Reading from compressed files in Dataflow

问题:

Is there a way (or any kind of hack) to read input data from compressed files? My input consists of a few hundreds of files, which are produced as compressed with gzip and uncompressing them is somewhat tedious.

Thanks, Genady

回答1:

Reading from compressed text sources is now supported in Dataflow (as of this commit). Specifically, files compressed with gzip and bzip2 can be read from by specifying the compression type:

TextIO.Read.from(myFileName).withCompressionType(TextIO.CompressionType.GZIP)

However, if the file has a .gz or .bz2 extension, you don't have do do anything: the default compression type is AUTO, which examines file extensions to determine the correct compression type for a file. This even works with globs, where the files that result from the glob may be a mix of .gz, .bz2, and uncompressed.

回答2:

The slower performance with my work around was most likely because Dataflow was putting most of the files in the same split so they weren't being processed in parallel. You can try the following to speed things up.

• Create a PCollection for each file by applying the Create transform multiple times (each time to a single file).
• Use the Flatten transform to create a single PCollection containing all the files from PCollections representing individual files.
• Apply your pipeline to this PCollection.

回答3:

I also found that for files that reside in the cloud store, setting the content type and content encoding appears to "just work" without the need for a workaround.

Specifically - I run

gsutil -m setmeta -h "Content-Encoding:gzip" -h "Content-Type:text/plain" <path>

回答4:

I just noticed that specifying the compression type is now available in the latest version of the SDK (v0.3.150210). I've tested it, and was able to load my GZ files directly from GCS to BQ without any problems.