Apache Arrow defines two formats for serializing data for interprocess communication (IPC):
a "stream" format and a "file" format, known as Feather.
RecordBatchStreamWriter and RecordBatchFileWriter are
interfaces for writing record batches to those formats, respectively.
For guidance on how to use these classes, see the examples section.
The RecordBatchFileWriter$create() and RecordBatchStreamWriter$create()
factory methods instantiate the object and take the following arguments:
sink An OutputStream
schema A Schema for the data to be written
use_legacy_format logical: write data formatted so that Arrow libraries
versions 0.14 and lower can read it. Default is FALSE. You can also
enable this by setting the environment variable ARROW_PRE_0_15_IPC_FORMAT=1.
metadata_version: A string like "V5" or the equivalent integer indicating
the Arrow IPC MetadataVersion. Default (NULL) will use the latest version,
unless the environment variable ARROW_PRE_1_0_METADATA_VERSION=1, in
which case it will be V4.
$write(x): Write a RecordBatch, Table, or data.frame, dispatching
to the methods below appropriately
$write_batch(batch): Write a RecordBatch to stream
$write_table(table): Write a Table to stream
$close(): close stream. Note that this indicates end-of-file or
end-of-stream--it does not close the connection to the sink. That needs
to be closed separately.
write_ipc_stream() and write_feather() provide a much simpler
interface for writing data to these formats and are sufficient for many use
cases. write_to_raw() is a version that serializes data to a buffer.
tf <- tempfile()
on.exit(unlink(tf))
batch <- record_batch(chickwts)
# This opens a connection to the file in Arrow
file_obj <- FileOutputStream$create(tf)
# Pass that to a RecordBatchWriter to write data conforming to a schema
writer <- RecordBatchFileWriter$create(file_obj, batch$schema)
writer$write(batch)
# You may write additional batches to the stream, provided that they have
# the same schema.
# Call "close" on the writer to indicate end-of-file/stream
writer$close()
# Then, close the connection--closing the IPC message does not close the file
file_obj$close()
# Now, we have a file we can read from. Same pattern: open file connection,
# then pass it to a RecordBatchReader
read_file_obj <- ReadableFile$create(tf)
reader <- RecordBatchFileReader$create(read_file_obj)
# RecordBatchFileReader knows how many batches it has (StreamReader does not)
reader$num_record_batches
#> [1] 1
# We could consume the Reader by calling $read_next_batch() until all are,
# consumed, or we can call $read_table() to pull them all into a Table
tab <- reader$read_table()
# Call as.data.frame to turn that Table into an R data.frame
df <- as.data.frame(tab)
# This should be the same data we sent
all.equal(df, chickwts, check.attributes = FALSE)
#> [1] TRUE
# Unlike the Writers, we don't have to close RecordBatchReaders,
# but we do still need to close the file connection
read_file_obj$close()