Read file in order in Google Cloud Dataflow

Question

I'm using Spotify Scio to read logs that are exported from Stackdriver to Google Cloud Storage. They are JSON files where every line is a single entry. Looking at the worker logs it seems like the file is split into chunks, which are then read in any order. I've already limited my job to exactly 1 worker in this case. Is there a way to force these chunks to be read and processed in order?

As an example (textFile is basically a TextIO.Read):

val sc = ScioContext(myOptions)
sc.textFile(myFile).map(line => logger.info(line))

Would produce output similar to this based on the worker logs:

line 5
line 6
line 7
line 8
<Some other work>
line 1
line 2
line 3
line 4
<Some other work>
line 9
line 10
line 11
line 12

What I want to know is if there's a way to force it to read lines 1-12 in order. I've found that gzipping the file and reading it with the CompressionType specified is a workaround but I'm wondering if there are any ways to do this that don't involve zipping or changing the original file.

Answer 1

Google Cloud Dataflow / Apache Beam currently do not support sorting or preservation of order in processing pipelines. The drawback of allowing for sorted output is that it outputting such a result for large datasets eventually bottlenecks on a single machine, which is not scalable for large datasets.