How to safely write to one file from many verticle instances in vert.x 3.2?

Question

Instead of using a logger or database server I'd like to append information to one file from possibly many verticle instances.

There are versions of methods for writing asynchronously to a file. Can I assume that vertx handles the synchronisation between the writes so that these dont interfere when using those versions of methods marked as ¨async¨ ? There seems to be a rule that one can rely on vertx providing all isolation between concurrent processing out of the box. But is that true in case of writing file access?

Could you please include a code snippet into the answer that shows how to open and write to one file from many verticle instances with finest possible granularity, e.g. for logging requests.

Answer 1

I wouldn't recommend writing to a single file with many different "writers". Regarding concurrent logging I would stick to the Single Writer principle.

Create a Verticle which subscribes to the Event Bus and listens for messages to be logged. Lets call this Verticle Logger which listens to system.logger.

EventBus eb = vertx.eventBus();

eb.consumer("system.logger", message -> {
  // write to file
});

Verticles which like to log something need to send a message to the Logger Verticle:

eventBus.send("system.logger", "foobar");

Appending to a existing file work something like this (didn't test):

vertx.fileSystem().open("file.log", new OpenOptions(), result -> {
    if (result.succeeded()) {
        Buffer buff = Buffer.buffer(message); // message from consume

        AsyncFile file = result.result();
        file.write(buff, buff.length() * i, ar -> {
            if (ar.succeeded()) {
                System.out.println("done");
            } else {
                System.err.println("write failed: " + ar.cause());
            }
        });
    } else {
        System.err.println("open file failed " + result.cause());
    }
});