faraday/docs/usage/streaming.md

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Sometimes you might need to receive a streaming response. You can do this with the on_data request option.

The on_data callback is a receives tuples of chunk Strings, and the total of received bytes so far.

This example implements such a callback:

# A buffer to store the streamed data
streamed = []

conn.get('/stream/10') do |req|
  # Set a callback which will receive tuples of chunk Strings,
  # the sum of characters received so far, and the response environment.
  # The latter will allow access to the response status, headers and reason, as well as the request info.
  req.options.on_data = Proc.new do |chunk, overall_received_bytes, env|
    puts "Received #{overall_received_bytes} characters"
    streamed << chunk
  end
end

# Joins all response chunks together
streamed.join

The on_data streaming is currently only supported by some adapters. To see which ones, please refer to Awesome Faraday comparative table or check the adapter documentation. Moreover, the env parameter was only recently added, which means some adapters may only have partial support (i.e. only chunk and overall_received_bytes will be passed to your block).