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From Jesse Wang <hello....@gmail.com>
Subject Re: Python and Java interoperability
Date Wed, 22 Jul 2020 05:48:50 GMT
 Hi Ryan,
Thanks for your reply.

On Tue, Jul 21, 2020 at 8:54 PM Ryan Murray <rymurr@dremio.com> wrote:

> Hey Jiaxing,
>
> You want to use the IPC mechanism to pass arrow buffers between
> languages[1]
>
> First get a buffer:
> ```
> import pyarrow as pa
>
> data = [
>     pa.array([1, 2, 3, 4]),
>     pa.array(['foo', 'bar', 'baz', None]),
>     pa.array([True, None, False, True])
> ]
> batch = pa.record_batch(data, names=['f0', 'f1', 'f2'])
> sink = pa.BufferOutputStream()
> writer = pa.ipc.new_stream(sink, batch.schema)
> writer.write_batch(batch)
> writer.close()
> buf = sink.getvalue()
> ```
>
> The buffer could be written to Redis, to a file etc. For redis I think
> `r.set("key", buf.hex())` is easiest, you don't have to worry about
> encoding.
>
> On the java side something like:
> ```
>     Jedis jedis = new Jedis();
>     String buf = jedis.get("key");
>     RootAllocator rootAllocator = new RootAllocator(Long.MAX_VALUE);
>     ByteArrayInputStream in = new
> ByteArrayInputStream(hexStringToByteArray(buf));
>     ArrowStreamReader stream = new ArrowStreamReader(in, rootAllocator);
>     VectorSchemaRoot vsr = stream.getVectorSchemaRoot();
>     stream.loadNextBatch()
> ```
> And the VectorSchemaRoot holds the correct Arrow Buffer.
>

I tested this and get the following exception thrown:
```
Exception in thread “main” java.lang.IllegalArgumentException
  at java.nio.ByteBuffer.allocate(ByteBuffer.java:334)
  at
org.apache.arrow.vector.ipc.message.MessageSerializer.readMessage(MessageSerializer.java:547)
  at
org.apache.arrow.vector.ipc.message.MessageChannelReader.readNext(MessageChannelReader.java:58)
  at
org.apache.arrow.vector.ipc.ArrowStreamReader.readSchema(ArrowStreamReader.java:132)
  at
org.apache.arrow.vector.ipc.ArrowReader.initialize(ArrowReader.java:178)
  at
org.apache.arrow.vector.ipc.ArrowReader.ensureInitialized(ArrowReader.java:169)
  at
org.apache.arrow.vector.ipc.ArrowReader.getVectorSchemaRoot(ArrowReader.java:62)
  at Main.main(Main.java:38)
```


> While Redis will work for this you might find a file or socket a bit more
> ergonomic in Arrow. The Plasma object store is also an option[2] which you
> can think of as a primitive Redis specifically for Arrow Buffers. Finally,
> if you are using Redis as a message bus you might find the Arrow RPC
> mechanism Arrow Flight is a good choice[3].
>
As for the Plasma, It seems it currently limited to a single host... (From
its source code arrow/cpp/src/plasma/io.cc, it used AF_UNIX socket only)


>
> [1]
> https://arrow.apache.org/docs/python/ipc.html#writing-and-reading-streams
> [2] https://arrow.apache.org/docs/python/plasma.html
> [3] https://arrow.apache.org/blog/2019/10/13/introducing-arrow-flight/
>
> On Tue, Jul 21, 2020 at 10:57 AM Jesse Wang <hello.wjx@gmail.com> wrote:
>
>> Hi,
>> I want to have a Java process read the content of DataFrames produced by
>> a Python process. The Java and Python processes run on different hosts.
>>
>> The solution I can think of is to have the Python process serialize the
>> DataFrame and save it to redis, and have the Java process parse the data.
>>
>> The solution I find serializes the DataFrame to 'pybytes':
>> (from
>> https://stackoverflow.com/questions/57949871/how-to-set-get-pandas-dataframes-into-redis-using-pyarrow
>> )
>> ```
>>    import pandas as pd
>>
>> import pyarrow as paimport redis
>>
>> df=pd.DataFrame({'A':[1,2,3]})
>> r = redis.Redis(host='localhost', port=6379, db=0)
>>
>> context = pa.default_serialization_context()
>> r.set("key", context.serialize(df).to_buffer().to_pybytes())
>> context.deserialize(r.get("key"))
>>    A0  11  22  3
>>
>> ```
>>
>> I wonder if this serialized 'pybytes' can be parsed at the Java end? If
>> not, how can I achieve this properly?
>>
>> Thanks!
>>
>> --
>>
>> Best Regards,
>> Jiaxing Wang
>>
>>
>

-- 

Best Regards,
Jiaxing Wang

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