GenStage and twitter stream

You haven’t overridden the default demand quantities. GenStage operates in batches, and the min / max batch sizes are 500 / 1000 by default, so unless you have more than that it isn’t gonna use more than one worker.

This is a good example of Default demand should be 1 · Issue #72 · elixir-lang/gen_stage · GitHub

Thanks for your answer @benwilson512

Actually in the consumer the init is this

{:consumer, tweets, subscribe_to: [{Twitter.Producer, max_demand: 20}]}

so the max_demand is 20. Also changing it to 1, the result stays the same: only one consumer receives the tweets, goes to sleep for 10s and once awake gets another tweet from the stream…

  [#PID<0.177.0>,
 "RT @review5x1: https://t.co/Fahn0npWhA #SummerSlam #CeremoniaDeClausura #ClosingCeremony #Tokio202 #FearTWD #Nexus6P #Nougat #Uefa2016 #ukz…"]
  [#PID<0.177.0>, "sleeping for 10s"]
    [#PID<0.177.0>, "awake"]
    [#PID<0.177.0>,
     "RT @OlympiaMschaft: Wir machen uns fertig für den Ausstieg. #WirfuerD #rio2016 https://t.co/iOZkuXjdZV"]
    [#PID<0.177.0>, "sleeping for 10s"]
    [#PID<0.177.0>, "awake"]
    [#PID<0.177.0>,
     "RT @BorisJohnson: Congratulations #Rio2016 for a fantastic Olympics &amp; to all @TeamGB for inspiring us again - London’s legacy lives on http…"]
    [#PID<0.177.0>, "sleeping for 10s"]

Can you please include a github repository with the code? I want to clone it and try to reproduce the error but I want to be sure to be using the same dependencies as you.

thanks @josevalim for the help.

Here is the github repo: https://github.com/alvises/elixir-twitter-genstage

I had hardcoded the twitter tokens so I had to invalidate them. You need to put your tokens in config/config.ex to be able to get the tweets from the twitter streams api

It is a bug in ExTwitter. The stream is discarding messages from the process inbox, which is a big no-no. This means that the subscriptions for the second and third stages are never consumed:

https://github.com/parroty/extwitter/blob/master/lib/extwitter/api/streaming.ex#L110-L111

ExTwitter should use a reference and make sure it only consumes messages it knows about. This must be reported as a bug.

Ideally ExTwitter would provide a GenStage producer, to avoid hijacking the process inbox, which won’t work with a GenStage since both GenStage and ExTwitter are trying to use the same process inbox. You can check the issues tracker so see if someone already requested this feature.

thank you!

I wrote a Twitter library from scratch (at that time, it was my first project to get startet with Elixir). It uses HTTPoison (wrapper for :hackney).

A few weeks ago, I’ve updated the streaming part of the lib to use GenStage.

You can have a look at the implementation here: twittex/lib/twittex/client/stream.ex at master · redrabbit/twittex · GitHub

Basically, the Twittex.Client.Stream receives incoming HTTP chunks from :hackney applying back-pressure at the TCP level with :hackney.stream_next/1.

The next thing I want to try out is using Flow and Flow.Window to implement features such as peak detection.

Thanks Mario!

Can you please tell me what do you mean with “applying back-pressure” ?

When my GenStage consumer(s) are too slow to handle the amount of incoming data (Tweets) the producer will stop ack the HTTP/TCP connection packages. The Twitter streaming endpoint will stop sending more data through the connection.

This is what whe want and follows the same pattern GenStage and Flow follow to consume data.

Without back-pressure, my producer would have to buffer the incoming Tweets or it’s internal message box would grow indefinitly. In both cases this would result in taking more and more memory, crashing the VM in the long term.