Metadata Injection transform Icon Metadata Injection


Metadata injection allows Apache Hop users to provide the required metadata for a pipeline at runtime. This can significantly reduce the need to develop slightly different versions of what is basically the same pipeline.

Supported Engines

Hop Engine



Not Supported


Not Supported


Not Supported


The Metadata Injection transform (MDI transform) inserts data and transform properties into a pipeline that is dynamically configured at runtime. This is typically used to enhance reusability of a pipeline.

E.g. Use Case: If you have 10 different CSV file definitions and don’t want to create 10 pipelines for each separate file, you can use ETL metadata injection. After the MDI transform is configured (mapping data and properties to the template/target pipeline), at run-time it will dynamically populate any non configured transform properties in the target pipeline. For example, in the template file, if you leave the Table input Database fields empty, and if configured in the MDI transform, it will populate the Table field and Stream fields dynamically.

Basically, the ETL Metadata Injection transform overrules the default behavior for the named pipeline. Not all incoming streams need to have the same layout. It is perfectly OK to provide input to the metadata injection transform from different streams with different layouts.

Sample files:

  • Configure MDI: read-and-inject-metadata.hpl

  • Read and fill in Template: read-file-template.hpl



Option Description

Transform name

Name of the transform.


Specify your template pipeline by entering in its path. Click Browse to display and enter the path details using the Virtual File System Browser.

If you select a pipeline that has the same root path as the current pipeline, the variable ${Internal.Transform.Current.Directory} will automatically be inserted in place of the common root path. For example, if the current pipeline’s path is /home/admin/pipeline.hpl and you select a pipeline in the folder /home/admin/path/sub.hpl then the path will automatically be converted to ${Internal.Transform.Current.Directory}/path/sub.hpl.

The ETL Metadata Injection transform features the two tabs with fields. Each tab is described below.

Inject Metadata Tab

This tab shows transforms and properties on the target/template file that will be modified at run-time. Map each property to a source that you would like to be populated dynamically.

Option Description

Target injection transform key

Lists the available fields in each transform of the template pipeline that can be injected with metadata.

Target description

Describes how the target fields relate to their target transforms.

Source transform

Lists the transform associated with the fields to be injected into the target fields as metadata.

Source field

Lists the fields to be injected into the target fields as metadata.

To specify the source field as metadata to be injected, perform the following transforms:

  1. In the Target injection transform key column, double-click the field for which you want to specify a source field. The Source field dialog box opens.

  2. Select a source field and click OK.

  3. Optionally, select Use constant value to specify a constant value for the injected metadata through one of the following actions:

    • Manually entering a value.

    • Using an internal variable to set the value (${Internal.transform.Unique.Count} for example).

    • Using a combination of manually specified values and parameter values (${FILE_PREFIX}_${FILE_DATE}.txt for example).

When specifying constant values for grouped lists of values like fields or filenames please note that there isn’t a good solution for that. Best practice is to use a Data Grid transform to inject a complete set of constant values. You can map those in this metadata injection transform. It will do its best to accommodate you by allowing you to inject a single row in the group with the specified constant value.

Injecting Metadata into the ETL Metadata Injection transform

For injecting metadata into the ETL Metadata Injection transform itself, the following exceptions apply:

  • To inject a method for how to specify a field (such as by FILENAME), set a PIPELINE_SPECIFICATION_METHOD constant to the field of an input transform. You can then map the field as a source to the PIPELINE_SPECIFICATION_METHOD constant in the ETL Metadata Injection transform.

  • The target field for the ETL Metadata Injection transform inserting the metadata into the original injection is defined by [GROUP NAME].[FIELD NAME]. For example, if the GROUP NAME is 'OUTPUT_FIELDS' and the FIELD NAME is 'OUTPUT_FIELDNAME', you would set the target field to 'OUTPUT_FIELDS.OUTPUT_FIELDNAME'.

Options Tab

Option Description

transform to read from (optional)

Optionally, select a transform in your template pipeline to pass data directly to a transform following the ETL Metadata Injection transform in your current pipeline.

Field name

If transform to read from is selected, enter the name of the field passed directly from the transform in the template pipeline.


If transform to read from is selected, select the type of the field passed directly from the transform in the template pipeline.


If transform to read from is selected, enter the length of the field passed directly from the transform in the template pipeline.


If transform to read from is selected, enter the precision of the field passed directly from the transform in the template pipeline.

Optional target file (hpl after injection)

For initial pipeline development or debugging, specify an optional file for creating and saving a pipeline of your template after metadata injection occurs. The resulting pipeline will be your template pipeline with the metadata already injected as constant values.

Streaming source transform

Select a source transform in your current pipeline to directly pass data to the Streaming target transform in the template pipeline.

Streaming target transform

Select the target transform in your template pipeline to receive data directly from the Streaming source transform.

Run resulting pipeline

Select to inject metadata and run the template pipeline. If this option is not selected, metadata injection occurs, but the template pipeline does not run.


  • Enter a pipeline filename in "Optional target file (hpl after injection)" so that a file will be generated after injection. This pipeline will contain all the properties after injection and can be viewed and executed for testing.