 * 8 because the 2 ^ 3 part is executed first, before multiplication.
  • ($1 * 2) ^ 3 processes the multiplication before exponentiation.
Operator Description
- Negation
! Logical not

Negation and Logical not have high precedence, so they always stick to their immediate neighbor, except when exponentiation is involved:

  • -$1 * 2 negates $1 first, and then multiplies.
  • -($1 * 2) multiplies, and then negates the result.
Operator Description
* Multiplication: $1 * 10
/ Division: $1 / 25 (Result is an integer if both arguments are integers, otherwise float)
% Modulo: $1 % 25

Multiplication, Division, and Modulo, having the same precedence, are executed from left to right, unless the order is altered by parentheses.

Operator Description
+ Addition for numeric values, concatenation for strings
- Subtraction

Addition and Subtraction are considered weaker operations compared to the operations in the previous group:

  • $1 + 2 * 3 results in $1 + 6 because 2 * 3 is executed first because of the higher precedence of multiplication.
  • ($1 + 2) * 3 prioritizes Addition before Multiplication.
Operator Description
< Less than
> Greater than
<= Less than or equal to
>= Greater than or equal to
== Equal to
!= Not equal to

Comparisons operate on numeric, Boolean, and string values. Because they have lower precedence than arithmetic operators, no parentheses are needed to compare results effectively:

  • $1 * 2 <= $2 is equivalent to ($1 * 2) <= $2.
Operator Description
|| Logical OR
&& Logical AND

Logical operators are used to chain conditions:

  • $1 > 100 && $2 > 200