I’ve recently onboarded into a vertx codebase and have seen usage of singletons like:

class MainVerticle extends AbstractVerticle {
    ...

    @Override
    public void start(Promise<Void> startPromise) {

        DatabaseClientWrapper.initialize(vertx);
        vertx.deployVerticle(new OtherVerticle(...));

    }

}

Where a singleton is initialized like the above, and then is later used like so:

class OtherVerticle extends AbstractVerticle {

    @Override
    public void start(Promise<Void> startPromise) {
        DbResult<T> result = DatabaseClientWrapper.getInstance().queryDatabase(...)
            .onComplete(ar -> {
                try {
                  processResult(ar);
                } catch (Exception e) {
                   handleFailure(e);
                }
            });
    }

}

My first question is if its ok to use a singleton like this? Will it cause any issues with multithreading?

Secondly, wouldn’t it be better to create a verticle for the database client, and then a verticle for the database service, and access the client around the application over the event bus?

Let me know what you think.

Tried researching different vertx patterns