Github user rdblue commented on a diff in the pull request:

    https://github.com/apache/spark/pull/20386#discussion_r164907626
  
    --- Diff: 
sql/core/src/main/java/org/apache/spark/sql/sources/v2/streaming/writer/StreamWriter.java
 ---
    @@ -32,40 +32,44 @@
     @InterfaceStability.Evolving
     public interface StreamWriter extends DataSourceWriter {
       /**
    -   * Commits this writing job for the specified epoch with a list of 
commit messages. The commit
    -   * messages are collected from successful data writers and are produced 
by
    -   * {@link DataWriter#commit()}.
    +   * Commits this writing job for the specified epoch.
        *
    -   * If this method fails (by throwing an exception), this writing job is 
considered to have been
    -   * failed, and the execution engine will attempt to call {@link 
#abort(WriterCommitMessage[])}.
    +   * When this method is called, the number of commit messages added by
    +   * {@link #add(WriterCommitMessage)} equals to the number of input data 
partitions.
    --- End diff --
    
    What does this mean? It isn't clear to me what "the number of input 
partitions" means, or why it isn't obvious that it is equal to the number of 
pending `WriterCommitMessage` instances passed to add.


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