[Rd] Risk of readRDS() not detecting race conditions with parallel saveRDS()?

[Henrik Bengtsson]

Hi Bill,

yes, emulating atomic writing by writing to a temporary file and then
renaming definitely lowers the risk for corruptions.  I actually take
a similar approach in the Aroma Project (aroma.affymetrix et al.),
R.utils::saveObject(), R.utils::downloadFile() and more, and it
provides a great protection against user-interrupts, power failures
and so on.  I've been considering adding it to R.cache as well.

However, I'm not sure that it is guaranteed to be truly atomic.  I'm
saying this because ~8 years I was running batch jobs on 50 computers
on a shared file system.  Each R process was looking for remaining
"job" directory (=one job) and if found, it renamed/moved it
immediately so no other process would find/grab the same job.
However, it turned out that occasionally two separate R processes (on
different machines) could grab and move that same directory at the
"same" time (holding on to the same file target), proceed with the
analysis and write the results to file (which then would contain
interweaved results from the two parallel runs).  From that I learned
that on certain NFS file systems, it can take up to 30 seconds(!)
before file updates are seen by all computers.  Of course, what you're
proposing is somewhat different - first creating a unique temporary
file for each process which is then renamed to a common file.  The
question is how this is affected by above file system delays etc.

So to summarize my strategy, I'd like to add all possible layers of
protection (that are not too expensive) against race conditions in
order to minimize any risks for errors and if errors still occur I'd
like to be able to detect them, and all this without assuming to much
about the file systems.  It's only as a last resort I want to turn to
coordinated approaches via a main server (mutex handler; TCPIP is
guaranteed to truly atomic everywhere) ...and I don't want to reinvent
cluster OSes.

Thanks for you feedback

/Henrik

On Sat, Sep 15, 2012 at 12:44 PM, William Dunlap <wdunlap at tibco.com> wrote:
> Why not write the RDS file more atomically - write it to a
> temporary file and rename that file to its final name when
> it is completely written?  E.g.,
>
> saveRDS.atomically
> function (object, file, ...)
> {
>     tfile <- tempfile(basename(file), dirname(file))
>     on.exit(if (file.exists(tfile)) unlink(tfile))
>     retval <- saveRDS(object, tfile, ...)
>     if (!file.rename(tfile, file)) { # perhaps want an if(file.exists(file))unlink(file) first
>         stop("Cannot rename temporary file ", tfile, " to ",
>             file)
>     }
>     invisible(retval)
> }
>
> (The file.rename may be tripped up by an overeager virus checker looking
> at the newly created tfile.  I don't know the best way to deal with that.)
>
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com
>
>
>> -----Original Message-----
>> From: r-devel-bounces at r-project.org [mailto:r-devel-bounces at r-project.org] On Behalf
>> Of Henrik Bengtsson
>> Sent: Saturday, September 15, 2012 10:22 AM
>> To: R-devel
>> Subject: [Rd] Risk of readRDS() not detecting race conditions with parallel saveRDS()?
>>
>> I hardly know anything about the format used in (non-compressed)
>> serialization/RDS, but hoping someone with more knowledge could give
>> me some feedback;
>>
>> Consider two R processes running in parallel on the same unknown file
>> system.  Both of them write and read to the same RDS file foo.rds
>> (without compression) at random times using saveRDS(object,
>> file="foo.rds", compress=FALSE) and object2 <-
>> readRDS(file="foo.rds").  This happens frequently enough such that
>> there is a risk for the two processes to write to the same "foo.rds"
>> file at the same time (here one needs to acknowledge that file updates
>> are not atomic nor instant).
>>
>> To simulate the event that two processes writes to the same file at
>> the same time (and non-atomically) results in a interweaved/appended
>> "foo.rds" file, I manually corrupted "foo.rds" by
>> inserting/dropping/replacing a single random byte.  It appears that
>> readRDS() will detect this simple event, by throwing an error on
>> "unknown input format", which is what I want.  My question is now, is
>> it reasonable to assume that if two or more processes happen to write
>> to the same RDS file at the same time, it is extremely unlikely (*)
>> that they would generate a file that would pass as valid by readRDS()?
>>  (*) extremely unlikely = if all of us would run this toy example we
>> would not end up with a non-detect but still corrupt "foo.rds" file
>> in, say, 10000 years.
>>
>> Background: The R.cache package allows memoization (caching of
>> results) to file such that the cache is persistent across R sessions.
>> The persistent part is achieved by writing cache files to the same
>> file directory.  This is safe when you run a single process, and even
>> if readRDS() would fail to read a cache file it is no big deal; the
>> memoization will just fail and the results will be recalculated and be
>> resaved.  The questions is what happens if you run this in parallel
>> and push it to the extreme; is there a risk that the memoization will
>> properly return but with invalid results.  I prefer not having to
>> synchronize this with a mutex/semaphore/common server, but instead
>> rely on this try-an-see approach (cf. the Ethernet protocol on shared
>> medium).  My guess (and hope) is that the risk is extremely unlikely
>> (*), but I'd like to hear if someone else thinks otherwise.
>>
>> Thanks,
>>
>> Henrik
>>
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