Preventing multiple users from doing the same action

Question

I have a swing desktop application that is installed on many desktops within a LAN. I have a mysql database that all of them talk to. At precisely 5 PM everyday, there is a thread that will wake up in each of these applications and try to back up files to a remote server. I would like to prevent all the desktop applications from doing the same thing.

The way I was thinking to do this was:

After waking up at 5PM , all the applications will try to write a row onto a MYSQL table. They will write the same information. Only 1 will succeed and the others will get a duplicate row exception. Whoever succeeds, then goes on to run the backup program.

My questions are:

1. Is this right way of doing things? Is there any better (easier) way? I know we can do this using sockets as well. But I dont want to go down that route... too much of coding also I would need to ensure that all the systems can talk to each other first (ping)

2. Will mysql support such as a feature. My DB is INNO DB. So I am thinking it does. Typically I will have about 20-30 users in the LAN. Will this cause a huge overhead for the DB to handle.

Answer 1

If you could put an intermediate class in between the applications and the database that would queue up the results and allow them to proceed in an orderly manner you'd have it knocked.

It sounds like the applications all go directly against the database. You'll have to modify the applications to avoid this issue.

I have a lot of questions about the design:

1. Why are they all writing "the same row"? Aren't they writing information for their own individual instance?
2. Why would every one of them have exactly the same primary key? If there was an auto increment or timestamp you would't have this problem.
3. What's the isolation set to on the database connection? If it's set to SERIALIZABLE, you'll force each one to wait until the previous one is done, at the cost of performance.
4. Could you have them all write files to a common directory and pick them up later in an orderly way?

I'm just brainstorming now.

Answer 2

It seems you want to backup server data not client data.

I recommend to use a 3-tier architecture using Java EE. You could use a Timer Service then to trigger the backup.

Though usually a backup program is an independent program eg started by a cron job on the server. But again: you'll need a server to do this properly, not just a shared folder.

Answer 3

Here is what I would suggest. Instead of having all clients wake up at the same time and trying to perform the backup, stagger the time at which they wake up.

So when a client wakes up - It will check some table in your DB (MYSQL) to see if a back up job has completed or is running currently. If the job has completed, the client will go on with its normal duties. You can decide how to handle the case when the job is running. - If the client finds that the back up job has not been run for the day, it will start the back up job. At the same time will modify the row to indicate that the back up job has started. Once the back up has completed the client will modify the table to indicate that the back up has completed.

This approach will prevent a spurt in network activity and can also provide a rudimentary form of failover. So if one client fails, another client at a later time can attempt the backup. (this is a bit more involved though. Basically it comes down to what a client should do when it sees that a back up job is on going).