I have a 3rd party program that is running continuously, and is logging events in a text file. I want to write a small Haskell program that reads the text file while the other program is running and warns me when certain events are logged.
I looked around and it seems as if, for Windows, readFile is single write OR multiple read - it does not allow single write and multiple read. As I understand it, this is to avoid side effects like the write changing the file after/during reads.
Is there some way for me to work around this constraint on locks? The log file is only appended, and I am only looking for specific rows in the file, so I really don't mind if I don't get the most recent write, as I am interested in eventual consistency and will keep checking the file.
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I have been asked to create a system for allowing third party companies to dump data into several of our tables. These third parties provide csv files on a periodic basis, and after doing some research it seemed like Oracle themselves had a standard tool for doing so, "sqlldr". I've since gotten it working to an acceptable degree, and we have a job scheduled to run that script once a day.
But one of the third parties supplies really dirty data, of the sort where I can't expect it to always load every row/record (looking like up to about 8% will fail). My boss asked me to forward "all output" from the first few tests to him, and like a moron I also sent the log file.
He has asked that this "report" be modified to include those exceptions that aren't unique constraints along with the line in the input file that caused the exception.
This means that I need data from the log file, but also from the (I believe) reject file in a single document. Rather than write a convoluted shell script to combine those two, does SQL*Loader itself allow any customization that might achieve the same thing? I've read through the Oracle documentation and haven't found anything that suggests this, but I've also learned not to trust it entirely either.
Is this possible? Ideally, the solution would allow me to add values to the reject file that don't exist in the original input file, but I'm also interested in any customization of the log file or reject file.
No.
I was going to stop there, but you can define the name of the log file, which might help with issue. Most automation with SQL*Loader involves wrapping it within shell scripts; aka "roll your own."
New to this forum - looks great!
I have some Processing code that periodically reads data wirelessly from remote devices and writes that data as bytes to a file, e.g. data.dat. I want to write an Objective C program on my Mac Mini using Xcode to read this file, parse the data, and act on the data if data values indicate a problem. My question is: can my two different programs access the same file asynchronously without a problem? If this is a problem can you suggest a technique that will allow these operations?
Thanks,
Kevin H.
Multiple processes can read from the same file at a time without any problem. A process can also read from a file while another writes without problem, although you'll have to take care to ensure that you read in any new data that was written. Multiple processes should not write to the same file at at the same time, though. The OS will let you do it, but the ordering of data will be undefined, and you'll like overwrite data—in general, you're gonna have a bad time if you do that. So you should take care to ensure that only one process writes to a file at a time.
The simplest way to protect a file so that only one process can write to it at a time is with the C function flock(), although that function is admittedly a bit rudimentary and may or may not suit your use case.
I want to be able to (programmatically) move (or copy and truncate) a file that is constantly in use and being written to. This would cause the file being written to would never be too big.
Is this possible? Either Windows or Linux is fine.
To be specific what I'm trying to do is log video with FFMPEG and create hour long videos.
It is possible in both Windows and Linux, but it would take cooperation between the applications involved. If the application that is writing the new data to the file is not aware of what the other application is doing, it probably would not work (well ... there is some possibility ... back to that in a moment).
In general, to get this to work, you would have to open the file shared. For example, if using the Windows API CreateFile, both applications would likely need to specify FILE_SHARE_READ and FILE_SHARE_WRITE. This would allow both (multiple) applications to read and write the file "concurrently".
Beyond sharing the file, though, it would also be necessary to coordinate the operations between the applications. You would need to use some kind of locking mechanism (either by locking some part of the file or some shared mutex/semaphore). Note that if you use file locking, you could lock some known offset in the file to act as a "semaphore" (it can even be a byte value beyond the physical end of the file). If one application were appending to the file at the same exact time that the other application were truncating it, then it would lead to unpredictable results.
Back to the comment about both applications needing to be aware of each other ... It is possible that if both applications opened the file exclusively and kept retrying the operations until they succeeded, then perform the operation, then close the file, it would essentially allow them to work without "knowledge" of each other. However, that would probably not work very well and not be very efficient.
Having said all that, you might want to consider alternatives for efficiency reasons. For example, if it were possible to have the writing application write to new files periodically, it might be more efficient than having to "move" the data constantly out of one file to another. Also, if you needed to maintain some portion of the file (e.g., move out the first 100 MB to another file and then move the second 100 MB to the beginning) that could be a fairly expensive operation as well.
logrotate would be a good option is linux, comes stock on just about any distro. I'm sure there's a similar windows service out there somewhere
I am running a batch job that has been going for many many hours, and the log file it is generating is increasing in size very fast and I am worried about disk space.
Is there any way through the command line, or otherwise, that I could hollow out that text file (set its contents back to nothing) with the utility still having a handle on the file?
I do not wish to stop the job and am only looking to free up disk space via this file.
Im on Vista, 64 bit.
Thanks for the help,
Well, it depends on how the job actually works. If it's a good little boy and it pipes it's log info out to stdout or stderr, you could redirect the output to a program that you write, which could then write the contents out to disk and manage the sizes.
If you have access to the job's code, you could essentially tell it to close the file after each write (hopefully it's an append) operation, and then you would have a timeslice in which you could actually wipe the file.
If you don't have either one, it's going to be a bit tough. If someone has an open handle to the file, there's not much you can do, IMO, without asking the developer of the application to find you a better solution, or just plain clearing out disk space.
Depends on how it is writing the log file. You can not just delete the start of the file, because the file handle has a offset of where to write next. It will still be writing at 100mb into the file even though you just deleted the first 50mb.
You could try renaming the file and hoping it just creates a new one. This is usually how rolling logs work.
You can use a rolling log class, which will wrap the regular file class but silently seek back to the beginning of the file when the file reaches a maximum designated size.
It is a very simple wrap, either write it yourself or try finding an implementation online.
Any programmatic techniques, portable or specific to NT and Linux that get the result of number of large files loading faster? I am after a 'ahead of time', a prior, whatever you prefer to call it mechanisms that I can control in code for two OS in a question.
Each file has to be processed in full, i.e. completely in size and sequentially for its contents. The aim is to speed up some batch file processing.
I don't know about NT, but one option on Linux would be to use madvise with the MADV_WILLNEED flag shortly before you actually need the next file to start reading it in early.
Alternately, a more portable option would be to simply manually do readahead in a separate thread from your buffer-processing thread - that is, read data in to fill an X MB buffer in thread A, process it as fast as you can in thread B.
I am not aware of a Win32 (NT) API similar to madvise().
However, I would suggest an approach.
First, pass the Win32 flag FILE_FLAG_SEQUENTIAL_SCAN to CreateFile(). This will allow the Windows operating system to perform better buffering of the file once you have opened it.
With FILE_FLAG_SEQUENTIAL_SCAN, your file parser may operate more quickly once the file is in memory. Unlike madvise() on Linux, the file will not begin loading into memory any earlier due to the use of the Win32 flag.
Next, we need to trigger the file to begin loading. Asynchronously read the first page of the file by calling ReadFileEx() with an OVERLAPPED structure and a FileIOCompletionRoutine function.
Your FileIOCompletionRoutine can simply return, or you can set the event in the overlapped structure -- read the MSDN details of ReadFileEx for details.
Since it would not be a critical failure if the pre-fetch hasn't completed when you actually read from the file, the easiest implementation would be to "fire and forget" -- execute the overlapped file read and then never check the result of it. Be sure that you read the data into valid buffers, though!
If you perform this operation for a file while reading the previous file, the result should be that the next file will commence paging in.
Be aware that this may slow your performance. As the next file begins to page in, the disk I/O to access that file will compete with disk I/O for the file you are currently parsing. If the two files are physically distant from each other on the same disk, the result of pre-fetching might be additional delay as the drive head seeks. Although modern drives have huge buffers which mitigate this, queuing the first page of a new file is likely to cause a head seek.
bdonlan's suggestion of a 'pre-fetch' thread which loads the files asynchronously from the processing would be a workable solution for Win32, also.