Reading a file that is being written to - Locking it?

Question

There is a file - stored on an external server which is updated very frequently by a vendor. My application polls this file every minute getting the values out. All I am doing is reading the file.

I am worried that by doing this I could inadvertently lock the file so it cant be written too by the vendor. Is this a possibility?

Kind regards

Answer 1

Further to Eric's answer - you could check the Last Modified Property of the temp file and only merge it with your 'working' file when it changes - that should protect you from read/write conflicts and only merge files just after the vendor has written to the temp. Though this is messy and mrab's comment is valid, a better solution should be found.

Answer 2

I have faced this problem several times, and as Peter Lawrey says there isn't any portable way to do this, and if your environment is Unix this should not be an issue at all as these concurrent access conditions are properly managed by the operating systems. However windows do not handle this at all (yes, that's the consequence of using an amateur OS for production work, lol).

Now that's said, there is a way to solve this if your vendor is flexible enough. They could write to a temp file and when finished move the temp file to the final destination. By doing this you avoid any concurrent access to the file between you and the vendor.

Another way is to precisely (difficult?) know the timing of your vendors update and avoid reading the file during some time frames. For instance if your vendor update the file every hour, avoid reading from five-to-the-hour to five-past-the-hour.

Hope it helps.

Answer 3

There is the Windows Shadow Copy service for volumes. This would allow to read the backup copy.

If the third party software is in java too, and uses a Logger, that should be tweakable: every minute writing to the next from 10 files or so.

I would try to relentlessly read the file (when modified since last read), till something goes wrong. Maybe you can make a test run with hundreds of reads in the weekend or at midnight, when no harm is done.

My answer: Maybe you need a local watch program, a watch service for a directoryr , that waits till the file is modified, and then makes a fast cooy; after that allowing the copy to be transmitted.