Error while connecting to AWS DocumentDB: Command isMaster not supported on $external database

Question

I’m trying to connect to AWS DocumentDB server using the Rust driver with MONGODB-AWS ($external) authentication. A simple “list collections” call fails during the initial handshake with:

Command isMaster not supported on $external database

Following code:

use std::path::PathBuf;
use mongodb::Client;
use mongodb::options::{AuthMechanism, ClientOptions, Credential, Tls, TlsOptions};

#[tokio::main]
async fn main() {
    let uri = "mongodb://localhost:27017/tpch?directConnection=true";

    let mut opts = ClientOptions::parse(uri).await.unwrap();

    opts.credential = Some(
        Credential::builder()
            .username("<access_key>".to_string())
            .password("<secret_access_key>".to_string())
            .source("$external".to_string())
            .mechanism(AuthMechanism::MongoDbAws)
            .build()
    );

    let ca_path = PathBuf::from("/Users/krinart/global-bundle.pem");
    opts.tls = Some(Tls::Enabled(
        TlsOptions::builder()
            .ca_file_path(Some(ca_path))
            .allow_invalid_hostnames(true)
            .build(),
    ));

    let client = Client::with_options(opts).unwrap();

    let db = client.database("tpch");
    println!("Collections: {:?}", db.list_collection_names().await.unwrap());
}

Returns:

Error { kind: Command(CommandError { code: 73, code_name: "", message: "Command isMaster not supported on $external database.", topology_version: None }), labels: {}, wire_version: None, source: None }

For the context this command works:

AWS_ACCESS_KEY_ID=<> AWS_SECRET_ACCESS_KEY=<> mongosh \
  --host localhost \
  --port 27017 \
  --tls \
  --tlsCAFile /Users/krinart/global-bundle.pem \
  --tlsAllowInvalidHostnames \
  --authenticationDatabase '$external' \
  --authenticationMechanism MONGODB-AWS \
  tpch

SYAM KRISHNA MG · Accepted Answer · 2025-08-21 06:09:46Z

0

This is because DocumentDB does not support isMaster; it utilizes hello instead, particularly in newer releases (v5.0). Ensure your driver version is compatible and uses hello, or upgrade the cluster to v5.0 for better API alignment with MongoDB

answered Aug 21 at 6:09

SYAM KRISHNA MG

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1 Comment

Viktor Ershov Aug 21 at 16:31

I use DocumentDB 5.0.0 and I get the same issue with hello.

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