We are considering removing Mongo support from Keycloak in 3.x. The reasons behind it is that there are a fair few issues in the current implementation, especially around consistency due to lack of transaction support in Mongo and often we update multiple documents. In many cases we rely on transactions to rollback to prevent partial updates, but this obviously doesn't work in Mongo.
With the fact that Mongo is already partially broken and the constant maintenance involved we're considering removing it and rather focus purely on the relational database back-end.
Another point to make is that we are not considering supporting Mongo in the supported version of Keycloak (Red Hat Single Sign-On). So we are never able to provide the same level of care and attention to it as we can for relational databases.
If we do decide to remove it we would make sure we provide a seamless and easy option to migrate from Mongo to a relational database!
I would like to gather some feedback from the community before doing anything. So please vote on the following Doodle:
http://doodle.com/poll/nnimebpkx774ppus
Also, comments on this post is more than welcome!
I'll end with a comment - Time spent by core developer on maintaining Mongo could be better spent on awesome new features, testing and bug fixing!
