r/man/RecordBatchWriter.Rd

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/record-batch-writer.R
\docType{class}
\name{RecordBatchWriter}
\alias{RecordBatchWriter}
\alias{RecordBatchStreamWriter}
\alias{RecordBatchFileWriter}
\title{RecordBatchWriter classes}
\description{
Apache Arrow defines two formats for \href{https://arrow.apache.org/docs/format/Columnar.html#serialization-and-interprocess-communication-ipc}{serializing data for interprocess communication (IPC)}:
a "stream" format and a "file" format, known as Feather.
\code{RecordBatchStreamWriter} and \code{RecordBatchFileWriter} are
interfaces for writing record batches to those formats, respectively.

For guidance on how to use these classes, see the examples section.
}
\section{Factory}{


The \code{RecordBatchFileWriter$create()} and \code{RecordBatchStreamWriter$create()}
factory methods instantiate the object and take the following arguments:
\itemize{
\item \code{sink} An \code{OutputStream}
\item \code{schema} A \link{Schema} for the data to be written
\item \code{use_legacy_format} logical: write data formatted so that Arrow libraries
versions 0.14 and lower can read it? Default is \code{FALSE}. You can also
enable this by setting the environment variable \code{ARROW_PRE_0_15_IPC_FORMAT=1}.
\item \code{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 \code{ARROW_PRE_1_0_METADATA_VERSION=1}, in
which case it will be V4.
}
}

\section{Methods}{

\itemize{
\item \verb{$write(x)}: Write a \link{RecordBatch}, \link{Table}, or \code{data.frame}, dispatching
to the methods below appropriately
\item \verb{$write_batch(batch)}: Write a \code{RecordBatch} to stream
\item \verb{$write_table(table)}: Write a \code{Table} to stream
\item \verb{$close()}: close stream. Note that this indicates end-of-file or
end-of-stream--it does not close the connection to the \code{sink}. That needs
to be closed separately.
}
}

\examples{
\donttest{
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
# 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)
# Unlike the Writers, we don't have to close RecordBatchReaders,
# but we do still need to close the file connection
read_file_obj$close()
}
}
\seealso{
\code{\link[=write_ipc_stream]{write_ipc_stream()}} and \code{\link[=write_feather]{write_feather()}} provide a much simpler
interface for writing data to these formats and are sufficient for many use
cases. \code{\link[=write_to_raw]{write_to_raw()}} is a version that serializes data to a buffer.
}