Questionable write reliability for MongoDB (?)

I have a “standard” Meteor+MongoDB application deployed to Heroku and Compose, respectively.

One of my methods does two simple things: it inserts a set of data into two different collections.

However, looking into the database, I noticed that one collection has like 987 documents, while the other has 986.

The application is not under heavy load by any means. It’s making me very uncomfortable because I could no longer trust the two collections, and end-users might experience data loss.

Has anyone experienced this kind of problem, and does anyone what steps should be taken to at least try to guarantee 100% writes?

Have you set writeConcern to 1 or 2?

I think that should force MongoDB to acknowledge a write.

But, I’m not an expert.