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Re: [DISCUSS] FLIP-27: Refactor Source Interface


Hi Thomas,

The iterator-like API was also the first thing that came to me. But it
seems a little confusing that hasNext() does not mean "the stream has not
ended", but means "the next record is ready", which is repurposing the well
known meaning of hasNext(). If we follow the hasNext()/next() pattern, an
additional isNextReady() method to indicate whether the next record is
ready seems more intuitive to me.

Similarly, in poll()/take() pattern, another method of isDone() is needed
to indicate whether the stream has ended or not.

Compared with hasNext()/next()/isNextReady() pattern,
isDone()/poll()/take() seems more flexible for the reader implementation.
When I am implementing a reader, I could have a couple of choices:

   - A thread-less reader that does not have any internal thread.
   - When poll() is called, the same calling thread will perform a bunch of
      IO asynchronously.
      - When take() is called, the same calling thread will perform a bunch
      of IO and wait until the record is ready.
   - A reader with internal threads performing network IO and put records
   into a buffer.
      - When poll() is called, the calling thread simply reads from the
      buffer and return empty result immediately if there is no record.
      - When take() is called, the calling thread reads from the buffer and
      block waiting if the buffer is empty.

On the other hand, with the hasNext()/next()/isNextReady() API, it is less
intuitive for the reader developers to write the thread-less pattern.
Although technically speaking one can still do the asynchronous IO to
prepare the record in isNextReady(). But it is inexplicit and seems
somewhat hacky.

Thanks,

Jiangjie (Becket) Qin



On Mon, Nov 5, 2018 at 6:55 AM Thomas Weise <thw@xxxxxxxxxx> wrote:

> Couple more points regarding discovery:
>
> The proposal mentions that discovery could be outside the execution graph.
> Today, discovered partitions/shards are checkpointed. I believe that will
> also need to be the case in the future, even when discovery and reading are
> split between different tasks.
>
> For cases such as resharding of a Kinesis stream, the relationship between
> splits needs to be considered. Splits cannot be randomly distributed over
> readers in certain situations. An example was mentioned here:
> https://github.com/apache/flink/pull/6980#issuecomment-435202809
>
> Thomas
>
>
> On Sun, Nov 4, 2018 at 1:43 PM Thomas Weise <thw@xxxxxxxxxx> wrote:
>
> > Thanks for getting the ball rolling on this!
> >
> > Can the number of splits decrease? Yes, splits can be closed and go away.
> > An example would be a shard merge in Kinesis (2 existing shards will be
> > closed and replaced with a new shard).
> >
> > Regarding advance/poll/take: IMO the least restrictive approach would be
> > the thread-less IO model (pull based, non-blocking, caller retrieves new
> > records when available). The current Kinesis API requires the use of
> > threads. But that can be internal to the split reader and does not need
> to
> > be a source API concern. In fact, that's what we are working on right now
> > as improvement to the existing consumer: Each shard consumer thread will
> > push to a queue, the consumer main thread will poll the queue(s). It is
> > essentially a mapping from threaded IO to non-blocking.
> >
> > The proposed SplitReader interface would fit the thread-less IO model.
> > Similar to an iterator, we find out if there is a new element (hasNext)
> and
> > if so, move to it (next()). Separate calls deliver the meta information
> > (timestamp, watermark). Perhaps advance call could offer a timeout
> option,
> > so that the caller does not end up in a busy wait. On the other hand, a
> > caller processing multiple splits may want to cycle through fast, to
> > process elements of other splits as soon as they become available. The
> nice
> > thing is that this "split merge" logic can now live in Flink and be
> > optimized and shared between different sources.
> >
> > Thanks,
> > Thomas
> >
> >
> > On Sun, Nov 4, 2018 at 6:34 AM Guowei Ma <guowei.mgw@xxxxxxxxx> wrote:
> >
> >> Hi,
> >> Thanks Aljoscha for this FLIP.
> >>
> >> 1. I agree with Piotr and Becket that the non-blocking source is very
> >> important. But in addition to `Future/poll`, there may be another way to
> >> achieve this. I think it may be not very memory friendly if every
> advance
> >> call return a Future.
> >>
> >> public interface Listener {
> >>      public void notify();
> >> }
> >>
> >> public interface SplitReader() {
> >>      /**
> >>       * When there is no element temporarily, this will return false.
> >>       * When elements is available again splitReader can call
> >> listener.notify()
> >>       * In addition the frame would check `advance` periodically .
> >>       * Of course advance can always return true and ignore the listener
> >> argument for simplicity.
> >>       */
> >>      public boolean advance(Listener listener);
> >> }
> >>
> >> 2.  The FLIP tells us very clearly that how to create all Splits and how
> >> to create a SplitReader from a Split. But there is no strategy for the
> user
> >> to choose how to assign the splits to the tasks. I think we could add a
> >> Enum to let user to choose.
> >> /**
> >>   public Enum SplitsAssignmentPolicy {
> >>     Location,
> >>     Workload,
> >>     Random,
> >>     Average
> >>   }
> >> */
> >>
> >> 3. If merge the `advance` and `getCurrent`  to one method like `getNext`
> >> the `getNext` would need return a `ElementWithTimestamp` because some
> >> sources want to add timestamp to every element. IMO, this is not so
> memory
> >> friendly so I prefer this design.
> >>
> >>
> >>
> >> Thanks
> >>
> >> Piotr Nowojski <piotr@xxxxxxxxxxxxxxxxx> 于2018年11月1日周四 下午6:08写道:
> >>
> >>> Hi,
> >>>
> >>> Thanks Aljoscha for starting this, it’s blocking quite a lot of other
> >>> possible improvements. I have one proposal. Instead of having a method:
> >>>
> >>> boolean advance() throws IOException;
> >>>
> >>> I would replace it with
> >>>
> >>> /*
> >>>  * Return a future, which when completed means that source has more
> data
> >>> and getNext() will not block.
> >>>  * If you wish to use benefits of non blocking connectors, please
> >>> implement this method appropriately.
> >>>  */
> >>> default CompletableFuture<?> isBlocked() {
> >>>         return CompletableFuture.completedFuture(null);
> >>> }
> >>>
> >>> And rename `getCurrent()` to `getNext()`.
> >>>
> >>> Couple of arguments:
> >>> 1. I don’t understand the division of work between `advance()` and
> >>> `getCurrent()`. What should be done in which, especially for connectors
> >>> that handle records in batches (like Kafka) and when should you call
> >>> `advance` and when `getCurrent()`.
> >>> 2. Replacing `boolean` with `CompletableFuture<?>` will allow us in the
> >>> future to have asynchronous/non blocking connectors and more
> efficiently
> >>> handle large number of blocked threads, without busy waiting. While at
> the
> >>> same time it doesn’t add much complexity, since naive connector
> >>> implementations can be always blocking.
> >>> 3. This also would allow us to use a fixed size thread pool of task
> >>> executors, instead of one thread per task.
> >>>
> >>> Piotrek
> >>>
> >>> > On 31 Oct 2018, at 17:22, Aljoscha Krettek <aljoscha@xxxxxxxxxx>
> >>> wrote:
> >>> >
> >>> > Hi All,
> >>> >
> >>> > In order to finally get the ball rolling on the new source interface
> >>> that we have discussed for so long I finally created a FLIP:
> >>>
> https://cwiki.apache.org/confluence/display/FLINK/FLIP-27%3A+Refactor+Source+Interface
> >>> >
> >>> > I cc'ed Thomas and Jamie because of the ongoing work/discussion about
> >>> adding per-partition watermark support to the Kinesis source and
> because
> >>> this would enable generic implementation of event-time alignment for
> all
> >>> sources. Maybe we need another FLIP for the event-time alignment part,
> >>> especially the part about information sharing between operations (I'm
> not
> >>> calling it state sharing because state has a special meaning in Flink).
> >>> >
> >>> > Please discuss away!
> >>> >
> >>> > Aljoscha
> >>> >
> >>> >
> >>>
> >>>
>