Upload CSV stream from Ruby to S3

Question

I am dealing with potentially huge CSV files which I want to export from my Rails app, and since it runs on Heroku, my idea was to stream these CSV files directly to S3 when generating them.

Now, I have an issue, in that Aws::S3 expects a file in order to be able to perform an upload, while in my Rails app I would like to do something like:

S3.bucket('my-bucket').object('my-csv') << %w(this is one line)

How can I achieve this?

Answer 1

You can use s3 multipart upload that allows upload by splitting large objects to multiple chunks. https://docs.aws.amazon.com/AmazonS3/latest/dev/mpuoverview.html

Multipart upload requires more complex coding but aws-sdk-ruby V3 supports upload_stream method which seems to execute multipart upload internally and it's very easy to use. Maybe exact solution for this use case. https://docs.aws.amazon.com/sdk-for-ruby/v3/api/Aws/S3/Object.html#upload_stream-instance_method

client = Aws::S3::Client.new(
  region: 'ap-northeast-1',
  credentials: your_credential
)

obj = Aws::S3::Object.new('your-bucket-here', 'path-to-output', client: client)

require "csv"
obj.upload_stream do |write_stream|
  [
    %w(this is first line),
    %w(this is second line),
    %w(this is third line),
  ].each do |line|
    write_stream << line.to_csv
  end
end

this,is,first,line
this,is,second,line
this,is,third,line

The argument to the upload_stream block can usually be used as an IO object, which allows you to chain and wrap CSV generation as you would for a file or other IO object:

obj.upload_stream do |write_stream|
  CSV(write_stream) do |csv|
    [
      %w(this is first line),
      %w(this is second line),
      %w(this is third line),
    ].each do |line|
      csv << line
    end
  end
end

Or for example, you could compress the CSV while you generate and upload it, using a tempfile to reduce memory footprint:

obj.upload_stream(tempfile: true) do |write_stream|
  # When uploading compressed data, use binmode to avoid an encoding error.
  write_stream.binmode

  Zlib::GzipWriter.wrap(write_stream) do |gzw|
    CSV(gzw) do |csv|
      [
        %w(this is first line),
        %w(this is second line),
        %w(this is third line),
      ].each do |line|
        csv << line
      end
    end
  end
end

Edited: In the compressed example code, you have to add binmode to fix the following error:

Aws::S3::MultipartUploadError: multipart upload failed: "\x8D" from ASCII-8BIT to UTF-8

Answer 2

s3 = Aws::S3::Resource.new(region:'us-west-2')
obj = s3.bucket.object("#{FOLDER_NAME}/#{file_name}.csv")
file_csv = CSV.generate do |csv|
    csv << ActionLog.column_names
    ActionLog.all.each do |action_log|
      csv << action_log.attributes.values
    end
  end
  obj.put body: file_csv

file_csv = CSV.generate is to create a string of CSV data in Ruby. After creating this string of CSV, we put to S3 using bucket, with the path

#{FOLDER_NAME}/#{file_name}.csv

In my code, I export all the data to an ActionLog model.