Yea I like DelayTimer, or SleepTimer, or WaitTimer or some such.
OutputTime is always an event time timestamp so it isn't even allowed
to be set outside the window (or you'd end up with an element assigned
to a window that it isn't within, since OutputTime essentially
represents reserving the right to output an element with that timestamp)
Kenn
On Mon, Feb 26, 2024 at 3:19 PM Robert Burke <rob...@frantil.com> wrote:
Agreed that a retroactive behavior change would be bad, even if
tied to a beam version change. I agree that it meshes well with
the general theme of State & Timers exposing underlying primitives
for implementing Windowing and similar. I'd say the distinction
between the two might be additional complexity for users to grok,
and would need to be documented well, as both operate in the
ProcessingTime domain, but differently.
What to call this new timer then? DelayTimer?
"A DelayTimer sets an instant in ProcessingTime at which point
computations can continue. Runners will prevent the EventTimer
watermark from advancing past the set OutputTime until Processing
Time has advanced to at least the provided instant to execute the
timers callback. This can be used to allow the runner to constrain
pipeline throughput with user guidance."
I'd probably add that a timer with an output time outside of the
window would not be guaranteed to fire, and that OnWindowExpiry is
the correct way to ensure cleanup occurs.
No solution to the Looping Timers on Drain problem here, but i
think that's ultimately an orthogonal discussion, and will
restrain my thoughts on that for now.
This isn't a proposal, but exploring the solution space within our
problem. We'd want to break down exactly what different and the
same for the 3 kinds of timers...
On Mon, Feb 26, 2024, 11:45 AM Kenneth Knowles <k...@apache.org>
wrote:
Pulling out focus points:
On Fri, Feb 23, 2024 at 7:21 PM Robert Bradshaw via dev
<dev@beam.apache.org> wrote:
> I can't act on something yet [...] but I expect to be able
to [...] at some time in the processing-time future.
I like this as a clear and internally-consistent feature
description. It describes ProcessContinuation and those timers
which serve the same purpose as ProcessContinuation.
On Fri, Feb 23, 2024 at 7:21 PM Robert Bradshaw via dev
<dev@beam.apache.org> wrote:
> I can't think of a batch or streaming scenario where it
would be correct to not wait at least that long
The main reason we created timers: to take action in the
absence of data. The archetypal use case for processing time
timers was/is "flush data from state if it has been sitting
there too long". For this use case, the right behavior for
batch is to skip the timer. It is actually basically incorrect
to wait.
On Fri, Feb 23, 2024 at 3:54 PM Robert Burke
<lostl...@apache.org> wrote:
> It doesn't require a new primitive.
IMO what's being proposed *is* a new primitive. I think it is
a good primitive. It is the underlying primitive to
ProcessContinuation. It would be user-friendly as a kind of
timer. But if we made this the behavior of processing time
timers retroactively, it would break everyone using them to
flush data who is also reprocessing data.
There's two very different use cases ("I need to wait, and
block data" vs "I want to act without data, aka NOT wait for
data") and I think we should serve both of them, but it
doesn't have to be with the same low-level feature.
Kenn
On Fri, Feb 23, 2024 at 7:21 PM Robert Bradshaw via dev
<dev@beam.apache.org> wrote:
On Fri, Feb 23, 2024 at 3:54 PM Robert Burke
<lostl...@apache.org> wrote:
>
> While I'm currently on the other side of the fence, I
would not be against changing/requiring the semantics of
ProcessingTime constructs to be "must wait and execute" as
such a solution, and enables the Proposed "batch" process
continuation throttling mechanism to work as hypothesized
for both "batch" and "streaming" execution.
>
> There's a lot to like, as it leans Beam further into the
unification of Batch and Stream, with one fewer exception
(eg. unifies timer experience further). It doesn't require
a new primitive. It probably matches more with user
expectations anyway.
>
> It does cause looping timer execution with processing
time to be a problem for Drains however.
I think we have a problem with looping timers plus drain
(a mostly
streaming idea anyway) regardless.
> I'd argue though that in the case of a drain, we could
updated the semantics as "move watermark to infinity"
"existing timers are executed, but new timers are ignored",
I don't like the idea of dropping timers for drain. I
think correct
handling here requires user visibility into whether a
pipeline is
draining or not.
> and ensure/and update the requirements around
OnWindowExpiration callbacks to be a bit more insistent on
being implemented for correct execution, which is
currently the only "hard" signal to the SDK side that the
window's work is guaranteed to be over, and remaining
state needs to be addressed by the transform or be garbage
collected. This remains critical for developing a good
pattern for ProcessingTime timers within a Global Window too.
+1
>
> On 2024/02/23 19:48:22 Robert Bradshaw via dev wrote:
> > Thanks for bringing this up.
> >
> > My position is that both batch and streaming should
wait for
> > processing time timers, according to local time (with
the exception of
> > tests that can accelerate this via faked clocks).
> >
> > Both ProcessContinuations delays and
ProcessingTimeTimers are IMHO
> > isomorphic, and can be implemented in terms of each
other (at least in
> > one direction, and likely the other). Both are an
indication that I
> > can't act on something yet due to external constraints
(e.g. not all
> > the data has been published, or I lack sufficient
capacity/quota to
> > push things downstream) but I expect to be able to (or
at least would
> > like to check again) at some time in the
processing-time future. I
> > can't think of a batch or streaming scenario where it
would be correct
> > to not wait at least that long (even in batch inputs,
e.g. suppose I'm
> > tailing logs and was eagerly started before they were
fully written,
> > or waiting for some kind of (non-data-dependent)
quiessence or other
> > operation to finish).
> >
> >
> > On Fri, Feb 23, 2024 at 12:36 AM Jan Lukavský
<je...@seznam.cz> wrote:
> > >
> > > For me it always helps to seek analogy in our
physical reality. Stream
> > > processing actually has quite a good analogy for
both event-time and
> > > processing-time - the simplest model for this being
relativity theory.
> > > Event-time is the time at which events occur _at
distant locations_. Due
> > > to finite and invariant speed of light (which is
actually really
> > > involved in the explanation why any stream
processing is inevitably
> > > unordered) these events are observed (processed) at
different times
> > > (processing time, different for different
observers). It is perfectly
> > > possible for an observer to observe events at a rate
that is higher than
> > > one second per second. This also happens in reality
for observers that
> > > travel at relativistic speeds (which might be an
analogy for fast -
> > > batch - (re)processing). Besides the invariant
speed, there is also
> > > another invariant - local clock (wall time) always
ticks exactly at the
> > > rate of one second per second, no matter what. It is
not possible to
> > > "move faster or slower" through (local) time.
> > >
> > > In my understanding the reason why we do not put any
guarantees or
> > > bounds on the delay of firing processing time timers
is purely technical
> > > - the processing is (per key) single-threaded, thus
any timer has to
> > > wait before any element processing finishes. This is
only consequence of
> > > a technical solution, not something fundamental.
> > >
> > > Having said that, my point is that according to the
above analogy, it
> > > should be perfectly fine to fire processing time
timers in batch based
> > > on (local wall) time only. There should be no way of
manipulating this
> > > local time (excluding tests). Watermarks should be
affected the same way
> > > as any buffering in a state that would happen in a
stateful DoFn (i.e.
> > > set timer holds output watermark). We should
probably pay attention to
> > > looping timers, but it seems possible to define a
valid stopping
> > > condition (input watermark at infinity).
> > >
> > > Jan
> > >
> > > On 2/22/24 19:50, Kenneth Knowles wrote:
> > > > Forking this thread.
> > > >
> > > > The state of processing time timers in this mode
of processing is not
> > > > satisfactory and is discussed a lot but we should
make everything
> > > > explicit.
> > > >
> > > > Currently, a state and timer DoFn has a number of
logical watermarks:
> > > > (apologies for fixed width not coming through in
email lists). Treat
> > > > timers as a back edge.
> > > >
> > > > input --(A)----(C)--> ParDo(DoFn) ----(D)---> output
> > > > ^ |
> > > > |--(B)-----------------|
> > > > timers
> > > >
> > > > (A) Input Element watermark: this is the watermark
that promises there
> > > > is no incoming element with a timestamp earlier
than it. Each input
> > > > element's timestamp holds this watermark. Note
that *event time timers
> > > > firing is according to this watermark*. But a
runner commits changes
> > > > to this watermark *whenever it wants*, in a way
that can be
> > > > consistent. So the runner can absolute process
*all* the elements
> > > > before advancing the watermark (A), and only
afterwards start firing
> > > > timers.
> > > >
> > > > (B) Timer watermark: this is a watermark that
promises no timer is set
> > > > with an output timestamp earlier than it. Each
timer that has an
> > > > output timestamp holds this watermark. Note that
timers can set new
> > > > timers, indefinitely, so this may never reach
infinity even in a drain
> > > > scenario.
> > > >
> > > > (C) (derived) total input watermark: this is a
watermark that is the
> > > > minimum of the two above, and ensures that all
state for the DoFn for
> > > > expired windows can be GCd after calling
@OnWindowExpiration.
> > > >
> > > > (D) output watermark: this is a promise that the
DoFn will not output
> > > > earlier than the watermark. It is held by the
total input watermark.
> > > >
> > > > So a any timer, processing or not, holds the total
input watermark
> > > > which prevents window GC, hence the timer must be
fired. You can set
> > > > timers without a timestamp and they will not hold
(B) hence not hold
> > > > the total input / GC watermark (C). Then if a
timer fires for an
> > > > expired window, it is ignored. But in general a
timer that sets an
> > > > output timestamp is saying that it may produce
output, so it *must* be
> > > > fired, even in batch, for data integrity. There
was a time before
> > > > timers had output timestamps that we said that you
*always* have to
> > > > have an @OnWindowExpiration callback for data
integrity, and
> > > > processing time timers could not hold the
watermark. That is changed now.
> > > >
> > > > One main purpose of processing time timers in
streaming is to be a
> > > > "timeout" for data buffered in state, to
eventually flush. In this
> > > > case the output timestamp should be the minimum of
the elements in
> > > > state (or equivalent). In batch, of course, this
kind of timer is not
> > > > relevant and we should definitely not wait for it,
because the goal is
> > > > to just get through all the data. We can justify
this by saying that
> > > > the worker really has no business having any idea
what time it really
> > > > is, and the runner can just run the clock at
whatever speed it wants.
> > > >
> > > > Another purpose, brought up on the Throttle
thread, is to wait or
> > > > backoff. In this case it would be desired for the
timer to actually
> > > > cause batch processing to pause and wait. This
kind of behavior has
> > > > not been explored much. Notably the runner can
absolutely process all
> > > > elements first, then start to fire any enqueued
processing time
> > > > timers. In the same way that state in batch can
just be in memory,
> > > > this *could* just be a call to sleep(). It all
seems a bit sketchy so
> > > > I'd love clearer opinions.
> > > >
> > > > These two are both operational effects - as you
would expect for
> > > > processing time timers - and they seem to be in
conflict. Maybe they
> > > > just need different features?
> > > >
> > > > I'd love to hear some more uses of processing time
timers from the
> > > > community.
> > > >
> > > > Kenn
> >
