When you add a transformation and connect it to a destination supporting cloud mode, RudderStack:

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

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;
}

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

Applying transformation on a batch of events

You can also 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;
}

Limitations

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