Safe Data Saving

Question

Hi, I'm developing an application which reads data from a set of XML files, stores the data in the application as objects, allows the manipulation of these objects through the interface and then can write the XML files back again with the new data.
Currently, the entire XML file is simply re-written every time the save is used, which is fine. The problem I am having however, is that the objects (and therefore the XML) refer to images (by name) which are all stored in a single directrory. Whenever the user clicks "save" every image is moved to a temporary folder, then as the XML is written, copied back into the correct folder under a new name (I am simply using the File.createTempFile(...) function to generate unique filenames. All of this is fine (albeit inefficent) and works.
The only problem is when there is an error in saving (either writing the XML or copying the files) I have run out of ideas for methods to ensure the data integrity when such an error occurs. I successfully prevent the XML file being overwritten, but the Image names have changed due to the process and make it unusable. I am looking for ideas on how to ensure the save is atomic (i.e. either succeeds or fails, and isn't left in an unpredictable state)?
(Hope that explanation makes sense, I can post the save function if needed, but it's fairly ugly)

Answer 1

One quick and dirty way of achieving this is using file names to mark the state of the conversion. For example, name the configuration file config.xml.new when you are writing it, then delete config.xml, then rename config.xml.new to config.xml.
On startup, check for config.xml - if it is there, delete config.xml.new (if it exists). If it is not there, look for config.xml.new and rename it to config.xml.
The same strategy can be used with your directory names.
One final comment: If you are serious about achieving ACID semantecs, you need to be sure to sync the file (FileChannel.force(true)) before you close it. Otherwise the OS can cache the changes you made, and a power outage will corrupt your data. syncing to disk like this is quite slow, so use it sparingly.
- K