# Transform data (transforms)
Transform data (transforms)

*Transforms* are mathematical expressions that map asset properties' data points from one form to another. A transform expression consists of asset property variables, literals, operators, and functions. The transformed data points hold a one-to-one relationship with the input data points. AWS IoT SiteWise calculates a new transformed data point each time any of the input properties receives a new data point.

**Note**  
For property updates with the same timestamp, output values may be overwritten by updates from other incoming properties.

For example, if your asset has a temperature measurement stream named `Temperature_C` with units in Celsius, you can convert each data point to Fahrenheit with the formula `Temperature_F = 9/5 * Temperature_C + 32`. Each time AWS IoT SiteWise receives a data point in the `Temperature_C` measurement stream, the corresponding `Temperature_F` value is calculated within a few seconds and available as the `Temperature_F` property.

If your transform contains more than one variable, the data point that arrives earlier initiates the computation immediately. Consider an example where a parts manufacturer uses a transform to monitor product quality. Using a different standard based on the part type, the manufacturer uses the following measurements to represent the process:
+ `Part_Number` - A string that identifies the part type.
+ `Good_Count` - An integer that increases by one if the part meets the standard.
+ `Bad_Count` - An integer that increases by one if the part doesn't meet the standard.

The manufacturer also creates a transform, `Quality_Monitor`, that equals ` if(eq(Part_Number, "BLT123") and (Bad_Count / (Good_Count + Bad_Count) > 0.1), "Caution", "Normal")`.

This transform monitors the percentage of bad parts produced for a specific part type. If the part number is BLT123 and the percentage of bad parts exceeds 10 percent (0.1), the transform returns `"Caution"`. Otherwise, the transform returns `"Normal"`.

**Note**  
If `Part_Number` receives a new data point before other measurements, the `Quality_Monitor` transform uses the new `Part_Number` value and the latest `Good_Count` and `Bad_Count` values. To avoid errors, reset `Good_Count` and `Bad_Count` before the next manufacturing run.
Use [metrics](metrics.md) if you want to evaluate expressions only after all variables receive new data points.

**Topics**
+ [

## Define transforms (console)
](#define-transforms-console)
+ [

## Define transforms (AWS CLI)
](#define-transform-cli)

## Define transforms (console)


When you define a transform for an asset model in the AWS IoT SiteWise console, you specify following parameters:
+ <a name="asset-property-name-console"></a>**Name** – The property's name.
+ <a name="asset-property-unit-console"></a>**Unit** – (Optional) The scientific unit for the property, such as mm or Celsius.
+ **Data type** – The data type of the transform, which can be **Double** or **String**.
+ **External ID** – (Optional) This is a user-defined ID. For more information, see [Reference objects with external IDs](object-ids.md#external-id-references) in the *AWS IoT SiteWise User Guide*.
+ **Formula** – The transform expression. Transform expressions can't use aggregation functions or temporal functions. To open the auto complete feature, start typing or press the down arrow key. For more information, see [Use formula expressions](formula-expressions.md).
**Important**  <a name="transform-input-rules"></a>
Transforms can input properties that are integer, double, Boolean, or string type. Booleans convert to `0` (false) and `1` (true).  
Transforms must input one or more properties that aren't attributes and any number of attribute properties. AWS IoT SiteWise calculates a new transformed data point each time the input property that isn't an attribute receives a new data point. New attribute values don't launch transform updates. The same request rate for asset property data API operations applies for transform computation results.  
<a name="formula-output-rules"></a>Formula expressions can only output double or string values. Nested expressions can output other data types, such as strings, but the formula as a whole must evaluate to a number or string. You can use the [jp function](expression-string-functions.md#jp-definition) to convert a string to a number. The Boolean value must be 1 (true) or 0 (false). For more information, see [Undefined, infinite, and overflow values](expression-tutorials.md#undefined-values).

For more information, see [Create an asset model (console)](create-asset-models.md#create-asset-model-console).

## Define transforms (AWS CLI)


When you define a transform for an asset model with the AWS IoT SiteWise API, you specify the following parameters:
+ <a name="asset-property-name-cli"></a>`name` – The property's name.
+ <a name="asset-property-unit-cli"></a>`unit` – (Optional) The scientific unit for the property, such as mm or Celsius.
+ `dataType` – The data type of the transform, which must be `DOUBLE` or `STRING`.
+ `externalId` – (Optional) This is a user-defined ID. For more information, see [Reference objects with external IDs](object-ids.md#external-id-references) in the *AWS IoT SiteWise User Guide*.
+ `expression` – The transform expression. Transform expressions can't use aggregation functions or temporal functions. For more information, see [Use formula expressions](formula-expressions.md).
+ `variables` – The list of variables that defines the other properties of your asset to use in the expression. Each variable structure contains a simple name to use in the expression and a `value` structure that identifies which property to link to that variable. The `value` structure contains the following information:
  + `propertyId` – The ID of the property from which to input values. You can use the property's name instead of its ID.
**Important**  <a name="transform-input-rules"></a>
Transforms can input properties that are integer, double, Boolean, or string type. Booleans convert to `0` (false) and `1` (true).  
Transforms must input one or more properties that aren't attributes and any number of attribute properties. AWS IoT SiteWise calculates a new transformed data point each time the input property that isn't an attribute receives a new data point. New attribute values don't launch transform updates. The same request rate for asset property data API operations applies for transform computation results.  
<a name="formula-output-rules"></a>Formula expressions can only output double or string values. Nested expressions can output other data types, such as strings, but the formula as a whole must evaluate to a number or string. You can use the [jp function](expression-string-functions.md#jp-definition) to convert a string to a number. The Boolean value must be 1 (true) or 0 (false). For more information, see [Undefined, infinite, and overflow values](expression-tutorials.md#undefined-values).

**Example transform definition**  
The following example demonstrates a transform property that converts an asset's temperature measurement data from Celsius to Fahrenheit. This object is an example of an [AssetModelProperty](https://docs.aws.amazon.com/iot-sitewise/latest/APIReference/API_AssetModelProperty.html) that contains a [Transform](https://docs.aws.amazon.com/iot-sitewise/latest/APIReference/API_Transform.html). You can specify this object as a part of the [CreateAssetModel](https://docs.aws.amazon.com/iot-sitewise/latest/APIReference/API_CreateAssetModel.html) request payload to create a transform property. For more information, see [Create an asset model (AWS CLI)](create-asset-models.md#create-asset-model-cli).  

```
{
...
"assetModelProperties": [
...
{
  "name": "Temperature F",
  "dataType": "DOUBLE",
  "type": {
    "transform": {
      "expression": "9/5 * temp_c + 32",
      "variables": [
        {
          "name": "temp_c",
          "value": {
            "propertyId": "Temperature C"
          }
        }
      ]
    }
  },
  "unit": "Fahrenheit"
}
],
...
}
```

**Example transform definition that contains three variables**  
The following example demonstrates a transform property that returns a warning message (`"Caution"`) if more than 10 percent of the BLT123 parts don't meet the standard. Otherwise, it returns an information message (`"Normal"`).  

```
{
...
"assetModelProperties": [
...
{
"name": "Quality_Monitor",
"dataType": "STRING",
"type": {
    "transform": {
        "expression": "if(eq(Part_Number,"BLT123") and (Bad_Count / (Good_Count + Bad_Count) > 0.1), "Caution", "Normal")",
        "variables": [
            {
                "name": "Part_Number",
                "value": {
                    "propertyId": "Part Number"
                }
            },
            {
                "name": "Good_Count",
                "value": {
                    "propertyId": "Good Count"
                }
            },
            {
                "name": "Bad_Count",
                "value": {
                    "propertyId": "Bad Count"
                }
            }
        ]
    }
}
}
...
}
```