MongoDB's oplog is kind of similar to MariaDB's binary log:

http://docs.mongodb.org/manual/core/replica-set-oplog/

At my last job, we had a project that needed to replicate from MongoDB
to PostgreSQL. To do that, we used a tool called mosql from Stripe
which does that by tailing the oplog and transforming the data to
something usable by Postgres:

https://github.com/stripe/mosql

I don't know how many of our users need to replicate from MongoDB to
MariaDB, but some product ideas are:

1.) A maxscale plugin that tails a MongoDB oplog and transforms and
stores the data in MariaDB.

2.) Some kind of oplog-to-binlog converter (could we also convert in
the other direction?).

3.) A storage engine that keeps tables automatically up-to-date by
tailing a MongoDB oplog.

By the way, the ruby gem that Stripe uses to tail the oplog is
licensed with an MIT license:

https://github.com/stripe/mongoriver