Ability to send custom control tuples within window may be useful, for
example, for sessions tracking, where session boundaries are not aligned
with window boundaries and 500 ms latency is not acceptable for an
application.
Thank you,
Vlad
On 6/25/16 10:52, Thomas Weise wrote:
It should not matter from where the control tuple is triggered. It will be
good to have a generic mechanism to propagate it and other things can be
accomplished outside the engine. For example, the new comprehensive support
for windowing will all be in Malhar, nothing that the engine needs to know
about it except that we need the control tuple for watermark propagation
and idempotent processing.
I also think the main difference to other tuples is the need to send it to
all partitions. Which is similar to checkpoint window tuples, but not the
same. Here, we probably also need the ability for the user to control
whether such tuple should traverse the entire DAG or not. For a batch use
case, for example, we may want to send the end of file to the next
operator, but not beyond, if the operator has asynchronous processing logic
in it.
For any logic to be idempotent, the control tuple needs to be processed at
a window boundary. Receiving the control tuple in the window callback would
avoid having to track extra state in the operator. I don't think that's a
major issue, but what is the use case for processing a control tuple within
the window?
Thomas
On Sat, Jun 25, 2016 at 6:19 AM, Pramod Immaneni <[email protected]>
wrote:
For the use cases you mentioned, I think 1) and 2) are more likely to
be controlled directly by the application, 3) and 4) are more likely
going to be triggered externally and directly handled by the engine
and 3) is already being implemented that way (apexcore-163).
The control tuples emitted by an operator would be sent to all
downstream partitions isn't it and that would be the chief distinction
compared to data (apart from the payload) which would get partitioned
under normal circumstances. It would also be guaranteed that
downstream partitions will receive control tuples only after the data
that was sent before it so we could send it immediately when it is
emitted as opposed to window boundaries.
However during unification it is important to know if these control
tuples have been received from all upstream partitions before
proceeding with a control operation. One could wait till end of the
window but that introduces a delay however small, I would like to add
to the proposal that the platform only hand over the control tuple to
the unifier when it has been received from all upstream partitions
much like how end window is processed but not wait till the actual end
of the window.
Regd your concern about idempotency, we typically care about
idempotency at a window level and doing the above will still allow the
operators to preserve that easily.
Thanks
On Jun 24, 2016, at 11:22 AM, David Yan <[email protected]> wrote:
Hi all,
I would like to propose a new feature to the Apex core engine -- the
support of custom control tuples. Currently, we have control tuples such
as
BEGIN_WINDOW, END_WINDOW, CHECKPOINT, and so on, but we don't have the
support for applications to insert their own control tuples. The way
currently to get around this is to use data tuples and have a separate
port
for such tuples that sends tuples to all partitions of the downstream
operators, which is not exactly developer friendly.
We have already seen a number of use cases that can use this feature:
1) Batch support: We need to tell all operators of the physical DAG when
a
batch starts and ends, so the operators can do whatever that is needed
upon
the start or the end of a batch.
2) Watermark: To support the concepts of event time windowing, the
watermark control tuple is needed to tell which windows should be
considered late.
3) Changing operator properties: We do have the support of changing
operator properties on the fly, but with a custom control tuple, the
command to change operator properties can be window aligned for all
partitions and also across the DAG.
4) Recording tuples: Like changing operator properties, we do have this
support now but only at the individual physical operator level, and
without
control of which window to record tuples for. With a custom control
tuple,
because a control tuple must belong to a window, all operators in the DAG
can start (and stop) recording for the same windows.
I can think of two options to achieve this:
1) new custom control tuple type that takes user's serializable object.
2) piggy back the current BEGIN_WINDOW and END_WINDOW control tuples.
Please provide your feedback. Thank you.
David
