zip_kit

Allows streaming, non-rewinding ZIP file output from Ruby.

zip_kit is a successor to and continuation of zip_tricks, which was inspired by zipline. I am grateful to WeTransfer for allowing me to develop zip_tricks and for sharing it with the community.

Allows you to write a ZIP archive out to a File, Socket, String or Array without having to rewind it at any point. Usable for creating very large ZIP archives for immediate sending out to clients, or for writing large ZIP archives without memory inflation.

The original gem (zip_tricks) handled all the zipping needs (millions of ZIP files generated per day), for WeTransfer, it is widely compatible with a large number of unarchiving end-user applications.

How does it work? How is it different from Rubyzip?

Check out the implementation details on the design of the library, and we have a separate reference on why you might want to use ZipKit over Rubyzip and vice versa.

Requirements

Ruby 2.6+ syntax support is required, as well as a a working zlib (all available to jRuby as well).

Diving in: send some large CSV reports from Rails

The included Railtie will automatically include ZipKit::RailsStreaming into the ActionController::Base class. You will then have a zip_kit_stream method available which accepts a block:

class ZipsController < ActionController::Base
  def download
    zip_kit_stream do |zip|
      zip.write_file('report1.csv') do |sink|
        CSV(sink) do |csv_write|
          csv_write << Person.column_names
          Person.all.find_each do |person|
            csv_write << person.attributes.values
          end
        end
      end
      zip.write_file('report2.csv') do |sink|
        ...
      end
    end
  end
end

The block receives the ZipKit::Streamer object you can write your files through.

The write_file method will use some heuristics to determine whether your output file would benefit from compression, and pick the appropriate storage mode for the file accordingly.

If you want some more conveniences you can also use zipline which will automatically process and stream attachments (Carrierwave, Shrine, ActiveStorage) and remote objects via HTTP.

RailsStreaming does not require ActionController::Live and will stream without it. See ZipKit::RailsStreaming#zip_kit_stream for more details on this. You can use it together with Live just fine if you need to.

Writing into streaming destinations

Any object that accepts bytes via either << or write methods can be a write destination. For example, here is how to upload a sizeable ZIP to S3 - the SDK will happily chop your upload into multipart upload parts:

bucket = Aws::S3::Bucket.new("mybucket")
obj = bucket.object("big.zip")
obj.upload_stream do |write_stream|
  ZipKit::Streamer.open(write_stream) do |zip|
    zip.write_file("file.csv") do |sink|
      File.open("large.csv", "rb") do |file_input|
        IO.copy_stream(file_input, sink)
      end
    end
  end
end

Writing through streaming wrappers

Any object that writes using either << or write can write into a sink. For example, you can do streaming output with builder which calls << on its target every time a complete write call is done:

zip.write_file('employees.xml') do |sink|
  builder = Builder::XmlMarkup.new(target: sink, indent: 2)
  builder.people do
    Person.all.find_each do |person|
      builder.person(name: person.name)
    end
  end
end

The output will be compressed and output into the ZIP file on the fly. Same for CSV:

zip.write_file('line_items.csv') do |sink|
  CSV(sink) do |csv|
    csv << ["Line", "Item"]
    20_000.times do |n|
      csv << [n, "Item number #{n}"]
    end
  end
end

Automatic storage mode (stored vs. deflated)

The ZIP file format allows storage in both compressed and raw storage modes. The raw ("stored") mode does not require decompression and unarchives faster.

ZipKit will buffer a small amount of output and attempt to compress it using deflate compression. If this turns out to be significantly smaller than raw data, it is then going to proceed with all further output using deflate compression. Memory use is going to be very modest, but it allows you to not have to think about the appropriate storage mode.

Deflate compression will work great for JSONs, CSVs and other text- or text-like formats. For example, here is how to output direct to STDOUT (so that you can run $ ruby archive.rb > file.zip in your terminal):

ZipKit::Streamer.open($stdout) do |zip|
  zip.write_file('mov.mp4.txt') do |sink|
    File.open('mov.mp4', 'rb'){|source| IO.copy_stream(source, sink) }
  end
  zip.write_file('long-novel.txt') do |sink|
    File.open('novel.txt', 'rb'){|source| IO.copy_stream(source, sink) }
  end
end

If you want to use specific storage modes, use write_deflated_file and write_stored_file instead of write_file.

Send a ZIP from a Rack response

zip_kit provides an OutputEnumerator object which will yield the binary chunks piece by piece, and apply some amount of buffering as well. Return the headers and the body to your webserver and you will have your ZIP streamed! The block that you give to the OutputEnumerator will receive the ZipKit::Streamer object and will only start executing once your response body starts getting iterated over - when actually sending the response to the client (unless you are using a buffering Rack webserver, such as Webrick).

body = ZipKit::OutputEnumerator.new do | zip |
  zip.write_file('mov.mp4') do |sink|
    File.open('mov.mp4', 'rb'){|source| IO.copy_stream(source, sink) }
  end
  zip.write_file('long-novel.txt') do |sink|
    File.open('novel.txt', 'rb'){|source| IO.copy_stream(source, sink) }
  end
end

[200, body.streaming_http_headers, body]

Send a ZIP file of known size, with correct headers

Sending a file with data descriptors is not always desirable - you don't really know how large your ZIP is going to be. If you want to present your users with proper download progress, you would need to set a Content-Length header - and know ahead of time how large your download is going to be. This can be done with ZipKit, provided you know how large the compressed versions of your file are going to be. Use the ZipKit::SizeEstimator to do the pre-calculation - it is not going to produce any large amounts of output, and will give you a to-the-byte value for your future archive:

bytesize = ZipKit::SizeEstimator.estimate do |z|
 z.add_stored_entry(filename: 'myfile1.bin', size: 9090821)
 z.add_stored_entry(filename: 'myfile2.bin', size: 458678)
end

zip_body = ZipKit::OutputEnumerator.new do | zip |
  zip.add_stored_entry(filename: "myfile1.bin", size: 9090821, crc32: 12485)
  zip << read_file('myfile1.bin')
  zip.add_stored_entry(filename: "myfile2.bin", size: 458678, crc32: 89568)
  zip << read_file('myfile2.bin')
end

hh = zip_body.streaming_http_headers
hh["Content-Length"] = bytesize.to_s

[200, hh, zip_body]

Writing ZIP files using the Streamer bypass

You do not have to "feed" all the contents of the files you put in the archive through the Streamer object. If the write destination for your use case is a Socket (say, you are writing using Rack hijack) and you know the metadata of the file upfront (the CRC32 of the uncompressed file and the sizes), you can write directly to that socket using some accelerated writing technique, and only use the Streamer to write out the ZIP metadata.

ZipKit::Streamer.open(io) do | zip |
  # raw_file is written "as is" (STORED mode).
  # Write the local file header first..
  zip.add_stored_entry(filename: "first-file.bin", size: raw_file.size, crc32: raw_file_crc32)

  # Adjust the ZIP offsets within the Streamer
  zip.simulate_write(my_temp_file.size)

  # ...and then send the actual file contents bypassing the Streamer interface
  io.sendfile(my_temp_file)
end

Other usage examples

Check out the examples/ directory at the root of the project. This will give you a good idea of various use cases the library supports.

Computing the CRC32 value of a large file

BlockCRC32 computes the CRC32 checksum of an IO in a streaming fashion. It is slightly more convenient for the purpose than using the raw Zlib library functions.

crc = ZipKit::StreamCRC32.new
crc << next_chunk_of_data
...

crc.to_i # Returns the actual CRC32 value computed so far
...
# Append a known CRC32 value that has been computed previosuly
crc.append(precomputed_crc32, size_of_the_blob_computed_from)

You can also compute the CRC32 for an entire IO object if it responds to #eof?:

crc = ZipKit::StreamCRC32.from_io(file) # Returns an Integer

Reading ZIP files

The library contains a reader module, play with it to see what is possible. It is not a complete ZIP reader but it was designed for a specific purpose (highly-parallel unpacking of remotely stored ZIP files), and as such it performs it's function quite well. Please beware of the security implications of using ZIP readers that have not been formally verified (ours hasn't been).

Contributing to zip_kit

Check out the latest main to make sure the feature hasn't been implemented or the bug hasn't been fixed yet.
Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it.
Fork the project.
Start a feature/bugfix branch.
Commit and push until you are happy with your contribution.
Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.