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

I updated the FLIP [1] with some Javadoc for the SplitReader to outline what I had in mind with the interface. Sorry for not doing that earlier, it's not quite clear how the methods should work from the name alone.

The gist of it is that advance() should be non-blocking, so isDone/advance()/getCurrent() are very similar to isDone()/poll()/take() that I have seen mentioned.

[1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-27%3A+Refactor+Source+Interface <https://cwiki.apache.org/confluence/display/FLINK/FLIP-27:+Refactor+Source+Interface>

> On 5. Nov 2018, at 11:05, Biao Liu <mmyy1110@xxxxxxxxx> wrote:
> Thanks Aljoscha for bringing us this discussion!
> 1. I think one of the reason about separating `advance()` and
> `getCurrent()` is that we have several different types returned by source.
> Not just the `record`, but also the timestamp of record and the watermark.
> If we don't separate these into different methods, the source has to return
> a tuple3 which is not so user friendly. The prototype of Aljoscha is
> acceptable to me. Regarding the specific method name, I'm not sure which
> one is better. Both of them are reasonable for me.
> 2. As Thomas and Becket mentioned before, I think a non-blocking API is
> necessary. Moreover, IMO we should not offer a blocking API. It doesn't
> help but makes things more complicated.
> 3. About the thread model.
> I agree with Thomas about the thread-less IO model. A standard workflow
> should look like below.
>  - If there is available data, Flink would read it.
>  - If there is no data available temporary, Flink would check again a
> moment later. Maybe waiting on a semaphore until a timer wake it up.
> Furthermore, we can offer an optional optimization for source which has
> external thread. Like Guowei mentioned, there can be a listener which the
> reader can wake the framework up as soon as new data comes. This can solve
> Piotr's concern about efficiency.
> 4. One more thing. After taking a look at the prototype codes. Off the top
> of my head, the implementation is more fit for batch job not streaming job.
> There are two types of tasks in prototype. First is a source task that
> discovers the splits. The source passes the splits to the second task which
> process the splits one by one. And then the source keeps watch to discover
> more splits.
> However, I think the more common scenario of streaming job is:
> there are fixed splits, each of the subtasks takes several splits. The
> subtasks just keep processing the fixed splits. There would be continuous
> datum in each split. We don't need a source task to discover more splits.
> It can not be finished in streaming job since we don't want the processing
> task finished even there are no more splits.
> So IMO we should offer another source operator for the new interface. It
> would discover all splits when it is opening. Then picks the splits belong
> to this subtask. Keep processing these splits until all of them are
> finished.
> Becket Qin <becket.qin@xxxxxxxxx> 于2018年11月5日周一 上午11:00写道:
>> 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