As far as I know, the current implementation of file sinks is the only reason why the flag IGNORE_MISSING for copying even exists - there's no other compelling reason to justify it. We implement "rename" as "copy, then delete" (in a single DoFn), so for idempodency of this operation we need to ignore the copying of a non-existent file.
I think the right way to go would be to change the implementation of renaming to have a @RequiresStableInput (or reshuffle) in the middle, so it's made of 2 individually idempotent operations: 1) copy, which would fail if input is missing, and would overwrite output if it exists -- reshuffle -- 2) delete, which would not fail if input is missing. That way first everything is copied (possibly via multiple attempts), and then old files are deleted (possibly via multiple attempts). On Wed, Jan 31, 2018 at 2:26 PM Udi Meiri <[email protected]> wrote: > I agree that overwriting is more in line with user expectations. > I believe that the sink should not ignore errors from the filesystem > layer. Instead, the FileSystem API should be more well defined. > Examples: rename() and copy() should overwrite existing files at the > destination, copy() should have an ignore_missing flag. > > On Wed, Jan 31, 2018 at 1:49 PM Raghu Angadi <[email protected]> wrote: > >> Original mail mentions that output from second run of word_count is >> ignored. That does not seem as safe as ignoring error from a second attempt >> of a step. How do we know second run didn't run on different output? >> Overwriting seems more accurate than ignoring. Does handling this error at >> sink level distinguish between the two (another run vs second attempt)? >> >> >> On Wed, Jan 31, 2018 at 12:32 PM, Udi Meiri <[email protected]> wrote: >> >>> Yeah, another round of refactoring is due to move the rename via >>> copy+delete logic up to the file-based sink level. >>> >>> On Wed, Jan 31, 2018, 10:42 Chamikara Jayalath <[email protected]> >>> wrote: >>> >>>> Good point. There's always the chance of step that performs final >>>> rename being retried. So we'll have to ignore this error at the sink level. >>>> We don't necessarily have to do this at the FileSystem level though. I >>>> think the proper behavior might be to raise an error for the rename at the >>>> FileSystem level if the destination already exists (or source doesn't >>>> exist) while ignoring that error (and possibly logging a warning) at the >>>> sink level. >>>> >>>> - Cham >>>> >>>> >>>> On Tue, Jan 30, 2018 at 6:47 PM Reuven Lax <[email protected]> wrote: >>>> >>>>> I think the idea was to ignore "already exists" errors. The reason >>>>> being that any step in Beam can be executed multiple times, including the >>>>> rename step. If the rename step gets run twice, the second run should >>>>> succeed vacuously. >>>>> >>>>> >>>>> On Tue, Jan 30, 2018 at 6:19 PM, Udi Meiri <[email protected]> wrote: >>>>> >>>>>> Hi, >>>>>> I've been working on HDFS code for the Python SDK and I've noticed >>>>>> some behaviors which are surprising. I wanted to know if these behaviors >>>>>> are known and intended. >>>>>> >>>>>> 1. When renaming files during finalize_write, rename errors are >>>>>> ignored >>>>>> <https://github.com/apache/beam/blob/3aa2bef87c93d2844dd7c8dbaf45db75ec607792/sdks/python/apache_beam/io/filebasedsink.py#L232>. >>>>>> For example, if I run wordcount twice using HDFS code I get a warning the >>>>>> second time because the file already exists: >>>>>> >>>>>> WARNING:root:Rename not successful: >>>>>> hdfs://beam-temp-counts2-7cb0a78005f211e8b6a08851fb5da245/1059f870-d64f-4f63-b1de-e4bd20fcd70a.counts2 >>>>>> -> hdfs://counts2-00000-of-00001, libhdfs error in renaming >>>>>> hdfs://beam-temp-counts2-7cb0a78005f211e8b6a08851fb5da245/1059f870-d64f-4f63-b1de-e4bd20fcd70a.counts2 >>>>>> to hdfs://counts2-00000-of-00001 with exceptions Unable to rename >>>>>> '/beam-temp-counts2-7cb0a78005f211e8b6a08851fb5da245/1059f870-d64f-4f63-b1de-e4bd20fcd70a.counts2' >>>>>> to '/counts2-00000-of-00001'. >>>>>> >>>>>> For GCS and local files there are no rename errors (in this case), >>>>>> since the rename operation silently overwrites existing destination >>>>>> files. >>>>>> However, blindly ignoring these errors might make the pipeline to report >>>>>> success even though output files are missing. >>>>>> >>>>>> 2. Output files (--ouput) overwrite existing files. >>>>>> >>>>>> 3. The Python SDK doesn't use Filesystems.copy(). The Java SDK >>>>>> doesn't use Filesystem.Rename(). >>>>>> >>>>>> Thanks, >>>>>> - Udi >>>>>> >>>>> >>>>> >>
