title: Outer Joins in Spark Structured Streaming
body: <p>Is there a way to perform <code>outer_join</code> (specifically left outer join) in Spark Structured Streaming? I noticed this <a href="https://issues.apache.org/jira/browse/SPARK-22053" rel="nofollow noreferrer">JIRA #22053</a> that enables inner join for streaming datasets (and it works great!). I was wondering if there was something similar for <code>outer_join</code> as well. 
  I tried using the same logic from the JIRA for outer join and haven't been able to get the right output. Thanks!</p>