I will submit a patch once I get set up for that. My crystal ball says
that some people will rely on sequential rather than seeking access to File
data. E.g. a utility that can either write the same data to a File or to
stdout, piped to another process that can read a File from stdin and
therefore can't seek() it. Or someone listening to a file with inotify()
as it is being written and not wanting to trip over the footer once the
writer writes it. As a couple of examples. In any case we have an
end-of-stream marker so might as well use it. :)
On Wed, May 22, 2019 at 8:56 AM Wes McKinney <wesmck...@gmail.com> wrote:
> The file format isn't intended to be used in a streaming setting. Use
> RecordBatchStreamWriter if you need to be able to read a dataset as a
> stream
>
> That being said I don't have a problem with writing the EOS in the
> file, but the current implementation is not "wrong".
>
> On Wed, May 22, 2019 at 8:37 AM John Muehlhausen <j...@jgm.org> wrote:
> >
> > I believe the change involves updating the File format notes as above, as
> > well as something like the following. The format also mentions "there is
> > no requirement that dictionary keys should be defined in a
> DictionaryBatch
> > before they are used in a RecordBatch, as long as the keys are defined
> > somewhere in the file." I'm not sure this is a good idea if the file is
> > enormous. The option to process a file as a stream is how we keep memory
> > usage reasonable?
> >
> > I will try to figure out a workflow for recommending changes to the code
> > base.... merge requests etc. Any tips appreciated.
> >
> > Status Close() override {
> > * // Close stream for compatibility with sequential readers*
> > * RETURN_NOT_OK(UpdatePosition());*
> > * int32_t end_of_stream_marker = 0;*
> > * RETURN_NOT_OK(Write(&end_of_stream_marker, sizeof(int32_t)));*
> >
> > // Write file footer
> > RETURN_NOT_OK(UpdatePosition());
> > int64_t initial_position = position_;
> > RETURN_NOT_OK(WriteFileFooter(*schema_, dictionaries_,
> record_batches_,
> > sink_));
> >
> > // Write footer length
> > RETURN_NOT_OK(UpdatePosition());
> > int32_t footer_length = static_cast<int32_t>(position_ -
> > initial_position);
> > if (footer_length <= 0) {
> > return Status::Invalid("Invalid file footer");
> > }
> >
> > RETURN_NOT_OK(Write(&footer_length, sizeof(int32_t)));
> >
> > // Write magic bytes to end file
> > return Write(kArrowMagicBytes, strlen(kArrowMagicBytes));
> > }
> >
> >
> > On Tue, May 21, 2019 at 11:19 PM Micah Kornfield <emkornfi...@gmail.com>
> > wrote:
> >
> > > This seems like a reasonable change. Is there any reason that we
> shouldnt
> > > always append EOS?
> > >
> > > On Tuesday, May 21, 2019, John Muehlhausen <j...@jgm.org> wrote:
> > >
> > > > Wes,
> > > >
> > > > Check out reader.cpp. It seg faults when it gets to the next
> > > > message-that-is-not-a-message... it is a footer. But I have no way
> to
> > > > know this in reader.cpp because I'm piping the File in via stdin.
> > > >
> > > > In seeker.cpp I seek to the end and figure out where the footer is
> (this
> > > > is a py-arrow-written file) and indeed it is at the offset where my
> > > > "streamed File" reader bombed out. If EOS were mandatory at this
> > > location
> > > > it would have been fine... I would have said "oh, time for the
> footer!"
> > > >
> > > > Basically what I'm saying is that we can't assume that File won't be
> > > > processed as a stream. In an actual non-file stream it is either
> EOS or
> > > > end-of-stream. But with a file-as-stream there is more data and we
> have
> > > to
> > > > know it isn't the stream anymore.
> > > >
> > > > Otherwise we've locked the File use-cases into those where the File
> isn't
> > > > streamed -- i.e. is seekable. See what I'm saying? For reader.cpp
> to
> > > have
> > > > been functional it would have had to read the entire File into a
> buffer
> > > > before parsing, since it could not seek(). This could be easily
> avoided
> > > > with a mandatory EOS in the File format. Basically:
> > > >
> > > > <magic number "ARROW1">
> > > > <empty padding bytes [to 8 byte boundary]>
> > > > <STREAMING FORMAT>
> > > > *<EOS if not in stream>*
> > > > <FOOTER>
> > > > <FOOTER SIZE: int32>
> > > > <magic number "ARROW1">
> > > >
> > > > -John
> > > >
> > > > On Tue, May 21, 2019 at 4:44 PM Wes McKinney <wesmck...@gmail.com>
> > > wrote:
> > > >
> > > >> hi John,
> > > >>
> > > >> I'm not sure I follow. The EOS you're referring to is part of the
> > > >> streaming format. It's designed to be readable using an InputStream
> > > >> interface that does not support seeking at all. You can see the core
> > > >> logic where messages are popped off the InputStream here
> > > >>
> > > >> https://github.com/apache/arrow/blob/6f80ea4928f0d26ca175002f2e9f51
> > > >> 1962c8b012/cpp/src/arrow/ipc/message.cc#L281
> > > >>
> > > >> If the end of the byte stream is reached, or EOS (0) is encountered,
> > > >> then the stream reader stops iteration.
> > > >>
> > > >> - Wes
> > > >>
> > > >> On Tue, May 21, 2019 at 4:34 PM John Muehlhausen <j...@jgm.org>
> wrote:
> > > >> >
> > > >> > https://arrow.apache.org/docs/format/IPC.html#file-format
> > > >> >
> > > >> > <EOS [optional]: int32>
> > > >> >
> > > >> > If this stream marker is optional in the file format, doesn't this
> > > >> prevent
> > > >> > someone from reading the file without being able to seek() it,
> e.g. if
> > > >> it
> > > >> > is "piped in" to a program? Or otherwise they'll have to stream
> in
> > > the
> > > >> > entire thing before they can start parsing?
> > > >> >
> > > >> > Any reason it can't be mandatory for a File?
> > > >> >
> > > >> > -John
> > > >>
> > > >
> > >
>
