Realtime - Listen to changes in documents

How can we listen to changes (CRUD) in documents related to a particular index?

For now the realtime channels you can subscribe are very broad ones.
https://appwrite.io/docs/realtime#channels

So, for your use case you'll probably need to subscribe to the databases.[ID].collections.[ID].documents channel.
Then, You can check the events to see that it matches what you're looking for.

Okay. Thanks @Binyamin

Also, is there any way to create a transaction and then execute it in one go to prevent inconsistencies ?

Not for now, you can upvote it here: https://github.com/appwrite/appwrite/issues/2788

Okay great.