Hi, While I'm not using the C++ version of Arrow, the issue you're talking about is a very common concern.
There are a few points to discuss here: 1. Generally, Parquet files cannot be appended to. You could of course load the file to memory, add more information and re-save, but that's not really what you're looking for... tools like `parquet-tools` can concatenate files together by creating a new file with two (or more) row groups, but that's not a very good solution either. Having multiple row groups in a single file is sometimes desirable, but in this case would just create a less compressed file, most probably. 2. The other concern is reliability - having a process that holds a big batch in memory and then spills them to disk every X minutes/rows/bytes is bound to have issues when things crash/get stuck/need to go down for maintenance. You probably want to have as close to "exactly once" guarantees as possible (the holy grail...). One common solution for this is to write to Kafka, and a have a consumer that periodically reads a batch of messages and stores them to file. This is nowadays provided by Kafka Connect <https://www.confluent.io/blog/apache-kafka-to-amazon-s3-exactly-once/>, thankfully. Anyway, the "exactly once" part stops at this point, and for anything that happens downstream you'd need 3. Then, you're back to the question of many many files per day... there is no magical solution to this. You may need to have a scheduled task that reads files every X hours (or every day?), and re-partitions the data in the way that makes the most sense for processing/querying later - perhaps by date, perhaps by customer, both, etc. There are various tools that help in this. Elad On Wed, May 26, 2021 at 7:32 PM Xander Dunn <[email protected]> wrote: > I have a very long-running (months) program that is streaming in data > continually, processing it, and saving it to file using Arrow. My current > solution is to buffer several million rows and write them to a new .parquet > file each time. This works, but produces 1000+ files every day. > > If I could, I would just append to the same file for each day. I see an > `arrow::fs::FileySystem::OpenAppendStream` - what file formats does this > work with? Can I append to .parquet or .feather files? Googling seems to > indicate these formats can't be appended to. > > Using the `parquet::StreamWriter > <https://arrow.apache.org/docs/cpp/parquet.html?highlight=writetable#writetable>`, > could I continually stream rows to a single file throughout the day? What > happens if the program is unexpectedly terminated? Would everything in the > currently open monolithic file be lost? I would be streaming rows to a > single .parquet file for 24 hours. > > Thanks, > Xander > >
