Which of the following methods will result in an invalid stream-static join?

A stream-static join is a specific operation in data processing that combines a static DataFrame with a streaming DataFrame based on common keys. In the context of the question, the focus is on the types of joins that can be performed, and the methods indicated can have different implications regarding their validity when working with streaming data.

The outer join, which is the method identified in the answer, is not allowed when performing stream-static joins because outer joins intend to include all records from both data sources—the static and the streaming. In streaming scenarios, including all records indefinitely can lead to ambiguity and complications in stream processing, especially since the streaming dataset keeps changing over time.

Inner and left joins, on the other hand, are valid as they can effectively map the streaming data against the static data and return either matched records or all records from the streaming source with matches from the static dataset, respectively. This behavior aligns well with the requirements of streaming applications, which typically need to handle real-time data efficiently without creating complications associated with unbounded result sets.

Thus, the method leading to an invalid stream-static join is the outer join because of its potential to create an unbounded output in a streaming context, which does not conform to established guidelines within stream processing frameworks.