It doesn't appear that Java writes an EOS (4-byte 0) after the last record batch in ArrowFileWriter.java [1] so may want to open a JIRA to make this simple change.
Note that this addition (not even a change) is backward compatible, old readers will be unaffected [1]: https://github.com/apache/arrow/blob/master/java/vector/src/main/java/org/apache/arrow/vector/ipc/ArrowFileWriter.java#L67 On Wed, May 22, 2019 at 12:24 PM John Muehlhausen <j...@jgm.org> wrote: > > https://github.com/apache/arrow/pull/4372 > > First contribution attempt... sorry in advance if I'm not coloring inside > the lines! > > On Wed, May 22, 2019 at 9:06 AM John Muehlhausen <j...@jgm.org> wrote: > > > 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 > >> > > >> > >> > > > > >> > > > >> > >
