PC Ext: Explanation of .PBB and .0LK Files (128913)

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A Microsoft Mail for PC Networks mail system may contain more than one EXTERNAL.EXE to send mail to either a remote user or an external postoffice. Likewise, multiple EXTERNAL.EXE programs may serve identical functions (for example, operate against the same group of postoffices or remote users). If more than one EXTERNAL.EXE is used against the same postoffices, the programs must be able to access and generate mail items without conflicting with one another.

The .0LK file is used to control access to a specific mail outbound queue, and the .PBB file is used to ensure that the same mail item can be sent to multiple destinations by different Externals at the same time.

OLK Files

For outbound mail, EXTERNAL.EXE "locks" the mailbag containing the outbound mail by creating a semaphore file in the P1 subdirectory. The creation and use of semaphore files in the P1 subdirectory keeps multiple EXTERNAL.EXE programs from trying to do the same operation against the same mailbag. Only the EXTERNAL.EXE that currently owns the lock file can send outbound mail from the mailbag. When the session is completed, the lock is removed.

The semaphore locking mechanism is a three step process. The first step is the creation of a file with the mailbag name for a prefix, and .0LK for a suffix. The file is created in the P1 subdirectory. The second step is to verify that there are no older types of lock files. The third step is to open the file with read/write, deny-all permissions. If these steps succeed, the EXTERNAL.EXE program can service the mailbag. When the session is complete, the file is closed and deleted.

If the lock file is stranded for any reason (system hang, machine rebooted in middle of session, etc.), this file-stranding event does not automatically create a problem. This will depend on the status of the file at the server.

The existence of the file is not a problem in itself, because the next EXTERNAL.EXE to attempt a lock may be able to open the stranded semaphore lock file, perform the mail transfer operations, and remove the semaphore file when finished. If none of the External programs can use the lock file, it will have to be manually closed through the server administrative functions.

PBB Files

For each message, only one .MAI file is created. Therefore, even though a single message may be sent to many users, only one .MAI file has been created for that message. When multiple EXTERNAL.EXE programs deliver the same mail message from a single postoffice to different users (either different remote users or different postoffices), each EXTERNAL.EXE program references that single .MAI file from each of the remote users (or postoffices) individual mailbag files. For example, Remote User One and Remote User Two, each have their own mailbag file. Both of those mailbag files point to a single .MAI message file. Because each remote user has his or her own mailbag, two separate EXTERNAL.EXE programs may deliver that single message to both users at the same time.

To send that single message to each remote user or postoffice, the EXTERNAL.EXE program must generate a file containing the addressing information for that .MAI message file. Each instance of EXTERNAL.EXE must generate a unique addressing-information file for each remote user or postoffice.

These unique addressing-information files have the format of XXXXXXXX.PBB where XXXXXXXX is the hexadecimal identifier (hex-id) of the .MAI file and the BB is a two letter identifier generated as described in the following paragraphs. An example of such a file is 000005AF.PBB. EXTERNAL.EXE generates and places the PBB files in the P1 subdirectory of the postoffice.

During a remote or dispatch session, EXTERNAL.EXE generates a unique P1 Locking ID or extension before generating any outgoing mail. It does this via an algorithm that tries to create a file LOCKP1ID.xxx, where xxx is an extension that can be anything from .PBB to .PZZ. If LOCKP1ID.PBB is already in use, an attempt to create "LOCKP1ID.PBC" is done. If this cannot be done, then the extension is again incremented until all possibilities are exhausted. There are at least 500 possible combinations, so the probability of that many concurrent sessions occurring is incredibly small.

Once the P1 lock (that is, extension) is generated for the session, EXTERNAL.EXE uses it for the filename extension for generating the addressing-information file. This allows for multiple EXTERNAL.EXEs to dispatch the same piece of mail to multiple remote recipients. You will get filenames like xxx.PBB and xxx.PBC for the same piece of outgoing mail if a concurrent transmission is occurring (the prefix will be the same as the .MAI).

When the session is complete, EXTERNAL.EXE will clean up by deleting any generated files, including the lock file(s).