DESIGN OF SCHEME TO KEEP REMOTE, UNSYNCHRONIZED NGAS DB SYNCHRONIZED
IN CASE OF REMOVAL OF FILE ENTRIES:
====================================================================
2003-05-16, Jens Knudstrup


o An XML document NgasRemoveStatus.xml is introduced, which contains

	1. A list of all files deleted from the disk.
	2. A list for each DB server/DB (<DB Server>.<DB Name> = <DB ID>), 
           indicating if the files were deleted from that DB.

The lay-out of this NGAS Remove Status is as follows:

<NgamsStatus>
  <DiskStatus ...>
    <FileStatus .../>
    ...
  </DiskStatus>
  <FileList Id="<DB ID>" Comment="File remove status">
    <FileStatus .../>
    ...
  </FileList>
  ...
</NgamsStatus>

This file is stored as:

<NGAS Mt Pt>/<Disk Mt Pt>/NgasRemoveStatus.xml


o When executing a REMDISK command on a disk, the NgasRemoveStatus.xml
file is not be removed.


o For each file being deleted, a small status document is generated
in the directory:

<NGAS Mt Pt>/<Disk Mt Pt>/staging/NgasRemoveStatus/

The format of this document is:

<NgamsStatus>
  <FileList Id="<DB ID>" Comment="File removal list">
    <FileStatus ... />
  </FileList>
</NgamsStatus>

The of these temporary files will be an ISO8601 timestamp + an unique index
(extension: "xml").

A function will be provided which can merge these file removal status
documents into the NgasRemoveStatus.xml in a secure way (i.e., no risk
of losing information).


o When executing a REMDISK command the 
"<NGAS Mt Pt>/<Disk Mt Pt>/staging/NgasRemoveStatus/" directory should not
be removed.


o When NG/AMS goes Online it is checked (by the Janitor Thread) if there are 
Removal Status Documents pending to be merged into NgasRemoveStatus.xml 
and if all files marked as deleted actually are deleted in the connected DB.
Latter is checked by comparing the two lists in the NgasRemoveStatus.xml
document. If entries are found to be deleted, these are deleted in the
NGAS DB and Removal Status Documents created accordingly.


o A synchronization scheme should be implemented to ensure that the
Data Consistency Checking is not scheduled before the Janitor Thread
has executed at least once to ensure that the DB is up to date before
starting the checking to avoid that false error messages are being send out.


o After a REMFILE or REMDISK command has been executed the Janitor Thread
should be triggered to have the NgasRemoveStatus.xml file updated
instantaneously.

--- oOo ---