HDFS file watcher

Question

Can I have file watcher on HDFS?

Scenario: The files are landing on HDFS continuously.I want to start a Spark Job once the number of files reached a threshold(it can be number of files or size of the files).

Is it possible to implement file watcher on HDFS to achieve this . If yes, then can anyone suggest the way to do it?What are the different options available? Can the Zookeeper or the Oozie do it?

Any help will be appreciated.Thanks.

Answer 1

Oozie coordinator can do this. Oozie coordinator actions can be triggered based on data availability. Write a data triggered coordinator. The coordinator actions are triggered based on the done-flag. done-flag is nothing but an empty file. So when your threshold is reached write an empty file into the directory.

Answer 2

Hadoop 2.6 introduced DFSInotifyEventInputStream that you can use for this. You can get an instance of it from HdfsAdmin and then just call .take() or .poll() to get all the events. Event types include delete, append and create which should cover what you're looking for.

Here's a basic example. Make sure you run it as the hdfs user as the admin interface requires HDFS root.

public static void main( String[] args ) throws IOException, InterruptedException, MissingEventsException
{
    HdfsAdmin admin = new HdfsAdmin( URI.create( args[0] ), new Configuration() );
    DFSInotifyEventInputStream eventStream = admin.getInotifyEventStream();
    while( true ) {
        EventBatch events = eventStream.take();
        for( Event event : events.getEvents() ) {
            System.out.println( "event type = " + event.getEventType() );
            switch( event.getEventType() ) {
                case CREATE:
                    CreateEvent createEvent = (CreateEvent) event;
                    System.out.println( "  path = " + createEvent.getPath() );
                    break;
                default:
                    break;
            }
        }
    }
}

Here's a blog post that covers it in more detail:

http://johnjianfang.blogspot.com/2015/03/hdfs-6634-inotify-in-hdfs.html?m=1