On Dec 7, 2005, at 1:16 PM, Joel Guillod wrote:
Wanted functions:
- To be able to synchronize each set (in order to have mirrors sets
of the xml documents);
This can be rather simple to accomplish. Add a last modified date
stamp to each document. Have a master db with the up-to-date version
of all documents. On a regular basis, maybe every minute or longer,
look to see what data sets acquired a last modified date that is
greater than the last stored last modified date. If a document has
been modified, send it to the master server. Periodically, every so
often, look to see if the master has any documents older than your
last last modified date. If so, grab those and replace what you have
internally.
Of course, all this postulates that your data does not change rapidly
OR that you infrequently have two people modifying the same data set
at the same time. If that is not a valid assumption, then you'll need
to institute a locking mechanism. When someone wants to change
something, make them press a button to indicate the desire to lock
that record. Grab the updates, send a record lock (with a time stamp)
to the master, and then give them some number of minutes to make the
desired changes. If the changes are not made within the desired
amount of time, have the master unlock the record and have the client
not save the changes (unless no one else has made a change to the
record which you can tell by the timestamp at the master).
But the key to it all is to date time stamp everything. Also, you are
using a date time stamp but keep in mind that clocks can get out of
sync so treat the date timestamp as an incrementing marker, not as
the actual time. Thus, when you update to the master, the master
creates the timestamp and that is what is used throughout the system.
Kee Nethery
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