Mongodb replicator is a java based library to stream changes (change-data-capture aka CDC) from MongoDB database and provides an application hook for the developers to Replication Handlers for custom transformation & specific destination related implementation. Sample implementation is here.

• Stream ordered change-list entries from master oplog
• Works with both non-sharded & sharded MongoDB clusters
• Takes care of master-slave flips & listens to master always
• Takes care of shard additions/removals in a sharded setup
• Express filters as your interested data set and only those filtered events will reach ReplicationHandler
• Provides check-point hooks which can be implemented based on the needed

• The change-list entries across shards in a sharded-cluster can have data ordering issues if chunk migration is turned on. In a sharded-cluster oplog streaming during chunk migration is not synchronized. However, this can be worked around by assuming partial ordering guarantees from this stream & have another level of ordering (using entity versions). For a standalone replicaset, this is fully functional.
• Runs on a single node. Needs to have a zookeeper or hazelcast based co-ordination to make it highly-available & distributed.

• Can be used as streaming pipe for Hot-Cold store implementation where cold has to be eventually consistent & Hot is MongoDB
• Can be used as sourcing process for change propogation
• Can be used for low-latency event sourcings & further processing
• Can be used also for XDCR (cross-dc-replication)