On 2/27/24 19:22, Robert Bradshaw via dev wrote:
On Mon, Feb 26, 2024 at 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.
Good point calling out the distinction between "I need to wait in case
there's more data." and "I need to wait for something external." We
can't currently distinguish between the two, but a batch runner can
say something definitive about the first. Feels like we need a new
primitive (or at least new signaling information on our existing
primitive).
Runners signal end of data to a DoFn via (input) watermark. Is there a
need for additional information?
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
