I have a text file and multiple threads/processes will write to it (it's a log file).
The file gets corrupted sometimes because of concurrent writings.
I want to use a file writing mode from all of threads which is sequential at file-system level itself.
I know it's possible to use locks (mutex for multiple processes) and synchronize writing to this file but I prefer to open the file in the correct mode and leave the task to System.IO.
Is it possible ? what's the best practice for this scenario ?
Your best bet is just to use locks/mutexex. It's a simple approach, it works and you can easily understand it and reason about it.
When it comes to synchronization it often pays to start with the simplest solution that could work and only try to refine if you hit problems.
To my knowledge, Windows doesn't have what you're looking for. There is no file handle object that does automatic synchronization by blocking all other users while one is writing to the file.
If your logging involves the three steps, open file, write, close file, then you can have your threads try to open the file in exclusive mode (FileShare.None), catch the exception if unable to open, and then try again until success. I've found that tedious at best.
In my programs that log from multiple threads, I created a TextWriter descendant that is essentially a queue. Threads call the Write or WriteLine methods on that object, which formats the output and places it into a queue (using a BlockingCollection). A separate logging thread services that queue--pulling things from it and writing them to the log file. This has a few benefits:
Threads don't have to wait on each other in order to log
Only one thread is writing to the file
It's trivial to rotate logs (i.e. start a new log file every hour, etc.)
There's zero chance of an error because I forgot to do the locking on some thread
Doing this across processes would be a lot more difficult. I've never even considered trying to share a log file across processes. Were I to need that, I would create a separate application (a logging service). That application would do the actual writes, with the other applications passing the strings to be written. Again, that ensures that I can't screw things up, and my code remains simple (i.e. no explicit locking code in the clients).
you might be able to use File.Open() with a FileShare value set to None, and make each thread wait if it can't get access to the file.
Related
So, I'd like to write a logger in c# for an app I'm working on. However, since I love efficiency, I don't want to be opening and closing a log file over and over again during execution.
I think I'd like to write all events to RAM and then write to the log file once when the app exits. Would this be a good practice? If so, how should I implement it?
If this is not a good practice, what would be?
(And I'm not using Windows' event log at this time.)
However, since I love efficiency, I don't want to be opening and closing a log file over and over again during execution
No. It's since you love premature optimizations.
I think I'd like to write all events to RAM and then write to the log file once when the app exits. Would this be a good practice? If so, how should I implement it?
If you love efficiency, why do you want to waste a lot of memory for log entries?
If this is not a good practice, what would be?
It is if you want to lose all logs when your application crashes (since it cannot write the log to disk then). Why did you create the log in the first place?
You'll have to think of some issues you might encounter:
System being shut down while your application runs -> no log files
An application crash might not invoke your write method
If the log grows large (how long does your application run?), you might get memory problems
If the log grows large, and there is not enough space on the drive, not a single log line will be written
You could simply keep the file open while your application runs (with at least FileShare.Read so you can monitor it), or consider writing batches of log lines, invoking the write method after a group of methods, or even using a timer.
Well if your app crashes, you lose all your logs. Better if you flush the logs to disk at an appropriate moment.. Lazy write:
Queue off the log entries to a seperate logger thread, (ie. store them in some class and queue the class instance to a producer-consumer queue). In the logger thread, wait on the input queue with a timeout. If a log entry comes in, store it in a local cache queue.
If (timeout fires) or (some high water mark of logs stored is reached) then write all cached log entries to the file and flush file buffers.
Rgds,
Martin
I am trying to implement file based autoincrement identity value (at int value stored in TXT file) and I am trying to come up with the best way to handle concurrency issues. This identity will be used for unique ID for my content. When saving new content this file gets opened, the value gets read, incremented, new content is saved and the incremented value is written back to the file (whether we store the next available ID or the last issued one doesn't really matter). While this is being done another process might come along and try to save new content. The previous process opens the file with FileShare.None so no other process will be able to read the file until it is released by the first process. While the odds of this happening are minimal it could still happen.
Now when this does happen we have two options:
wait for the file to become available -
Emulate waiting on File.Open in C# when file is locked
we are talking about miliseconds here, so I guess this wouldn't be an issue as long as something strange happens and file never becomes available, then this solution would result in an infinite loop, so not an ideal solution
implement some sort of a queue and run all operations on files within a queue. My user experience requirements are such that at the time of saving/modifying files user should never be informed about exceptions or that something went wrong - he would get informed about them through a very friendly user interface later when operations would fail on the queue too.
At the moment of writing this, the solution should work within ASP.NET MVC application (both synchronously and async thru AJAX) but, if possible, it should use the concepts that could also work in Silverlight or Windows Forms or WPF application.
With regards to those two options which one do you think is better and for the second option what are possible technologies to implement this?
The ReaderWriterLockSlim class seems like a good solution for synchronizing access to the shared resource.
I have 3 processes each of which listens to a data feed. All the 3 processes need to read and update the same file after receiving data. The 3 processes keep running whole day. Obviously, each process needs to have exclusive read/write lock on the file.
I could use "named mutex" to protect the read/write of the file or I can open the file using FileShare.None.
Would these 2 approaches work? Which one is better?
The programe is written in C# and runs on Windows.
Use a named mutex for this. If you open the file with FileShare.None in one process, the other processes will get an exception thrown when they attempt to open the file, which means you have to deal with waiting and retrying etc. in these processes.
I agree with the named mutex. Waiting / retrying on file access is very tedious and exception-prone, whereas the named mutex solution is very clean and straightforward.
"monitor" is one of you choise since it is the solution of synchronization problem.
I have several processes running concurrently that I want to log to the same file.
We have been using Enterprise Library 4.1 Logging Application Block (with a RollingFlatFileTraceListener), and it works fine, apart from the fact that it prepends a GUID to the log file name when two processes try to write to the log file at the same time (a quirk of System.Diagnostics.TextWriterTraceListener I believe).
I've tried various things, including calling Logger.Writer.Dispose() after writing to the log file, but it's not ideal to do a blocking call each time a log entry is being written.
The EntLib forums suggest using MSMQ with a Distributor Service, but that is not an option as MSMQ is not allowed at my company.
Is there another way I can quickly and easily log from multiple threads/processes to the same file?
Sorry to say but the answer is no. The File TraceListeners lock the output file so only one TraceListener can log to a file.
You can try other Trace Listeners that are not file based (e.g. Database, Event Log).
Another option I can think of would be to write your own logging service (out of process) that would log to the file and accepts LogEntries. Then create a custom trace listener that sends a message to your service.
It might not be a good idea since you would have a bit of custom development plus it could impact performance since it is an out of process call. Basically you are setting up your own simplified-pseudo-distributor-service.
EntLib locks the log file when it writes to it. Therefore, 2 processes cannot write to the same log file.
When we have had this problem, that we needed to log from many difference places, to the same place, we have used database logging.
If you are 100% stuck logging to a text file, then you could log to individual log files, and then write a program to merge these files.
I know this is old, but if you are still curious. log4net supports this:
http://logging.apache.org/log4net/release/faq.html#How do I get multiple process to log to the same file?
The problem occurs when the App Pool Recycles and allows for Overlapping Threads. The closing thread has it still open, and the new thread gets the error. Try disabling the overlapping recycling behavior in IIS, or create your own version of the text writer.
how to write to a text file that can be accessed by multiple sources (possibly in a concurrent way) ensuring that no write operation gets lost?
Like, if two different processes are writing in the same moment to the file, this can lead to problems. The simples solution (not very fast and not very elegant) would be locking the file while beginning the process (create a .lock file or similar) and release it (delete the lock) while the writing is done.
When beginning to write, i would check if the .lock file exists and delay the writing till the file is released.
What is the recommended pattern to follow for this kind of situation?
Thanks
EDIT
I mean processes, like different programs from different clients, different users and so on, not threads within the same program
Consider using a simple database. You will get all this built-in safety relatively easy.
The fastest way of synchronizing access between processes is to use Mutexes / Semaphores. This thread answers how to use them, to simulate read-writer lock pattern:
Is there a global named reader/writer lock?
I suggest using the ReaderWriterLock. It's designed for multiple readers but ensures only a single writer can writer data at any one time MSDN.
I would look at something like the Command Pattern for doing this. Concurrent writing would be a nightmare for data integrity.
Essentially you use this pattern to queue your write commands so that they are done in order of request.
You should also use the ReaderWriterLock to ensure that you can read from the file while writing occurs. This would be a second line of defense behind the command pattern so that only one thread could write to the file at a given time.
You can try lock too. It's easy - "lock ensures that one thread does not enter a critical section while another thread is in the critical section of code. If another thread attempts to enter a locked code, it will wait (block) until the object is released."
http://msdn.microsoft.com/en-us/library/c5kehkcz%28VS.71%29.aspx
I would also recommend you look for examples of having multiple readers and only 1 writer in a critical section heres a short paper with a good solution http://arxiv.org/PS_cache/cs/pdf/0303/0303005v1.pdf
Alternatively you could look at creating copies of the file each time it is requested and when it comes time to write any changes to the file you merge with the original file.