How can I know Mongodb query runs on which node?

When connecting to a MongoDB replication cluster, I want to know which node the query runs on.

I tried to use explain() in mongo shell, but the Java driver doesn't seem to support this command.

How can I achieve this using MongoDB Java Driver?

Solution

You can try utilizing Command Monitoring from MongoDB Java Driver;

public class CustomCommandListener implements CommandListener {
    @Override
    public void commandStarted(final CommandStartedEvent event) {
        System.out.println(String.format("Sent command '%s:%s' with id %s to database" +
            " '%s' on connection '%s' to server '%s'",
            event.getCommandName(),
            event.getCommand().get(event.getCommandName()),
            event.getRequestId(),
            event.getDatabaseName(),
            event.getConnectionDescription().getConnectionId(),
            event.getConnectionDescription().getServerAddress()));
    }

    @Override
    public void commandSucceeded(CommandSucceededEvent event) {
        //ignore
    }

    @Override
    public void commandFailed(CommandFailedEvent event) {
        //ignore
    }
}

There are also CommandSucceededEvent and CommandFailedEvent, but regardless of its result, you can get some details with the CommandStartedEvent as above.

Then pass this custom listener into your MongoClient settings;

ClusterSettings clusterSettings = ClusterSettings.builder().hosts(hostList).build();
MongoClientSettings settings = MongoClientSettings.builder()
    .addCommandListener(new CustomCommandListener())
    .clusterSettings(clusterSettings)
    // other settings
    .build();
MongoClient client = MongoClients.create(settings);