Question

I’m implementing an iPhone app with the “following” feature on Java/GAE.
This feature is similar to Instagram’s in that a user can follow other users’ events. Events can be new photos posted, liking photos, following other users, etc.

Let say we have two entities: Follow and Event, both of which have a reference to the user id.

So I’m thinking about using OR queries where we can list events of target users. But GAE does not support OR queries, and issuing separate queries would be also inefficient as the number of queries can be quite large (e.g. 100+ follows is a norm).

What is a common practice for doing this?

Answer 1

You have to stop thinking in the relational model 😉 We do something similar on our app. Keep a List of UIDs that are “subscribed” to the user who is being followed in the followed user’s entity. When the user does something that requires notification create an event entity for each user who is following the user (yes, a popular user could create a whole bunch of entries, this could be offloaded to a Task Queue).

When a user checks for notifications you can fetch them by his UID. Since this is indexed it will return very quickly. When you fetch the events, delete them or mark them read.

Remember, joins and other filtered queries are very slow, but fetching massive numbers of indexed records is not. If something is going to create a large number of records, offload it to a Task Queue.