martin-traverse commented on issue #794:
URL: https://github.com/apache/arrow-java/issues/794#issuecomment-3047867486
Hm - I have found ArrowReader / ArrowWriter to be a bit opinionated.
Particularly an issue with reader because you have less control (when bytes are
going to arrive) but also writer makes a lot of assumptions e.g. dictionaries
are static, no deltas etc. Neither gives you much control of what is happening
at the file level. I ended up overriding and shading some of the supporting
classes in order to use them (I have patches to submit which could reduce the
need for this).
I think the goal with ArrowReader / ArrowWriter was to abstract away the
different formats (file and stream), so perhaps that abstraction is partly what
causes the loss of control. My preference would be for a more explicit API that
directly maps to the file structure, it is always possible to layer
generalisation on top for a specific pattern.
Here is a quick summary of what is the same / different:
Writer:
/// These can be the same as ArrowWriter
void writeBatch();
long bytesWritten();
void close;();
// These are different
void writeHeader(); // Explicit control instead of start() / end(),
which may or may not trigger writes
void resetBatch(VectorSchemaRoot batch); // Allow streaming pattern
Reader:
// These are the same as ArrowReader:
VectorSchemaRoot getVectorSchemaRoot();
long bytesRead();
void close();
// These are also the same, from DictionaryProvider
Set<Long> getDictionaryIds();
Dictionary lookup(long id);
// These are different - Arrow has readSchema() and initialize(), which
may or may not trigger reads
void readHeader(); // Explicit read control
Schema getSchema(); // Explicit get - does not trigger reading
// These are aloo different - explicit control over reading
boolean readBatch();
boolean hasNextBatch();
long nextBatchPosition();
long nextBatchSize();
One minor detail on naming - ArrowReader has `loadNaextBatch()` which is
possibly more descriptive, but I think there is value in sticking with just one
scheme - Load / save, read / write, get / put etc. otherwise it gets confusing.
Happy to take a steer if you feel differently on any of this. Otherwise if
you are happy lmk and I'll make a start :)
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