Is the logfile cleared or moved and a new one created? If it's just cleared, you *will* miss some data eventually.

Since i don''t know what the data fields mean, it is rather hard to give advice? Is there some unique identifier for each line? If not, then it's rather hard to find a working solution - it can be done, but it depends on a number of small implementation details.

A general way would be to either have the above mentioned unique identifier or to bluntly count lines and hope for the best. If the first line in the file doesn't match the first line of the file you "remembered" from the last run, open an older file and try again. Then, when you found the correct file, work on that. When finished, check for newer files and work on them in correct order. As Last step, "remember" the first line of the last file you worked on.