Demystifying cron job through AWS lambda in Serverless

Some services like video upload to social media like Facebook, YouTube, etc via API need some time to process the request as it takes time to finish the processing of a video by Facebook or YouTube. At Craftsmen, for one of our customers we have a service API where users can send us requests with a video URL, and in which social media platform the video should be uploaded, our API will upload the video to the provided social media platform in a short moment.

But the problem is that we don’t know when the processing from these social media platforms will be finished. Let’s first see our workflow to upload a video.

Flowchart for uploading a video to YouTube

So you can see that the request is directly passed to AWS Fargate because it may take more than 30 seconds to upload the video and any API request will last till 29 seconds and after that, it will return response even if the request is still processing. Now one problem arises is that the Fargate task only feeds the video to the social media platform’s API like Facebook graph API and YouTube data API. We don’t actually know if the video upload is successful or not as processing may take at least 2–3 minutes.

So we introduced a CRON lambda based solution to overcome the problem. The basic idea is shown in the flowchart below:

CRON lambda flowchart

The main theme is that, whenever a new request comes to the AWS Fargate task, after uploading it, we save the video information in the main DB as a processing state and add a new item in another DB called Polling DB. Our CRON lambda runs after every 5 minutes and checks if there is any new item in the Polling DB. If so, then it tries to fetch the info. about the item from the social media API and update the main DB accordingly.

The code for the CRON lambda is simple. To make a lambda run as a CRON job, we just need to specify in the YML like below:

Name of your lambda:
 handler: source to your handler lambda
 events:
  - schedule: rate(5 minutes)

rate (5 minutes) means that the CRON will run after every 5 minutes. We use node JS and TypeScript. To test it offline, you will need a plugin called serverless-offline-scheduler. You will find it in the npm library.

Main code is below:

The driver function is the main.ts file. At first, it tries to fetch all the items whose polling status is false and the poll count is less than 5. That means we will retry the process for each item at most 5 times. If the poll count is greater than or equal to 5, we consider that the process was failed. So we update the info for that item in the main DB as unprocessed and delete the item from the polling table. Thus the polling table will never get overpopulated with items as we are deleting items from thereafter the process is done.

If the polling status is false and the poll count is less than 5, that means we need to process the item. So we store the items in an array. Then immediately after that, we update the polling status of the items stored in the array to true. We do this because if the CRON running currently takes more than 5 minutes, another new CRON will also start running and it will try to fetch all the items in the Polling DB. So we set this polling status to true so that the CRON lambdas that will run later can understand that these items are being processed by another CRON.

Then we try to see for each id if there exists any video in social media. If so, that means the video processing is done by the social media servers. So we update the info for that item in the main DB as processed and delete the item from the polling table as it’s work is done. If we can’t find any item in the social media with that id, then we update the polling status to false for that item in the Polling DB and increase the poll count by one. Thus another CRON which will run later will fetch this item also for processing, as it has polling status false.

This is how we used lambda CRON in our project to overcome the problems we had. To test it offline, you need to install serverless-offline-scheduler and run the command SLS schedule or serverless schedule. It will run only the CRON lambda in offline.