FAQ

How can I achieve data locality?

For any MongoDB deployment, the Mongo Spark Connector sets the preferred location for a DataFrame or Dataset to be where the data is:

• For a non sharded system, it sets the preferred location to be the hostname(s) of the standalone or the replica set.
• For a sharded system, it sets the preferred location to be the hostname(s) of the shards.

To promote data locality,

• Ensure there is a Spark Worker on one of the hosts for non-sharded system or one per shard for sharded systems.
• Use a nearest read preference to read from the local mongod.
• For a sharded cluster, you should have a mongos on the same nodes and use localThreshold configuration to connect to the nearest mongos. To partition the data by shard use the ShardedPartitioner Configuration.

How do I resolve Unrecognized pipeline stage name Error?

In MongoDB deployments with mixed versions of mongod, it is possible to get an Unrecognized pipeline stage name: '$sample' error. To mitigate this situation, explicitly configure the partitioner to use and define the Schema when using DataFrames.