JsonEmitter

NOTE this project may appear inactive, but that's just because it's done (I think). Enjoy!

JsonEmitter is a library for efficiently generating very large bits of JSON in Ruby. Need to generate a JSON array of 10,000 database records without eating up all your RAM? No problem! Objects? Nested structures? JsonEmitter has you covered.

Use JsonEmitter in your Rack/Rails/Sinatra/Grape API to stream large JSON responses without worrying (much) about RAM or HTTP timeouts. Use it to write large JSON objects to your filesystem, S3, or ~~a 3D printer~~ anywhere else!

HTTP Chunked Responses

These examples will use the Order enumerator to generate chunks of JSON and send them to the client as more chunks are generated. No more than 500 orders will be in memory at a time, regardless of how many orders there are. And only small portions of the JSON will be in memory at once, no matter how much we're generating. (NOTE There are limits to how long something will stream - your app server, nginx/apache, your browser, etc. may cut it off eventually.)

enumerator = Order.
  where("created_at >= ?", 1.year.ago).
  find_each(batch_size: 500)

Rails

class OrdersController < ApplicationController
  def index
    headers["Content-Type"] = "application/json"
    headers["Last-Modified"] = Time.now.ctime.to_s
    self.response_body = JsonEmitter.array(enumerator, rack: env) { |order|
      order.to_h
    }
  end
end

Sinatra

get "/orders" do
  content_type :json
  JsonEmitter.array(enumerator, rack: env) { |order|
    order.to_h
  }
end

Grape

get :orders do
  stream JsonEmitter.array(enumerator, rack: env) { |order|
    ApiV1::Entities::Order.new(order)
  }
end

Rack

map "/orders" do
  run ->(env) {
    stream = JsonEmitter.array(enumerator, rack: env) { |order|
      order.to_h
    }
    [200, {"Content-Type" => "application/json"}, stream]
  }
end

Sending objects

You may also stream Hashes as JSON objects. Keys must be Strings or Symbols, but values may be anything: literals, Enumerators, Arrays, other Hashes, or Procs that return any of those.

JsonEmitter.object({
  orders: Order.find_each.lazy.map { |order|
    {id: order.id, desc: order.description}
  },
  big_text: ->() { load_tons_of_text },
}, rack: env)

Rack middleware won't work!

IMPORTANT Your Rack middleware will be finished by the time your JSON is built! So if you're depending on middleware to set Time.zone, report exceptions, etc. it won't work here. Fortunately, you can use JsonEmitter.wrap and JsonEmitter.error as replacements.

Put these somewhere like config/initializers/json_emitter.rb.

JsonEmitter.wrap

# Ensure that ActiveRecord connections are returned to the connection pool
JsonEmitter.wrap do
  ->(app) { ActiveRecord::Base.with_connection(&app.call) }
end

JsonEmitter.wrap do
  # Get TZ at the call site
  current_tz = Time.zone
  
  # Return a Proc that restores the call site's TZ before building the JSON
  ->(app) {
    default_tz = Time.zone
    Time.zone = current_tz
    res = app.call
    Time.zone = default_tz
    res
  }
end

JsonEmitter.error

JsonEmitter.error do |ex, context|
  Airbrake.notify(ex, {
    request_path: context.request&.path,
    query_string: context.request&.query_string,
  })
end

Returning errors

When streaming an HTTP response, you can't change the response code once you start sending data. So if you hit an error after you start, you need another way to communicate errors to the client.

One way is to always steam an object that includes an errors field. Any errors will be collected while the Enumerator is running. After it's finished, they'll be added to the JSON object.

  def get_data
    errors = []
    enum = Enumerator.new { |y|
      finished = false
      until finished
        data, errs, finished = get_data_chunk
        if errs
          errors += errs
          finished = true
          next
        end
        data.each { x| y << x }
      end
    }
    return enum, -> { errors }
  end
  
  items_enum, errors_proc = get_data
  JsonEmitter.object({
    items: items_enum,
    errors: errors_proc,
  })

Non-HTTP uses

JsonEmitter.array takes an Enumerable and returns a stream that generates chunks of JSON.

JsonEmitter.array(enumerator).each { |json_chunk|
  # write json_chunk somewhere
}

Streams have a #write method for writing directly to a File or IO object.

File.open("~/out.json", "w+") { |f|
  JsonEmitter.array(enumerator).write f
}

License

MIT License. See LICENSE for details.

jhollinger / json-emitter