Error when connecting to Amazon DocumentDB due to default_source

[kbuman-embotech]

Versions/Environment

1. What version of Rust are you using? 1.76.0
2. What operating system are you using? Ubuntu 20.04.6 LTS
3. What versions of the driver and its dependencies are you using? mongodb@3.1.0,bson@2.12.0
4. What version of MongoDB are you using? 5.0.0
5. What is your MongoDB topology? Amazon DocumentDB

Bug description

In the case of AuthMechanism::MongoDbAws, the implementation of default_source<'a>(&'a self, uri_db: Option<&'a str>) -> &'a str[] in client/auth.rs returns $external, which is used to overwrite the target_db field of the hello command in handshake.rs. However, running hello and isMaster is not supported on $external in Amazon DocumentDB deployments:

"ok": 0,
"code": 73,
"errmsg": "Command isMaster not supported on $external database."

Expected behavior

I expect to be able to handshake and connect to Amazon DocumentDB clusters by using a connection string of the form mongodb://<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&authMechanism=MONGODB-AWS.

To Reproduce
Steps to reproduce the behavior:

1. Deploy an Amazon DocumentDB cluster.
2. Create the necessary IAM role and instance profile.
3. Associate an EC2 instance with the instance profile created in 2..
4. Login to the EC2 instance using SSH.
5. Use mongo-rust-driver to connect to the cluster created in 1. with a connection string of the form mongodb://<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&authMechanism=MONGODB-AWS.
6. Bug occurs.

Note
I am not entirely certain whether this is actually a bug in mongo-rust-driver, or a configuration issue on the part of AWS. I was able to connect to my cluster by making the hello command in the handshake target the admin database.

[isabelatkinson]

Hey @kbuman-embotech, MongoDB and DocumentDB are not equivalent, and the driver is only written to support interacting with MongoDB clusters. See here for more details on the differences between the two. Unfortunately we are not able to provide guidance for working with DocumentDB, but please let us know if you run into a similar issue when connected to MongoDB.

[kbuman-embotech]

Thanks a lot for the clarification @isabelatkinson. Much obliged.