On October 31, 2014, MongoDB Inc. introduced on their community blog three different approaches to solve a very common problem, social inboxes.
If you want to see the blog post, refer to http://blog.mongodb.org/post/65612078649/schema-design-for-social-inboxes-in-mongodb.
From the three presented schema designs, it is possible to see the application of all the scaling concepts we have seen until now in an easy and efficient way. In all of the cases, the concept of a fan out applies, in which the workload is distributed among the shards in parallel. Each approach has its own application according to the needs of the database client.
The three schema designs are: