How does Pig use Hadoop Globs in a 'load' statement?

Question

As I've noted previously, Pig doesn't cope well with empty (0-byte) files. Unfortunately, there are lots of ways that these files can be created (even within Hadoop utilitities).

I thought that I could work around this problem by explicitly loading only files that match a given naming convention in the LOAD statement using Hadoop's glob syntax. Unfortunately, this doesn't seem to work, as even when I use a glob to filter down to known-good input files, I still run into the 0-byte failure mentioned earlier.

Here's an example: Assume I have the following files in S3:

• mybucket/a/b/ (0 bytes)
• mybucket/a/b/myfile.log (>0 bytes)
• mybucket/a/b/yourfile.log (>0 bytes)

If I use a LOAD statement like this in my pig script:

myData = load 's3://mybucket/a/b/*.log as ( ... )

I would expect that Pig would not choke on the 0-byte file, but it still does. Is there a trick to getting Pig to actually only look at files that match the expected glob pattern?

Marciamarciano

Answer 1

This is a fairly ugly solution, but globs that don't rely on the * wildcard syntax appear to work. So, in our workflow (before calling our pig script), we list all of the files below the prefix we're interested, and then create a specific glob that consists of only the paths we're interested in.

For example, in the example above, we list "mybucket/a":

hadoop fs -lsr s3://mybucket/a

Which returns a list of files, plus other metadata. We can then create the glob from that data:

myData = load 's3://mybucket/a/b{/myfile.log,/yourfile.log}' as ( ... )

This requires a bit more front-end work, but allows us to specifically target files we're interested and avoid 0-byte files.

Update: Unfortunately, I've found that this solution fails when the glob pattern gets long; Pig ends up throwing an exception "Unable to create input slice".

Marciamarciano