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

Initially written and as a spiritual successor to zipline and now proudly powering it under the hood.

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.

zip_tricks currently handles all our zipping needs (millions of ZIP files generated per day), so we are pretty confident it is widely compatible with a large number of unarchiving end-user applications.


Ruby 2.1+ syntax support (keyword arguments with defaults) and a working zlib (all available to jRuby as well). jRuby might experience problems when using the reader methods due to the argument of IO#seek being limited to 32 bit sizes.

Diving in: send some large CSV reports from Rails

The easiest is to use the Rails' built-in streaming feature:

class ZipsController < ActionController::Base
  include ActionController::Live # required for streaming
  include ZipTricks::RailsStreaming

  def download
    zip_tricks_stream do |zip|
      zip.write_deflated_file('report1.csv') do |sink|
        CSV(sink) do |csv_write|
          csv << Person.column_names
          Person.all.find_each do |person|
            csv << person.attributes.values
      zip.write_deflated_file('report2.csv') do |sink|

Create a ZIP file without size estimation, compress on-the-fly during writes

Basic use case is compressing on the fly. Some data will be buffered by the Zlib deflater, but memory inflation is going to be very constrained. Data will be written to destination at fairly regular intervals. Deflate compression will work best for things like text files.

out = my_tempfile # can also be a socket do |zip|
  zip.write_stored_file('mov.mp4.txt') do |sink|'mov.mp4', 'rb'){|source| IO.copy_stream(source, sink) }
  zip.write_deflated_file('long-novel.txt') do |sink|'novel.txt', 'rb'){|source| IO.copy_stream(source, sink) }

Unfortunately with this approach it is impossible to compute the size of the ZIP file being output, since you do not know how large the compressed data segments are going to be.

Send a ZIP from a Rack response

Create a RackBody object and give it's constructor a block that adds files. The block will only be called when actually sending the response to the client (unless you are using a buffering Rack webserver, such as Webrick).

body = do | zip |
  zip.write_stored_file('mov.mp4') do |sink| # Those MPEG4 files do not compress that well'mov.mp4', 'rb'){|source| IO.copy_stream(source, sink) }
  zip.write_deflated_file('long-novel.txt') do |sink|'novel.txt', 'rb'){|source| IO.copy_stream(source, sink) }
[200, {'Transfer-Encoding' => 'chunked'}, body]

Send a ZIP file of known size, with correct headers

Use the SizeEstimator to compute the correct size of the resulting archive.

# Precompute the Content-Length ahead of time
bytesize = ZipTricks::SizeEstimator.estimate do |z|
 z.add_stored_entry(filename: 'myfile1.bin', size: 9090821)
 z.add_stored_entry(filename: 'myfile2.bin', size: 458678)

# Prepare the response body. The block will only be called when the response starts to be written.
zip_body = 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')

[200, {'Content-Length' => bytesize.to_s}, 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.

# io has to be an object that supports #<< 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)

  # then send the actual file contents bypassing the Streamer interface

  # ...and then adjust the ZIP offsets within the Streamer

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 =
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 = ZipTricks::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_tricks

  • Check out the latest master 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.
  • If you alter the ZipWriter, please take the time to run the test in the qa/ directory. Ensure that the generated (large) files open manually - see README_QA for more.


Copyright (c) 2019 WeTransfer. zip_tricks is distributed under the conditions of the Hippocratic License - See LICENSE.txt for further details.