Problem Statement:

Extend this design to support the download for batches stats upto 1L participants.

Solutions

 'Scroll API ' can be used to retrieve large numbers of results (or even all results) from a single search request, in much the same way as you would use a cursor on a traditional database.

Pros:

1. We can retrieve large data set 
2. We can slice the data based upon shards 

Cons:

1. We can not use scroll api for real time user request

Approach 1:

Start the service instantly to download the batches stats

Pros:

1. No extra efforts require to handle the request 

Cons:

1. Duplicate request will entertain multiple times

Approach 2:

Start the service in some specified time of day.

We can queue all the request coming for batch stats. Our scheduler will check the queue and start processing the request one by one 

Pros:

1. We can avoid duplicate requests

Cons:

1. We have to maintain a queue to handle all the request.
2. We have to schedule a scheduler.