# Striim 3.10.1 documentation

#### MongoDB setup

MongoDBReader reads data from the Replica Set Oplog, so to use it you must be running a replica set (see Deploy a Replica Set or Convert a Standalone to a Replica Set). For more information, see Replication and Replica Set Data Synchronization.

In InitialLoad mode, the user specified in MongoDBReader's Username property must have read access to all databases containing the specified collections. In Incremental mode, the user must have read access to the local database and the oplog.rs collection. The oplog is a capped collection, which means that the oldest data is automatically removed to keep it within the specified size. To support recovery, the oplog must be large enough to retain all data that may need to be recovered. See Oplog Size and Change Oplog Size for more information.

To support recovery (see Recovering applications), the replica set's oplog must be large enough to retain all the events generated while Striim is offline.