I think this is still unsafe in case exists(dst) (e.g. this is a re-run of
a pipeline) but src is missing due to some bad reason. However it's
probably better than what we have (e.g. we currently certainly don't
perform checksum checks).
On Thu, Feb 1, 2018 at 2:45 PM Udi Meiri <eh...@google.com> wrote:
> For GCS, I would do what I believe we already do.
> rename(src, dst):
> - if !exists(src) and exists(dst) return 0
> - if !exists(src) and !exists(dst) return error
> - if exists(src) and exists(dst) { if checksum(src) == checksum(dst)
> return 0 else delete(dst) }
> - Start a GCS copy from src to dst.
> - Wait for GCS copy to complete.
> - delete(src)
>
> For filesystems that don't have checksum() metadata, size() can be used
> instead.
>
> I've opened a bug to track this:
> https://issues.apache.org/jira/browse/BEAM-3600
>
> On Thu, Feb 1, 2018 at 2:25 PM Eugene Kirpichov <kirpic...@google.com>
> wrote:
>
>> Yes, IGNORE_MISSING_FILES is unsafe because it will - well - ignore files
>> that are missing for more ominous reasons than just this being a non-first
>> attempt at renaming src to dst. E.g. if there was a bug in constructing the
>> filename to be renamed, or if we somehow messed up the order of rename vs
>> cleanup, etc. - these situations with IGNORE_MISSING_FILES would lead to
>> silent data loss (likely caught by unit tests though - so this is not a
>> super serious issue).
>>
>> Basically I just can't think of a case when I was copying files and
>> thinking "oh man, I wish it didn't give an error if the stuff I'm copying
>> doesn't exist" - the option exists only because we couldn't come up with
>> another way to implement idempotent rename on GCS.
>>
>> What's your idea of how a safe retryable GCS rename() could work?
>>
>> On Wed, Jan 31, 2018 at 6:45 PM Udi Meiri <eh...@google.com> wrote:
>>
>>> Eugene, if I get this right, you're saying that IGNORE_MISSING_FILES is
>>> unsafe because it will skip (src, dst) pairs where neither exist? (it only
>>> looks if src exists)
>>>
>>> For GCS, we can construct a safe retryable rename() operation, assuming
>>> that copy() and delete() are atomic for a single file or pair of files.
>>>
>>>
>>>
>>> On Wed, Jan 31, 2018 at 4:00 PM Raghu Angadi <rang...@google.com> wrote:
>>>
>>>> On Wed, Jan 31, 2018 at 2:43 PM, Eugene Kirpichov <kirpic...@google.com
>>>> > wrote:
>>>>
>>>>> 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.
>>>>>
>>>>
>>>> Something like this is needed only in streaming, right?
>>>>
>>>> Raghu.
>>>>
>>>>
>>>>> 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 <eh...@google.com> 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 <rang...@google.com>
>>>>>> 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 <eh...@google.com>
>>>>>>> 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 <
>>>>>>>> chamik...@google.com> 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 <re...@google.com>
>>>>>>>>> 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 <eh...@google.com>
>>>>>>>>>> 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
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>
