Backup application with single instance functionality

Question

Hi dudes,

Currently working on an application, that help you to take backup of the files in you machine at the server (hosted by the company itself), so that you can recover data after any hdd crash. I have implemented Single Instance feature, across the users.

Single Instance : A file uploaded already at the server, wouldn't be uploaded again. Whenever any other instance of the exact file uploaded will not be actual upload but some database changes and linked to the same previously uploaded file.

Issue arise when same file (that has not already been uploaded before) is uploaded simultaneously by more than one users, On Start file wouldn't be detected for an instance (as database is updated only after successful upload/backup). All are running, at once. What will be the best way to implement single instance in this way.

I am thinking when I let all the instance upload as it is. So more than one instance of the file will reside at the server. But whenever another backup of the same file will be taken afterwards, I will remove all the previous instances and link them up with the one. This will not let user double uploads and also less complex on the cost of some disc space that too for a while probably (till next upload of the same file will be done)

Thanks for your thoughts in advance.

Answer 1

Calculate the hash (signature) of the file before upload and store it in the DB. Then - start uploading. if a similar file will be mark for uploading during the upload of the first file (you will know b/c you already saved the hash) - you will hold the 2nd file upload, until the first one finish successfully, and then link, if the 1st on fails, you can go to the 2nd source and upload it.