Azure Downloads and Zip Streaming

zip_streaming.rb

# Service object azure_api.rb for interfacing with azure blob storage. Handles getting a list of files as well as
# getting files by top_level_directory/prefix and multiple prefixes. Also caches requests to ensure a fast response.

require 'azure/storage/blob'

CACHE_LENGTH      = 3.hours
PREFIX_DELIMITER  = ','

class AzureApi
  def self.get_files_for(directory, prefix=nil)
    return self.list_blobs(directory) if prefix.nil?

    prefixes = prefix.split(PREFIX_DELIMITER)

    prefixes.flat_map do |prefix|
      self.list_blobs(File.join(directory, prefix, '/'))
    end
  end

  def self.list_blobs(prefix)
    Rails.cache.fetch("get_files_for_#{AZURE_CONTAINER}_#{prefix}", expires_in: CACHE_LENGTH) do
      AZURE_CLIENT.list_blobs(AZURE_CONTAINER, prefix: prefix)
    end
  end

  def self.download(path)
    Rails.cache.fetch("download_#{AZURE_CONTAINER}_#{path}", expires_in: CACHE_LENGTH) do
      AZURE_CLIENT.get_blob(AZURE_CONTAINER, path)
    end
  end
end

########################## STREAMING ACTION FOR CONTROLLER ##################  

def download_all_as_zip
    # Generates a zip of all files given a Azure directory and a prefix
    blobs = AzureApi.get_files_for(DIRECTORY, PREFIX)

    send_file_headers!(
      type:         "application/zip",
      disposition:  "attachment",
      filename:     "download_#{Time.now.to_i}.zip"
    )

    response.headers["Last-Modified"]     = Time.now.httpdate.to_s
    response.headers["X-Accel-Buffering"] = "no"

    writer = ZipTricks::BlockWrite.new do |chunk|
      response.stream.write(chunk)
    end

    ZipTricks::Streamer.open(writer) do |zip|
      blobs.each do |blob|
        # Download each individual blob, takes the full path of the blob
        _blob, content  = AzureApi.download(blob.name)

        zip.write_deflated_file(blob.name) do |file_writer|
          # Add blob content to file writer to stream back to browser
          file_writer << content
        end
      end
    end
  ensure
    response.stream.close
  end