The Reddit’s front page contain a list of recent posts from all communities. In most cases, you want to show posts from communities that the user is subscribe to, but at the same time, you want to avoid flooding the front page with posts from any single community. You also need this to be really fast.
It turns out that doing this in RavenDB is actually very easy. We are going to create a map/reduce index that will aggregate the few most recent posts per community, like so:
What this index will do is provide us with the five most recent posts in each community, as well as their date. This is an interesting example of a map/reduce index, because we are using both aggregation and fanout in the index.
The nice thing about this index is that we can project the results directly from it to the user. Let’s see how the queries will look like:
This is a simple query that does quite a lot. It gives us the most recent 15 posts across all the communities that the user care about, with no single community able to generate more than 5 posts. It sort them the posted date and fetch the actual posts in the same query. This is going to give you consistent performance regardless of how much data you have and how many updates your experience. The actual Reddit front page is a lot more complex, I’m sure, but this serve as a nice example of how you can do non trivial stuff in RavenDB’s indexes that simplify your life by a lot.