Transcoding with AWS

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Nowadays moving everything to the cloud becomes more and more popular. A lot of software companies move their technology stack to such infrastructure. One of the biggest players in this field is Amazon Web Services - AWS. That’s why I decided decided to adapt existing code from my previous project and move transcoding to write blog posts about that. In this series I process to cloud.

Overview of series

I decided to adapt code from my previous blog series about celery and rabbit-mq. I did that because code from this django application actually transcodes mp3 files to other formats. This series will be divided into these parts:

Moving static and media files to AWS

AWS transcoder operates only on files that are inside S3 bucket so first I need to change how these files are served in django.

Let’s say that I already had my account on AWS. Next step is to generate specific account using IAM. While creating a user I want to give him access to AWS S3:

Policy for IAM user

and after I download its credentials I can create S3 container - I have chosen Ireland because with Frankfurt I have a problem with uploading files to S3. After bucket creation, it’s time to add policy. The policy is JSON that tells AWS which user can access given bucket. More information about that can be found here. Adding policy is quite simple from S3 management view:

Policy for S3 bucket

This policy looks like this:

  "Version": "2008-10-17",
  "Statement": [
      "Sid": "PublicReadForGetBucketObjects",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      "Action": "s3:GetObject",
      "Resource": "AWS_RESOURCE"
      "Effect": "Allow",
      "Principal": {
        "AWS": "AWS_PRINCIPAL"
      "Action": "s3:*",
      "Resource": ["AWS_RESOURCE"]

Where AWS_PRINCIPAL is your IAM user in format of ARN resource: "arn:aws:iam::AMAZON_ID:user/IAM_USER" and AWS_RESOURCE: arn:aws:s3:::S3_BUCKET_NAME/*.

As I have this set up right now I can make changes in the application itself. To use S3 as a static and media files container I used django-storages. To make django-storages to work I have to add couple things in

import environ
env = environ.Env()

  # other applicaitons

    'Expires': 'Thu, 15 Apr 2010 20:00:00 GMT',
    'Cache-Control': 'max-age=86400',
AWS_S3_HOST = ''

I’m using here another package called django-environ. It allows me to get certain settings from environmental variables. I’m setting them in my virtualenvwrapper script inside $ENV_PATH/bin/postactivate:

export AWS_ACCESS_KEY_ID='key'
export AWS_SECRET_ACCESS_KEY='acces_id'

The last line with AWS_S3_HOST is really important here as boto - client that django-storages use underneath to connect to AWS doesn’t have default region set up. If this is not specified I upload files with redirection which don’t allow to transfer static files or upload any large media file.

As I have AWS settings set up there is time to change static files settings in

STATICFILES_STORAGE = 'audio_transcoder.storages.StaticStorage'
  os.path.join(BASE_DIR.root, 'static'),

I add custom StaticStorage as I want my static files to be under static in S3 bucket:

from django.conf import settings
from storages.backends.s3boto import S3BotoStorage

class StaticStorage(S3BotoStorage):
  location = settings.STATICFILES_LOCATION

To upload my static files I run python collectstatic. After a while I can see that my files are in a bucket:

Static files inside S3

Right now when I run my server I can see the location of my static files:

Static files loaded from S3

As static files are working it’s high time to use AWS for media files. Right now it’s simple - in settings I add:

DEFAULT_FILE_STORAGE = 'audio_transcoder.storages.MediaStorage'

with custom storage:

class MediaStorage(S3BotoStorage):
  location = settings.MEDIAFILES_LOCATION

Now when I upload my mp3 file it’s sent directly to S3 bucket under media location:

Media files in S3

As I have static and media files integrated with AWS now it’s time to transcode them. In this post, I will write a short example of how to integrate AWS ElasticTranscoder with Django application.

Basic terms

ElasticTranscoder allows you to transcode files from your S3 bucket to various formats. To set this service up first you have to create a pipeline. What pipeline is? It’s a workflow- how your transcoder should work. You can create a different pipeline for long content and different for short one. In my application I created the following pipeline:

Pipeline configuration

As I have my pipeline configured next step is to create jobs. Jobs are tasks for a transcoder that say which file I want to transcode, to what format or codec I want to do this:

Job details

PresetID is user created or already existing configuration that defines the format of transcoder output: is it mp4 or maybe flac? What resolution should video files have? All of this is set up in present.

As we know basic terms used in AWS Elastic Transcoder let’s jump into the code.


AWS has very good python API called boto3. Using that API and few examples from the internet I was able to create a simple class to create transcode job:

import os

from django.conf import settings

import boto3

class AudioTranscoder(object):

  def __init__(self, region_name='eu-west-1', pipeline_name='Audio Files'):
      self.region_name = region_name
      self.pipeline_name = pipeline_name
      self.transcoder = boto3.client('elastictranscoder', self.region_name)
      self.pipeline_id = self.get_pipeline()

  def get_pipeline(self):
      paginator = self.transcoder.get_paginator('list_pipelines')
      for page in paginator.paginate():
          for pipeline in page['Pipelines']:
              if pipeline['Name'] == self.pipeline_name:
                  return pipeline['Id']

  def start_transcode(self, filename):
      base_filename, _ = self.create_aws_filename(filename, '')
      wav_aws_filename, wav_filename = self.create_aws_filename(
          filename, extension='.wav'
      flac_aws_filename, flac_filename = self.create_aws_filename(
          filename, extension='.flac'
      mp4_aws_filename, mp4_filename = self.create_aws_filename(
          filename, extension='.mp4'
              'Key': base_filename,
              'FrameRate': 'auto',
              'Resolution': 'auto',
              'AspectRatio': 'auto',
              'Interlaced': 'auto',
              'Container': 'auto'
              'Key': wav_aws_filename,
              'PresetId': '1351620000001-300300'
              }, {
              'Key': flac_aws_filename,
              'PresetId': '1351620000001-300110'
              }, {
              'Key': mp4_aws_filename,
              'PresetId': '1351620000001-100110'
      return (wav_filename, flac_filename, mp4_filename)

  def create_aws_filename(filename, extension):
      aws_filename = os.path.join(
          settings.MEDIAFILES_LOCATION, filename + extension
      return aws_filename, os.path.basename(aws_filename)

transcoder = AudioTranscoder()

Going from the top - I specified my region_name as well as pipeline_name for boto3 to know to which region it should connect. In method get_pipeline I iterate through all available pipelines and return that has the same name as pipeline_name. In this function paginator is an object which holds on portion of data so user don’t have to wait until all available pipelines are fetched.

The main logic is in start_transcode method. At the beginning, I used helper function create_aws_filename that’s creating proper AWS file name like media/my_mp3.mp3 and returns that whole path with filename. After I created filenames for all of my files I’m calling create_job that creates a job basing on pipeline_id and base_filename. The job can have multiple outputs so I specified one for wav, flac and mp4 files. How is it used in code? Let’s go to view:

class UploadAudioFileView(FormView):
  # some code

  def form_valid(self, form):
      audio_file = AudioFile(
      wav_filename, flac_filename, mp4_filename = transcoder.start_transcode(

      audio_file.mp4_file = mp4_filename
      audio_file.flac_file = flac_filename
      audio_file.wav_file = wav_filename
      return HttpResponseRedirect(
          reverse('audio:detail', kwargs={'pk':})

In form_valid first I’m calling save() on AudioFile object which is uploading the file to S3 bucket. Then I’m using transcoder.start_transcode and basing on output from this function I match filenames to their respective fields. I know that this solution is not the best one as I have to call save twice and if you have a better way to do this I’m glad to hear it from you.

I’ve ran transcoder from django application using AWS python API. But there is also one more way to do the same - use AWS Lambda. Today I will write how to use this tool to trigger transcoding of uploaded files.

What is AWS Lambda

AWS Lambda is a service that allows you to run code against some event. What event may you say? For instance uploading a file to S3 bucket. In my example, I use this service to start transcode jobs. User upload file to media/music_file and then instead of firing up event from django application I trigger AWS Lambda function that does the same job.

Right now it’s time to jump into the code.

Setting up AWS Lambda for transcoder

When you want to create an AWS Lambda functions you can use a couple of predefined functions a.k.a blueprints. As a base, I used one called: s3-get-object-python. As you chosen your function now it’s time to add trigger so the function can run.

AWS Lambda configuration

And AWS Lambda function is created! But by default, it only gets content type of the object that is put in the S3 bucket. If I want to start to transcode job I can use following code:

import os
import boto3

def lambda_handler(event, context):
  transcoder = boto3.client('elastictranscoder', 'eu-west-1')
  pipeline_id = get_pipeline(transcoder, 'Audio Files')
  base_filename = os.path.basename(event['Records'][0]['s3']['object']['key'])
  output = transcoder.create_job(
          'Key': create_aws_filename('media', base_filename, ''),
          'FrameRate': 'auto',
          'Resolution': 'auto',
          'AspectRatio': 'auto',
          'Interlaced': 'auto',
          'Container' : 'auto'
          'Key': create_aws_filename('transcoded', base_filename, '.wav'),
          'PresetId': '1351620000001-300300'
          }, {
          'Key': create_aws_filename('transcoded', base_filename, '.flac'),
          'PresetId': '1351620000001-300110'
          }, {
          'Key': create_aws_filename('transcoded', base_filename, '.mp4'),
          'PresetId': '1351620000001-100110'
  return output

def get_pipeline(transcoder, pipeline_name):
      paginator = transcoder.get_paginator('list_pipelines')
      for page in paginator.paginate():
          for pipeline in page['Pipelines']:
              if pipeline['Name'] == pipeline_name:
                  return pipeline['Id']

def create_aws_filename(folder, filename, extension):
      aws_filename = os.path.join(
          folder, filename + extension
      return aws_filename

It’s code from my previous posts but modified in a few places so it can work in AWS Lambda. In this service, you can use python 2.7. The main function is called lambda_handler and takes an event from S3 in form of JSON and context which is python object. As you can see creating transcoder and pipeline_id are the same as previously. base_filename is taken from event JSON. Then I create transcode job and return its output.

As you may noticed I specified a different folder for outputs than for inputs. Why? Because this function has trigger for put in media. Then it starts transcoder jobs that are creating files on S3 bucket. If I specified the same location for output I can start recursion and AWS Lambda start triggering itself. It’s not a good idea and doesn’t try this at home unless you have a lot of money. That’s why it is so important to test your code before you run it. It’s possible - while saving your code you can add the event to test so your AWS Lambda the function will run against this test event:

  "Records": [
      "eventVersion": "2.0",
      "eventTime": "2016-12-15T21:20:44.231Z",
      "requestParameters": {
        "sourceIPAddress": "IP_ADDRESS"
      "s3": {
        "configurationId": "configurationId",
        "object": {
          "eTag": "eTag",
          "sequencer": "sequencer",
          "key": "media/5981d6e9-8e88-44a9-bd7b-f8dce886877b",
          "size": 571258
        "bucket": {
          "arn": "arn:aws:s3:::YOUR_BUCKET_NAME",
          "name": "YOUR_BUCKET_NAME",
          "ownerIdentity": {
            "principalId": "YOUR_BUCKET_ID"
        "s3SchemaVersion": "1.0"
      "responseElements": {
        "x-amz-id-2": "x-amz-id-2",
        "x-amz-request-id": "x-amz-request-id"
      "awsRegion": "eu-west-1",
      "eventName": "ObjectCreated:Put",
      "userIdentity": {
        "principalId": "AWS:USER_ID"
      "eventSource": "aws:s3"

Right now clicking test you can know if your function is behaving correctly:

AWS Lambda test function result

That’s all! Your function is working and creating transcode jobs. This is another way of accomplishing the same result -transcoding the files uploaded from Django.

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.

How message application should work

From previous I know that the last point of my application flow is to inform user that file is transcoded and ready to download. To do such thing I have to display message on every webpage that current user is. This message should have information about which file was processed. First I wanted to do this with existing django messaging framework but as it turns out is works only with request. As I decided to show message for different users as long as they dismiss this information I had to write my own small application.

Implementation in django

In my newly created application I created following model:

from django.db import models
from django.contrib.auth.models import User

class Message(models.Model):
    text = models.CharField(max_length=250)
    read = models.BooleanField(default=False)

    def __str__(self):
        return self.text

I decided to display my message only when it wasn’t read. Based on that right now I can use it in endpoint that works with AWS (audio_transcode/

def transcode_complete(request):
    # rest of code is in previous blog post
    if json_body['Message']['state'] == 'COMPLETED':
        audio_file = AudioFile.objects.get(
        text='Your file {} has been processed'.format(
    return HttpResponse('OK')

As my message is created right now comes time for displaying it to the user. To do that I have to add a message to template context. It can be done via creating your own context manager:

from .models import Message

def message_context_processor(request):
    if request.user.is_anonymous():
        return {'messages': []}
    return {'messages': Message.objects.filter(read=False)}

And registering it:

        # rest of options
        'OPTIONS': {
            'context_processors': [
            # rest of context processors

And adding a message as django template tag:

{% if messages %} {% for message in messages %}
  class="alert alert-success alert-dismissible"
  data-message-id="{{ }}"
  data-message-url="{% url 'messages:read-message' %}"
  <button type="button" class="close" data-dismiss="alert" aria-label="Close">
    <span aria-hidden="true">x</span>
  {{ message.text }}
{% endfor %} {% endif %}

Which renders as follows:

Transcode complete message

In the previous screenshot, there is an X that dismiss the message and make it read. To communicate with the backend I wrote quick jQuery script:

var csrftoken = Cookies.get("csrftoken");

function csrfSafeMethod(method) {
  // these HTTP methods do not require CSRF protection
  return /^(GET|HEAD|OPTIONS|TRACE)$/.test(method);
  beforeSend: function (xhr, settings) {
    if (!csrfSafeMethod(settings.type) && !this.crossDomain) {
      xhr.setRequestHeader("X-CSRFToken", csrftoken);

$(".alert").on("", function (event) {
    method: "POST",
    data: { message_id: },

Going from the top - django by default uses csrftoken so I have to get it that my request passes the authentication. I’m using here library called js-cookie. In ajaxSetup I tell jQuery to always send csrftokens while using ajax request. Below I add the event listener to an element that has .alert class. This event - is provided by bootstrap. On triggering this event I send ajax POST to url from data attribute in alert element -data-message-url. Data that I send is taken from data-message-id attribute on alerts div. How endpoint for receiving such messages looks like? See below:

from .models import Message
from django.http import HttpResponse

def read_message(request):
     message = Message.objects.get(id=request.POST['message_id']) = True
 return HttpResponse('OK')

Here I take message_id and set read to True and save message.

That’s all for this blog post and blog series! I know that in this design are particular flaws like: what is there will be more users than one? Everybody will see everyone messages. If you have idea how to fix that please write in comments below.

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