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JMF under the Magnifying Glass

Job Messaging Format is the messaging language, which JDF makes available. In brief, JMF is an information carrier whose task is to send messages between the management system and the production equipment. The actual messages are short and precise and are created, like JDF files, using an XML structure.

With these messages, the management system can steer the production equipment, dynamically update jobs, control the status of machines, manage lines, handle data exchange, and acquire status reports to more accurately perform after-calculations.

The idea is that the management system, with JMF’s help, will be able to achieve a type of “plug and play” compatibility between the production equipment that is part of the flow. This can come about by the management system sending special JMF messages to the equipment in the flow, which simply ask what the equipment is capable of and how it wants to receive JDF files. The equipment responds and an agreement is reached. Often, this short dialog is referred to as a handshake procedure.

Sending JDF & JMF
When one sends JDF files or JMF messages, it usually takes place over HTTP. The files are sent to a hot folder where the receiving program is in constant wait for incoming jobs. The major difference between sending JDF files directly and using JMF messages for sending JDF files is that the JMF method allows for receiving back status reports. If one chooses to send a JDF file with JMF, it can be done two ways: referring or MIME packaging. Which method one uses depends on, among other things, which method the receiver has support for.

Referring
As JMF cannot contain other documents it uses external references. In practice, what happens is one sends a JMF message with a link (URL) to where the JDF file is located. This works in the exact same way as when linking to a picture in a HTML document. The JDF file can then be retained in the management system, which gives more control over the file.

MIME Package
Packaging is handled identically to how an email program handles attachments. JMF messages are packaged together with one or several JDF files, plus eventual pictures and color profiles. The MIME package is then sent to the receiver who unpacks and processes the JMF file, which refers to the included JDF files.

When the receiving software receives a JMF message, it is placed in a line. In the line are all the jobs, which are to be executed. With help from different JMF commands/messages, it is possible, from the management system, to manipulate the line and pose questions to the equipment.

Below are several examples of commonly occurring JMF messages. Some of them manipulate the line while others are used for the handshake procedure mentioned above:

• SubmitQueueEntry
  This command starts a process and sends a JDF node to a line. This is the only way to command a JDF compatible machine to execute a JDF file.
   

• HoldQueue
  This command pauses the line.
   

• QueueStatus
  With this command, one can check the status of the line.
   

• ResumeQueue
  If one wants to restart the line, this is the command to use.
   

• NewJDF
  This command allows a machine to tell the management system that is has begun a job without having been given a JDF file. For example, one could imagine that prepress begins working with a file without it having come from MIS.
   

• KnownDevice
  Asks what equipment is connected to the receiving controller.
   

• KnownMessage
  Asks what type of messages the production equipment can receive.
   

• SubmissionMethod
  Asks how the files can be received (MIME or URL).
   

• ResourceQuery
  This command can be used, for example, to ask how much paper is left.

There is much more to be said about JMF and how it is used. If you are interested in learning more, please visit www.CIP4.org.