# Simple Mapping (sub-transformation)
As you build a transformation, you may notice a sequence of steps you want to repeat. This sequence can be turned into a *mapping*.
Like the [Mapping](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping.md) step, you can use the **Simple Mapping (sub-transformation)** step to run a reusable sub-transformation.
Compared to the Mapping step, **Simple Mapping (sub-transformation)** accepts **one input** and **one output**, so it behaves like other steps that read and write rows.
### Before you begin
In your sub-transformation, define input and output using these placeholder steps:
* [Mapping Input Specification](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping-input-specification.md)
* [Mapping Output Specification](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping-output-specification.md)
The mapping executed by the parent transformation through the **Simple Mapping (sub-transformation)** step is known as the *sub-transformation*.
You can run **Simple Mapping (sub-transformation)** in multiple copies, clusters, single-threading mode, and other execution models.
You can also use this step in advanced data routing scenarios (for example, **Filter Rows** and **Switch / Case**) and in partitioned or clustered transformations.
### Step name and transformation
| Field | Description |
| ------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Step name** | Specifies the unique name of the step on the canvas. A step can be placed on the canvas several times; however, it represents the same step multiple times. Default: `Simple Mapping (sub-transformation)`. |
| **Transformation** | Specify your mapping sub-transformation to execute. Select Browse to locate the transformation in the VFS browser.
If you select a transformation that has the same root path as the current transformation, the variable ${Internal.Entry.Current.Directory} is inserted automatically in place of the common root path. For example, if the current transformation path is /home/admin/transformation.ktr and you select a transformation in the folder /home/admin/path/sub.ktr, then the path is converted automatically to ${Internal.Entry.Current.Directory}/path/sub.ktr.
If you are working with a repository, specify the name of the transformation. If you are not working with a repository, specify the XML file name of the transformation. Transformations previously specified by reference are converted automatically to use the transformation name within the Pentaho Repository.
|
### Options
The **Simple Mapping (sub-transformation)** step includes the following tabs:
* **Parameters**
* **Input**
* **Output**
#### Parameters tab
Use the **Parameters** tab to define or pass Kettle variables to the sub-transformation.
| Option | Description |
| -------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Variable name** | Variable name to pass to the sub-transformation. |
| **String value (can include variable expressions)** | Value to assign to the variable name. You can include variable expressions. |
| **Inherit all variables from the parent transformation** | Makes all variables available in the parent transformation available in the sub-transformation, even if they are not explicitly listed in the **Parameters** tab. |
{% hint style="info" %}
If **Inherit all variables from the parent transformation** is not selected, only the variables listed in the **Parameters** tab are passed to the sub-transformation.
{% endhint %}
#### Input tab
Use this tab to map fields from the incoming stream to the fields in the [Mapping Input Specification](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping-input-specification.md) step inside the sub-transformation.
| Option | Description |
| --------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Fieldname from source step** | Name of a field in the incoming stream. |
| **Fieldname to mapping input step** | Name of the field as it is used inside the sub-transformation. |
| **Mapping** | Opens a dialog listing available fields in both the parent transformation and the sub-transformation so you can map fields. |
| **Update mapped fieldnames downstream** | Renames fields back to their original names when they reach the Mapping Output Specification step. This can make sub-transformations more transparent and reusable. |
**Add inputs to the table**
1. Select **Mapping**.
2. Select a **Source Field**.
3. Select a **Target Field**.
4. Select **Add**.
5. To remove a mapping, select **Delete**.
6. Select **OK**.
#### Output tab
Use this tab to map fields from the [Mapping Output Specification](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping-output-specification.md) step inside the sub-transformation to the output stream of the **Simple Mapping (sub-transformation)** step.
| Option | Description |
| ------------------------------- | ----------------------------------------------------------------------------------------------------- |
| **Fieldname from mapping step** | Field name as it is available in the Mapping Output Specification step inside the sub-transformation. |
| **Fieldname to target step** | Field name to use in the outgoing stream of the **Simple Mapping (sub-transformation)** step. |
| **Get Fields** | Populates the grid with all output fields from the sub-transformation. |
### Logging
To differentiate log lines from a mapping, edit `kettle.properties` and set `KETTLE_LOG_MARK_MAPPINGS` to `Y`.
When `KETTLE_LOG_MARK_MAPPINGS` is set to `Y`, PDI precedes log lines with the step name and mapping name.
### See also
* [Mapping](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping.md)
* [Mapping Input Specification](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping-input-specification.md)
* [Mapping Output Specification](/pdia-data-integration/pdi-transformation-steps-reference-overview/mapping-output-specification.md)
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