This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Hi, My directions do not require me to lock down the complete database in response to the client sending in a -1 argument to the lock() method. However, my server needs to shut down elegantly without damaging the file or losing information. When my server is told to start shutting down, when a client then calls the lock method, the server raises an exception: "Server shutting down, no lock requests are being accepted at this time." Then, my original idea was simply to monitor the number of threads attempting to complete from the LockManager. However, I noticed that the number of threads in the LockManager can be zero, and the server still isn't ready to shut down! So, what I will do is the following to safely shut down the server: 1. When it is time to shut down, the server will no longer accept lock requests, and raise an exception as discussed above. 2. Furthermore, the server will monitor disk writes; so, for instance, if no create, delete, or update method has been called in the last two minutes, the server will conclude that there are no outstanding writes. When both conditions above are true: 1. No threads in the lock manager, 2. No disk writes in the last two minutes, only then will the server assume that it is safe to shut down. Here are my test runs. I'm again using my version of George's Super Software Exerciser to drive the tests, but in this particular run, updates, creates, and deletes are all occuring. Note how the number of waiting threads drops to zero for a quite a few minutes in the middle of the run! Only to pick up again later. I suspect that this behavior is related to the Mac OS X operating system. Aside: in this test run, I've doubled the usual size of the database file to contain 62 records. Also, there were 620 instantiations of the tester class wherein each instantiation represents one Data instance (i.e., it contains one Data instance).
Hi Javini, I am just getting started on my project so you are way ahead of me.
1. When it is time to shut down, the server will no longer accept lock requests, and raise an exception as discussed above.
I like this approach and expect to do this myself when I get that far.
2. Furthermore, the server will monitor disk writes; so, for instance, if no create, delete, or update method has been called in the last two minutes, the server will conclude that there are no outstanding writes.
I generally don't like timed waits as they tend to delay things unneccessarily. It also seems like no matter how long you wait, there is no guarantee with this approach. It seems to me that you could simply lock every record in the database and be done much quicker than waiting for two minutes of complete idle time. Now on a very large database this would not scale at all but if we had a very large database for this assignment, a lot of other things would need to change too. So it seems to me that locking every record is reasonable and faster.
Joined: Dec 03, 2003
Hi, Thanks again for reading and responding to my posting. You have a good point about the waiting. What I think that I will do is have a preferences item specify how many minutes maximum the server will wait before shutting down. Also, a preferences item saying how much time must expire before it is assumed that no more writes will occur. Example: Maximum Wait Time: 15 minutes No More Writes Can Occur Time Assumed: 2 minutes
So, the shutdown sequence will look something like this. First, set a flag so that no new clients can obtain any locks. If the lock manager says no records are locked, then shutdown is possible. If the lock manager says that at least one record is locked, then the client holding this lock has the user-specified time to complete whatever operation was being conducted on that record. Finally, as it is possible, as shown on my computer, for the lock manager to be empty, and yet for 15 minutes of writes to continue, while I am waiting for the maximum user-specified wait time, I will periodically check for any writes having occurred. If the lock manager is empty, and no writes have occurred within so many minutes, let's say user-specified at 2 minutes, then it is safe to shutdown prior to the maximum allowable wait time of 15 minutes. The wait time, if found undesirable by the user, can be set to very low values. NO GUARANTEES -------------- Your point about there being no guarantees is very good. I may also have a status panel on the server, where it shows the number of currently held locks, and the last time a write occurred. The system administrator can use his own judgement as to when it is safe to shut down the server. This button may be called: Override and Shut Down Right Now. Because the system administrator can see what the situation is, there is no need to wait for any number of minutes. Thanks, Javini Javono [ March 27, 2004: Message edited by: Javini Javono ]