When you add a transformation and connect it to a cloud mode-supported destination, RudderStack does the following:

1. Tracks and collects events at the source.
2. Applies the transformation logic to your events.
3. Forwards the transformed event to your connected destination.

Applying transformation on a batch of events

You can perform any aggregation or roll-up operation on a batch of events using the transformBatch function instead of transformEvent, as shown:

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export function transformBatch(events, metadata) {
    return events;
}

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def transformBatch(events, metadata):
    return events

Debugging transformations

Once you add a transformation, you can capture any event-related information in the form of logs while running a test on your transformation. You can do this by including the log function in your transformation code.

An example of using the log function is shown below:

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export function transformEvent(event, metadata) {
  const meta = metadata(event);
  event.sourceId = meta.sourceId;

  log("Event Name is", event.event, ";", "Message ID is", event.messageId);
  log("Source ID is", meta.sourceId);

  return event;
}

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def transformEvent(event, metadata):
    meta = metadata(event)
    event['sourceId'] = meta['sourceId']
    
    log("Event Name is", event['event'], ";", "Message ID is", event['messageId'])
    log("Source ID is", meta['sourceId'])
    
    return event

On adding the above transformation and clicking Run Test, you can see the resulting log in the Logs section of the dashboard, as shown:

Limitations

The transformation fails if the following memory and time limits are exceeded: