Spring Data MongoDB Reactive: How to get the result of db.runCommand() - mongodb

Question

We're currently using spring data mongodb reactive as part of springboot application to connect to AWS Document DB.
I'm trying to enable change streams on the documentdb database on application startup. The idea is to enable it only if already not enabled. To this, I'm having the following code that runs the command using MongoClient from reactive package but I don't know how to read the response from running the query that checks if the change stream is already enabled.
#Service
public class InitDB {
private final MongoClient mongoClient;
private static final String enableChangeStreams = """
{modifyChangeStreams: 1,
database: "",
collection: "",
enable: true}
""";
private static final String isChangeStreamsEnabled = """
{aggregate: 1,
pipeline: [{$listChangeStreams: 1},
{$match: {$or: [{database: "", collection: ""}]}}
],
cursor:{}}
""";
#Autowired
public InitDB(final MongoClient mongoClient) {
this.mongoClient = mongoClient;
}
public void initializeDB() {
runCommand(isChangeStreamsEnabled);
}
private void runCommand(final String command) {
final MongoDatabase db = mongoClient.getDatabase("admin");
final Document document = Document.parse(command);
final Bson bson = document.toBsonDocument();
final Publisher<Document> commandResult = db.runCommand(bson);
commandResult.subscribe();
//How to fetch the command result response into a variable via subscribers?
final SubscriberHelpers.ObservableSubscriber subscriber = new SubscriberHelpers.ObservableSubscriber();
commandResult.subscribe(subscriber);
System.out.println(subscriber.isCompleted());
}
}
Can someone please guide on how to fetch the command result response into a variable via subscribers?
Based on the response, if the change stream is not enabled then I would want to run enableChangeStreams command (the code is yet to be built for this)

Answer 1

Since you are resorting to a blocking primitive using the SubscriberHelpers#ObervableSubscriber, you can use the await / get methods to block the executing context waiting for the MongoDB query response:
private void runCommand(final String command) {
final MongoDatabase db = mongoClient.getDatabase("admin");
final Document document = Document.parse(command);
final Bson bson = document.toBsonDocument();
final Publisher<Document> commandResult = db.runCommand(bson);
final SubscriberHelpers.ObservableSubscriber subscriber = new SubscriberHelpers.ObservableSubscriber();
commandResult.subscribe(subscriber);
// block waiting for command response
System.out.println(subscriber.get(1000, TimeUnit.MILLISECONDS));
}