MongoDB connections from AWS Lambda

I'm looking to create a RESTful API using AWS Lambda/API Gateway connected to a MongoDB database. I've read that connections to MongoDB are relatively expensive so it's best practice to retain a connection for reuse once its been established rather than making new connections for every new query.

This is pretty straight forward for normal applications as you can establish a connection during start up and reuse it during the applications lifetime. But, since Lambda is designed to be stateless retaining this connection seems to be less straight forward.

Therefore, I'm what would be the best way to approach this database connection issue? Am I forced to make new connections every time a Lambda function is invoked or is there a way to pool these connections for more efficient queries?

Thanks.