# MapReduce Output

The **MapReduce Output** step defines the key/value pairs for Hadoop output.

The output of this step becomes the Hadoop output. How Hadoop uses that output depends on how you configured the transformation.

* If this step is used in a **Mapper** transformation type and a combiner and/or reducer is configured, the output becomes the input pairs for the combiner and/or reducer.
* If this step is used in a **Mapper** transformation type and no combiner or reducer is configured, the output is passed using the submitting Hadoop job’s format.
* If this step is used in a **Combiner** transformation type and a reducer is configured, the output becomes the input pairs for the reducer.
* If this step is used in a **Combiner** transformation type and no reducer is configured, the output is passed using the submitting Hadoop job’s format.
* If this step is used in a **Reducer** transformation type, the output is passed using the submitting Hadoop job’s format.

The data type for keys and values must be defined before this step.

### Options

| Option          | Description                                                                                                                                               |
| --------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Step name**   | Specifies the unique name of the step on the canvas. You can place the step on the canvas multiple times; however, it represents the same step each time. |
| **Key field**   | The Hadoop output field that represents the MapReduce key.                                                                                                |
| **Value field** | The Hadoop output field that represents the MapReduce value.                                                                                              |

### Metadata injection support

All fields of this step support metadata injection. You can use it with [ETL metadata injection](/pdia-data-integration/pdi-transformation-steps-reference-overview/etl-metadata-injection.md) to pass metadata to your transformation at runtime.


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