How to use Deferred

[Enorio]

I'm having trouble understanding the flow of the Deferred class.
For example, this API is used in the AsyncKudu. How can I do this method?

public void foo(){
    //verify that the table exists;
    ...
    //if exists, open table;
    ...
    //write on the table;
   ...
}

More specifically, the AsyncKuduClient has this method to see if a table exists:

Deferred<Boolean> aux = kuduClient.tableExists(tableName);

What can I do after this?
And after that, how can I call this method asynchronously?

Thank you.

[Enorio]

I did it. I can reach the kudu table and write what I want in there. Although I get this error

java.lang.AssertionError: Tried to resume the execution of Deferred@2103498087(state=DONE, result=true, callback=, errback=)) although it's not in state=PAUSED. This occurred after the completion of Deferred@316763603(state=RUNNING, result=Written with success., callback=, errback=) which was originally returned by callback=com.foo.kudu.KuduDAOImpl$$Lambda$34/1065552698@465c6144@1180459332

Any idea what this is?

[manolama]

So for the first part, you use a Deferred like a Future so add a callback using one of the addCallback() methods and/or addErrorback(). Or call .join() on it.

For that error, it sounds like the deferred was executed twice. Make sure you are not calling aux.callback(some object) anywhere. Only the AsyncKuduClient should do that.

[Enorio]

The only way I could reach the kudu database and write there was like this
this is basically callbacks inside callbacks inside callbacks :D

public void asyncWrite(String tableName, Foo foo) {
        Deferred<Deferred<Boolean>> res = new Deferred<>();

        res.addCallback(new Callback<Deferred<Deferred<String>>, Deferred<Boolean>>() {
            @Override
            public Deferred<Deferred<String>> call(Deferred<Boolean> booleanDeferred){
                return booleanDeferred.addCallbacks(new Callback<Deferred<String>, Boolean>() {
                    @Override
                    public Deferred<String> call(Boolean aBoolean) throws Exception {
                        if (aBoolean) {
                            Deferred<String> stringDeferred = kuduClient.openTable(tableName)
                                    .addCallbacks(new Callback<String, KuduTable>() {
                                        @Override
                                        public String call(KuduTable kuduTable) throws KuduException {
                                            AsyncKuduSession session = kuduClient.newSession();
                                            session.setFlushMode(SessionConfiguration.FlushMode.AUTO_FLUSH_SYNC);

                                            Upsert upst = kuduTable.newUpsert();
                                            PartialRow row = upst.getRow();
                                            row.addString("house_id", foo.getId());
                                      
                                            session.apply(upst);
                                            session.close();
                                            LOGGER.info("Written with success.");
                                            return "Written with success.";
                                        }
                                    }, (Callback<Object, KuduException>) e -> {
                                        LOGGER.error(e.getMessage());
                                        return e;
                                    });
                            return stringDeferred;
                        } else {
                            throw new Exception("Table doesn't exists");
                        }

                    }
                }, (Callback<Object, Exception>) e -> {
                    System.out.println(e.getMessage());
                    return e;
                });
            }
        });

        //executing the callback
        res.callback(kuduClient.tableExists(tableName));
    }

[manolama]

ahh, so if you're using https://kudu.apache.org/apidocs/org/apache/kudu/client/AsyncKuduClient.html then you need to attach your callbacks to those method calls. E.g. res.callback(kuduClient.tableExists(tableName)); should be

kuduClient.tableExists(tableName).
addCallback(new Callback<Object, Boolean>() {
  @Override
  public Object call(Boolean exists) {
     return kuduClient.openTable(tableName).addCallback(
           new Callback<Object, KuduTable>() {
       @Override
       public Object call(KuduTable kuduTable) throws KuduException {
         AsyncKuduSession session = kuduClient.newSession();
         session.setFlushMode(SessionConfiguration.FlushMode.AUTO_FLUSH_SYNC);
         Upsert upst = kuduTable.newUpsert();
         PartialRow row = upst.getRow();
         row.addString("house_id", foo.getId());
                                      
        session.apply(upst);
        session.close();
        LOGGER.info("Written with success.");
         return null;
       }
     )};
  }
});