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.

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


deflambda_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(
      PipelineId=pipeline_id,
      Input={'Key': create_aws_filename('media', base_filename,''),'FrameRate':'auto','Resolution':'auto','AspectRatio':'auto','Interlaced':'auto','Container':'auto'},
      Outputs=[{'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


defget_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']defcreate_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:

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.

It’s the last blog post in this year - Merry Christmas and Happy New Year!

Special thanks to Kasia for being an editor for this post. Thank you.