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A control file is a special file that is sent along with one or more data files to tell applications that handle the data files how to handle them.  Control files are typically created by the same application that original sends files into a file transfer system.

The most common type of control file is a “trigger file“.  Trigger files are used to initiate further processing or retransmission of files already uploaded to or present on a separate file transfer system.

Other types of control files are often used to communicate final file names, intermediate processing applications, conditions of transfer/processing success or other metadata.

There is no standard format for control files, but many control files are either simple “flat files” with one file configuration entry in each line or simple XML files.

There are three general expectations made on file transfer systems by applications that submit control files.

Send and Forget (No Control File): When applications send files into a file transfer system they rely on the file transfer system to correctly determine when files can safely be processed or retransmitted.   This is often a safe assumption, especially when each file can be processed on its own and each file can be processed immediately.

Send and Commit: The use of a control file allows applications to commit a group of transmitted files to final acceptance, further processing or retransmission.  The use of a control file is often used here when two or more files need to be submitted together to allow final acceptance, further processing or retransmission.

Send, Commit and Confirm: If a file transfer application returns a status code or file after acting on a submitted control file then it can confirm both the original files sent and the contents of the control file.  The use of the extra confirmation step is often used when the submitting application wants to check up on the transfer status or history of its submitted files.  (In this case, either the submitted control file or resulting confirmation file will contain an ID the submitting application can use to perform lookups after the initial confirmation.)