Hi all,
Thanks Aljoscha for going forward with the side inputs and for the nice
proposal!
I'm also in favor of the implementation with N-ary input (3.) for the
reasons Ventura explained. I'm strongly against managing side inputs at
StreamTask level (2.), as it would create another abstraction for almost
the same purposes as a TwoInputOperator. Making use of the second input
of a 2-input operator (1.) could be useful for prototyping. I assume it
would be easier to implement a minimal solution with that, but I'm not
sure. If the N-ary input prototype is almost ready, then it's best to go
with that.
For side input readiness, it would be better to wait for the side input
to be completely ready. As Gyula has suggested, waiting only for the
first record does not differ much from not waiting at all. I would also
prefer user-defined readiness, but for the minimal solution we could fix
this for completely read side input and maybe go only for static side
inputs first.
I understand that we should push a minimal viable solution forward. The
current API and implementation proposal seems like a good start.
However, long term goals are also important, to avoid going in a wrong
direction. As I have not participated in the discussion let me share
also some longer term considerations in reply to the others. (Sorry for
the length.)
How would side inputs help the users? For the simple, non-windowed cases
with static input a CoFlatMap might be sufficient. The main input can be
buffered while the side input is consumed and stored in the operator
state. Thus, the user can decide inside the CoFlatMap UDF when to start
consuming the stream input (e.g. when the side input is ready). Of
course, this might be problematic to implement, so the side inputs API
could help the user with this pattern.
1)
First, marking the end of side input is not easy. Every side input
should broadcast some kind of EOF to the consuming operator. If we
generalize to non-static (slowly changing) inputs, then progress
tracking messages should be broadcast periodically. This is reminiscent
of the watermark time tracking for windows.
I agree with Gyula that we should have user defined side input
readiness. Although, couldn't we use windowing for this? It's not worth
having two separate time tracking mechanisms (one for windows, one for
side inputs). If the windowing is not flexible enough to handle such
cases, then what about exposing watermark tracking to the user? E.g. we
could have an extra user defined event handler in RichFunctions when
time progress is made. This generalizes the two progress tracking. Of
course, this approach requires more work so it's not for the minimal
viable solution.
2)
Second, exposing a buffer to the user helps a bit, but the users could
buffer the data simply in an operator state. How would a buffer help
more? Of course, the interface could have multiple implementations, such
as a spilling buffer, and the user could choose. That helps the "waiting
pattern".
I agree with Wenlong's suggestion that a blocking (or backpressure) must
be an option. It seems crucial to avoid consuming a large part of the
main input, that would take a lot of space. I suggest not to expose a
buffer, but to allow the users to control whether to read from the
different inputs. E.g. in the N-ary input operator UDF the user could
control this per input: startConsuming(), stopConsuming(). Then it's the
user's responsibility not to get into deadlocks, but the runtime handles
the buffering. For reading static side input, the user could stop
consuming the main input until she considers the side input ready.
User controlled backpressure would also benefit avoiding deadlock in
stream loops.
3)
I also agree with Wenlong's 2. point, that checkpointing should be
considered, but I don't think it's really important for the prototype.
If we maintain the side input in the state of the consuming operator
then the checkpoint would not stop once the static side input is
finished, because the main input goes on, the operator stays running.
Incremental checkpointing could prevent checkpointing static data at
every checkpoint.
Cheers,
Gabor
On 2017-03-09 16:59, Aljoscha Krettek wrote:
Hi Jamie,
actually the approach where the .withSideInput() comes before the user
function is only required for implementation proposal #1, which I like
the least. For the other two it can be after the user function, which is
also what I prefer.
Regarding semantics: yes, we simply wait for anything to be available.
For GlobalWindows, i.e. side inputs on a normal function where we simply
don't have windows, this means that we wait for anything. For the
windowed case, which I'm proposing as a second step we will wait for
side input in a window to be available that matches the main-input
window. For the keyed case we wait for something on the same key to be
available, for the broadcast case we wait for anything.
Best,
Aljoscha
On Thu, Mar 9, 2017, at 16:55, Jamie Grier wrote:
Hi, I think the proposal looks good. The only thing I wasn't clear on
was
which API is actually being proposed. The one where .withSideInput()
comes
before the user function or after. I would definitely prefer it come
after
since that's the normal pattern in the Flink API. I understood that
makes
the implementation different (maybe harder) but I think it helps keep the
API uniform which is really good.
Overall I think the API looks good and yes there are some tricky
semantics
here but in general if, when processing keyed main streams, we always
wait
until there is a side-input available for that key we're off to a great
start and I think that was what you're suggesting in the design doc.
-Jamie
On Thu, Mar 9, 2017 at 7:27 AM, Aljoscha Krettek <aljos...@apache.org>
wrote:
Hi,
these are all valuable suggestions and I think that we should implement
them when the time is right. However, I would like to first get a
minimal viable version of this feature into Flink and then expand on it.
I think the last few tries of tackling this problem fizzled out because
we got to deep into discussing special semantics and features. I think
the most important thing to agree on right now is the basic API and the
implementation plan. What do you think about that?
Regarding your suggestions, I have in fact a branch [1] from May 2016
where I implemented a prototype implementation. This has an n-ary
operator and inputs can be either bounded or unbounded and the
implementation actually waits for all bounded inputs to finish before
starting to process the unbounded inputs.
In general, I think blocking on an input is only possible while you're
waiting for a bounded input to finish. If all inputs are unbounded you
cannot block because you might run into deadlocks (in the processing
graph, due to back pressure) and also because blocking will also block
elements that might have a lower timestamp and might fall into a
different window which is already ready for processing.
Best,
Aljoscha
[1]
https://github.com/aljoscha/flink/commits/operator-ng-side-input-wrapper
On Tue, Mar 7, 2017, at 14:39, wenlong.lwl wrote:
Hi Aljoscha, thank you for the proposal, it is great to hear about the
progress in side input.
Following is my point of view:
1. I think there may be an option to block the processing of the main
input
instead of buffer the data in state because in production, the through
put
of the main input is usually much larger, and buffering the data before
the
side input may slow down the preparing of side input since the i-o and
computing resources are always limited.
2. another issue may need to be disscussed: how can we do checkpointing
with side input, because static side input may finish soon once started
which will stop the checkpointing.
3. I agree with Gyula that user should be able to determines when a side
input is ready? Maybe we can do it one step further: whether users can
determine a operator with multiple inputs to process which input each
time
or not? It would be more flexible.
Best Regards!
Wenlong
On 7 March 2017 at 18:39, Ventura Del Monte <venturadelmo...@gmail.com>
wrote:
Hi Aljoscha,
Thank you for the proposal and for bringing up again this discussion.
Regarding the implementation aspect,I would say the first way could
be easier/faster to implement but it could add some overhead when
dealing with multiple side inputs through the current 2-streams union
transform. I tried the second option myself as it has less overhead
but then the outcome was something close to a N-ary operator consuming
first each side input while buffering the main one.
Therefore, I would choose the third option as it is more generic
and might help also in other scenarios, although its implementation
requires more effort.
I also agree with Gyula, I think the user should be allowed to define
the
condition that determines when a side input is ready, e.g., load the
side
input first, incrementally update the side input.
Best,
Ventura
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On Mon, Mar 6, 2017 at 3:50 PM, Gyula Fóra <gyula.f...@gmail.com>
wrote:
Hi Aljoscha,
Thank you for the nice proposal!
I think it would make sense to allow user's to affect the readiness
of
the
side input. I think making it ready when the first element arrives is
only
slightly better then making it always ready from usability
perspective.
For
instance if I am joining against a static data set I want to wait
for the
whole set before making it ready. This could be exposed as a user
defined
condition that could also recognize bounded inputs maybe.
Maybe we could also add an aggregating (merging) side input type,
that
could work as a broadcast state.
What do you think?
Gyula
Aljoscha Krettek <aljos...@apache.org> ezt írta (időpont: 2017.
márc.
6.,
H, 15:18):
Hi Folks,
I would like to finally agree on a plan for implementing side
inputs in
Flink. There has already been an attempt to come to consensus [1],
which
resulted in two design documents. I tried to consolidate those two
and
also added a section about implementation plans. This is the
resulting
FLIP:
https://cwiki.apache.org/confluence/display/FLINK/FLIP-
17+Side+Inputs+for+DataStream+API
In terms of semantics I tried to go with the minimal viable
solution.
The part that needs discussing is how we want to implement this. I
outlined three possible implementation plans in the FLIP but what
it
boils down to is that we need to introduce some way of getting
several
inputs into an operator/task.
Please have a look at the doc and let us know what you think.
Best,
Aljoscha
[1]
https://lists.apache.org/thread.html/
797df0ba066151b77c7951fd7d603a
8afd7023920d0607a0c6337db3@1462181294@%3Cdev.flink.apache.org%3E
--
Jamie Grier
data Artisans, Director of Applications Engineering
@jamiegrier <https://twitter.com/jamiegrier>
ja...@data-artisans.com
