Krzysztof Żuraw

Transcoding with AWS- part four

  • #django
  • #aws

As I have my transcoder up and running now it’s time to let user know that their uploaded files were transcoded. To this occasion I will use AWS SNS service which allows me to send notification about completion of transcode job.

Setting up AWS SNS to work with AWS Transcoder

After logging to AWS console and selecting SNS I have to create a topic:

SNS topic

Topic is endpoint for other application in AWS to send their notifications. For my case I have to change it in AWS Transcoder pipeline settings:

Transcoder SNS subscription

Last thing I have to do was to create subscription for topic created above. They are a lot of types of subscription that you can find in SNS settings but I will be using HTTP request.

Receiving notifications from SNS service in Django

The flow of application will look like this:

  1. User upload a file
  2. File is sent to S3
  3. Transcode job is fired after uploading form view
  4. After transcode completion AWS transcoder sends SNS notification
  5. This notification is taken by SNS subscription and send to my endpoint
  6. After validating notification endpoint inform user that his or her files are transcoded

To receive HTTP notifications I have to create a endpoint in my Django application. First I add url in audio_transcoder/


Code for this endpoint looks as follows (audio_transcoder/

from django.views.decorators.csrf import csrf_exempt
from .utlis import convert_sns_str_to_json
from django.http import (

def transcode_complete(request):
    if request.method != 'POST':
        return HttpResponseNotAllowed(request.method)
    if request.META['HTTP_X_AMZ_SNS_TOPIC_ARN'] != settings.SNS_TOPIC_ARN:
        return HttpResponseForbidden('Not vaild SNS topic ARN')
    json_body = json.loads(request.body.decode('utf-8'), object_hook=convert_sns_str_to_json)
    if json_body['Message']['state'] == 'COMPLETED':
        # do something
    return HttpResponse('OK')

What is happening there? The first 2 ifs in transcode_complete are for checking if user sends POST request and as a SNS documentation says I have to make sure that message received are valid as everyone can send request to this endpoint.

In line with json_body I have to use helper that I pass to object_hook:

import json

def convert_sns_str_to_json(obj):
  value = obj.get('Message')
  if value and isinstance(value, str):
      obj['Message'] = json.loads(value)
  return obj

This small function is for converting nested strings received from SNS to python dicts. I know that every notification will have Message key so based on that I can load string and convert it to python dictionary.

The last if will be completed in next blog post.

Right now I have my endpoint up and running. But there is a problem - Amazon SNS needs to have access to that endpoint and I’m developing this application on my localhost. How to overcome such issue? I used ngrok which allows me to tunnel to my localhost from internet. How to use it? After downloading and unpacking you first run:

$ python transcoder/ runserver

And in other window:

$ ./ngrok http 9000

Ngrok will start and you can use url shown in console - for me:

With this url I go to AWS SNS subscription tab and add new subscription:

Creating a SNS subscription

After setting this up you will receive SNS message with link that you need to paste in browser to confirm subscription.

That’s all for today! In the next blog post I will take care about how to inform user that transcode job has completed. Feel free to comment - your feedback is always welcome.

The code that I have made so far is available on github. Stay tuned for next blog post from this series.