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From Thomas Weise <...@apache.org>
Subject Re: [DISCUSS] Proposal for adapting Malhar operators for batch use cases
Date Mon, 27 Feb 2017 16:22:12 GMT
I don't think this is a use case for count based window.

We have multiple files that are retrieved in a sequence and there is no
knowledge of the number of records per file. The requirement is to
aggregate each file separately and emit the aggregate when the file is read
fully. There is no concept of "end of something" for an individual key and
global window isn't applicable.

However, as already explained and implemented by Bhupesh, this can be
solved using watermark and window (in this case the window timestamp isn't
a timestamp, but a file sequence, but that doesn't matter.

Thomas


On Mon, Feb 27, 2017 at 8:05 AM, David Yan <davidyan@gmail.com> wrote:

> I don't think this is the way to go. Global Window only means the timestamp
> does not matter (or that there is no timestamp). It does not necessarily
> mean it's a large batch. Unless there is some notion of event time for each
> file, you don't want to embed the file into the window itself.
>
> If you want the result broken up by file name, and if the files are to be
> processed in parallel, I think making the file name be part of the key is
> the way to go. I think it's very confusing if we somehow make the file to
> be part of the window.
>
> For count-based window, it's not implemented yet and you're welcome to add
> that feature. In case of count-based windows, there would be no notion of
> time and you probably only trigger at the end of each window. In the case
> of count-based windows, the watermark only matters for batch since you need
> a way to know when the batch has ended (if the count is 10, the number of
> tuples in the batch is let's say 105, you need a way to end the last window
> with 5 tuples).
>
> David
>
> On Mon, Feb 27, 2017 at 2:41 AM, Bhupesh Chawda <bhupesh@datatorrent.com>
> wrote:
>
> > Hi David,
> >
> > Thanks for your comments.
> >
> > The wordcount example that I created based on the windowed operator does
> > processing of word counts per file (each file as a separate batch), i.e.
> > process counts for each file and dump into separate files.
> > As I understand Global window is for one large batch; i.e. all incoming
> > data falls into the same batch. This could not be processed using
> > GlobalWindow option as we need more than one windows. In this case, I
> > configured the windowed operator to have time windows of 1ms each and
> > passed data for each file with increasing timestamps: (file1, 1), (file2,
> > 2) and so on. Is there a better way of handling this scenario?
> >
> > Regarding (2 - count based windows), I think there is a trigger option to
> > process count based windows. In case I want to process every 1000 tuples
> as
> > a batch, I could set the Trigger option to CountTrigger with the
> > accumulation set to Discarding. Is this correct?
> >
> > I agree that (4. Final Watermark) can be done using Global window.
> >
> > ​~ Bhupesh​
> >
> > _______________________________________________________
> >
> > Bhupesh Chawda
> >
> > E: bhupesh@datatorrent.com | Twitter: @bhupeshsc
> >
> > www.datatorrent.com  |  apex.apache.org
> >
> >
> >
> > On Mon, Feb 27, 2017 at 12:18 PM, David Yan <davidyan@gmail.com> wrote:
> >
> > > I'm worried that we are making the watermark concept too complicated.
> > >
> > > Watermarks should simply just tell you what windows can be considered
> > > complete.
> > >
> > > Point 2 is basically a count-based window. Watermarks do not play a
> role
> > > here because the window is always complete at the n-th tuple.
> > >
> > > If I understand correctly, point 3 is for batch processing of files.
> > Unless
> > > the files contain timed events, it sounds to be that this can be
> achieved
> > > with just a Global Window. For signaling EOF, a watermark with a
> > +infinity
> > > timestamp can be used so that triggers will be fired upon receipt of
> that
> > > watermark.
> > >
> > > For point 4, just like what I mentioned above, can be achieved with a
> > > watermark with a +infinity timestamp.
> > >
> > > David
> > >
> > >
> > >
> > >
> > > On Sat, Feb 18, 2017 at 8:04 AM, Bhupesh Chawda <
> bhupesh@datatorrent.com
> > >
> > > wrote:
> > >
> > > > Hi Thomas,
> > > >
> > > > For an input operator which is supposed to generate watermarks for
> > > > downstream operators, I can think about the following watermarks that
> > the
> > > > operator can emit:
> > > > 1. Time based watermarks (the high watermark / low watermark)
> > > > 2. Number of tuple based watermarks (Every n tuples)
> > > > 3. File based watermarks (Start file, end file)
> > > > 4. Final watermark
> > > >
> > > > File based watermarks seem to be applicable for batch (file based) as
> > > well,
> > > > and hence I thought of looking at these first. Does this seem to be
> in
> > > line
> > > > with the thought process?
> > > >
> > > > ~ Bhupesh
> > > >
> > > >
> > > >
> > > > _______________________________________________________
> > > >
> > > > Bhupesh Chawda
> > > >
> > > > Software Engineer
> > > >
> > > > E: bhupesh@datatorrent.com | Twitter: @bhupeshsc
> > > >
> > > > www.datatorrent.com  |  apex.apache.org
> > > >
> > > >
> > > >
> > > > On Thu, Feb 16, 2017 at 10:37 AM, Thomas Weise <thw@apache.org>
> wrote:
> > > >
> > > > > I don't think this should be designed based on a simplistic file
> > > > > input-output scenario. It would be good to include a stateful
> > > > > transformation based on event time.
> > > > >
> > > > > More complex pipelines contain stateful transformations that depend
> > on
> > > > > windowing and watermarks. I think we need a watermark concept that
> is
> > > > based
> > > > > on progress in event time (or other monotonic increasing sequence)
> > that
> > > > > other operators can generically work with.
> > > > >
> > > > > Note that even file input in many cases can produce time based
> > > > watermarks,
> > > > > for example when you read part files that are bound by event time.
> > > > >
> > > > > Thanks,
> > > > > Thomas
> > > > >
> > > > >
> > > > > On Wed, Feb 15, 2017 at 4:02 AM, Bhupesh Chawda <
> > > bhupesh@datatorrent.com
> > > > >
> > > > > wrote:
> > > > >
> > > > > > For better understanding the use case for control tuples in
> batch,
> > ​I
> > > > am
> > > > > > creating a prototype for a batch application using File Input
and
> > > File
> > > > > > Output operators.
> > > > > >
> > > > > > To enable basic batch processing for File IO operators, I am
> > > proposing
> > > > > the
> > > > > > following changes to File input and output operators:
> > > > > > 1. File Input operator emits a watermark each time it opens
and
> > > closes
> > > > a
> > > > > > file. These can be "start file" and "end file" watermarks which
> > > include
> > > > > the
> > > > > > corresponding file names. The "start file" tuple should be sent
> > > before
> > > > > any
> > > > > > of the data from that file flows.
> > > > > > 2. File Input operator can be configured to end the application
> > > after a
> > > > > > single or n scans of the directory (a batch). This is where
the
> > > > operator
> > > > > > emits the final watermark (the end of application control tuple).
> > > This
> > > > > will
> > > > > > also shutdown the application.
> > > > > > 3. The File output operator handles these control tuples. "Start
> > > file"
> > > > > > initializes the file name for the incoming tuples. "End file"
> > > watermark
> > > > > > forces a finalize on that file.
> > > > > >
> > > > > > The user would be able to enable the operators to send only
those
> > > > > > watermarks that are needed in the application. If none of the
> > options
> > > > are
> > > > > > configured, the operators behave as in a streaming application.
> > > > > >
> > > > > > There are a few challenges in the implementation where the input
> > > > operator
> > > > > > is partitioned. In this case, the correlation between the
> start/end
> > > > for a
> > > > > > file and the data tuples for that file is lost. Hence we need
to
> > > > maintain
> > > > > > the filename as part of each tuple in the pipeline.
> > > > > >
> > > > > > The "start file" and "end file" control tuples in this example
> are
> > > > > > temporary names for watermarks. We can have generic "start
> batch" /
> > > > "end
> > > > > > batch" tuples which could be used for other use cases as well.
> The
> > > > Final
> > > > > > watermark is common and serves the same purpose in each case.
> > > > > >
> > > > > > Please let me know your thoughts on this.
> > > > > >
> > > > > > ~ Bhupesh
> > > > > >
> > > > > >
> > > > > >
> > > > > > On Wed, Jan 18, 2017 at 12:22 AM, Bhupesh Chawda <
> > > > > bhupesh@datatorrent.com>
> > > > > > wrote:
> > > > > >
> > > > > > > Yes, this can be part of operator configuration. Given
this,
> for
> > a
> > > > user
> > > > > > to
> > > > > > > define a batch application, would mean configuring the
> connectors
> > > > > (mostly
> > > > > > > the input operator) in the application for the desired
> behavior.
> > > > > > Similarly,
> > > > > > > there can be other use cases that can be achieved other
than
> > batch.
> > > > > > >
> > > > > > > We may also need to take care of the following:
> > > > > > > 1. Make sure that the watermarks or control tuples are
> consistent
> > > > > across
> > > > > > > sources. Meaning an HDFS sink should be able to interpret
the
> > > > watermark
> > > > > > > tuple sent out by, say, a JDBC source.
> > > > > > > 2. In addition to I/O connectors, we should also look at
the
> need
> > > for
> > > > > > > processing operators to understand some of the control
tuples /
> > > > > > watermarks.
> > > > > > > For example, we may want to reset the operator behavior
on
> > arrival
> > > of
> > > > > > some
> > > > > > > watermark tuple.
> > > > > > >
> > > > > > > ~ Bhupesh
> > > > > > >
> > > > > > > On Tue, Jan 17, 2017 at 9:59 PM, Thomas Weise <thw@apache.org>
> > > > wrote:
> > > > > > >
> > > > > > >> The HDFS source can operate in two modes, bounded or
> unbounded.
> > If
> > > > you
> > > > > > >> scan
> > > > > > >> only once, then it should emit the final watermark
after it is
> > > done.
> > > > > > >> Otherwise it would emit watermarks based on a policy
(files
> > names
> > > > > etc.).
> > > > > > >> The mechanism to generate the marks may depend on the
type of
> > > source
> > > > > and
> > > > > > >> the user needs to be able to influence/configure it.
> > > > > > >>
> > > > > > >> Thomas
> > > > > > >>
> > > > > > >>
> > > > > > >> On Tue, Jan 17, 2017 at 5:03 AM, Bhupesh Chawda <
> > > > > > bhupesh@datatorrent.com>
> > > > > > >> wrote:
> > > > > > >>
> > > > > > >> > Hi Thomas,
> > > > > > >> >
> > > > > > >> > I am not sure that I completely understand your
suggestion.
> > Are
> > > > you
> > > > > > >> > suggesting to broaden the scope of the proposal
to treat all
> > > > sources
> > > > > > as
> > > > > > >> > bounded as well as unbounded?
> > > > > > >> >
> > > > > > >> > In case of Apex, we treat all sources as unbounded
sources.
> > Even
> > > > > > bounded
> > > > > > >> > sources like HDFS file source is treated as unbounded
by
> means
> > > of
> > > > > > >> scanning
> > > > > > >> > the input directory repeatedly.
> > > > > > >> >
> > > > > > >> > Let's consider HDFS file source for example:
> > > > > > >> > In this case, if we treat it as a bounded source,
we can
> > define
> > > > > hooks
> > > > > > >> which
> > > > > > >> > allows us to detect the end of the file and send
the "final
> > > > > > watermark".
> > > > > > >> We
> > > > > > >> > could also consider HDFS file source as a streaming
source
> and
> > > > > define
> > > > > > >> hooks
> > > > > > >> > which send watermarks based on different kinds
of windows.
> > > > > > >> >
> > > > > > >> > Please correct me if I misunderstand.
> > > > > > >> >
> > > > > > >> > ~ Bhupesh
> > > > > > >> >
> > > > > > >> >
> > > > > > >> > On Mon, Jan 16, 2017 at 9:23 PM, Thomas Weise
<
> thw@apache.org
> > >
> > > > > wrote:
> > > > > > >> >
> > > > > > >> > > Bhupesh,
> > > > > > >> > >
> > > > > > >> > > Please see how that can be solved in a unified
way using
> > > windows
> > > > > and
> > > > > > >> > > watermarks. It is bounded data vs. unbounded
data. In Beam
> > for
> > > > > > >> example,
> > > > > > >> > you
> > > > > > >> > > can use the "global window" and the final
watermark to
> > > > accomplish
> > > > > > what
> > > > > > >> > you
> > > > > > >> > > are looking for. Batch is just a special
case of streaming
> > > where
> > > > > the
> > > > > > >> > source
> > > > > > >> > > emits the final watermark.
> > > > > > >> > >
> > > > > > >> > > Thanks,
> > > > > > >> > > Thomas
> > > > > > >> > >
> > > > > > >> > >
> > > > > > >> > > On Mon, Jan 16, 2017 at 1:02 AM, Bhupesh
Chawda <
> > > > > > >> bhupesh@datatorrent.com
> > > > > > >> > >
> > > > > > >> > > wrote:
> > > > > > >> > >
> > > > > > >> > > > Yes, if the user needs to develop a
batch application,
> > then
> > > > > batch
> > > > > > >> aware
> > > > > > >> > > > operators need to be used in the application.
> > > > > > >> > > > The nature of the application is mostly
controlled by
> the
> > > > input
> > > > > > and
> > > > > > >> the
> > > > > > >> > > > output operators used in the application.
> > > > > > >> > > >
> > > > > > >> > > > For example, consider an application
which needs to
> filter
> > > > > records
> > > > > > >> in a
> > > > > > >> > > > input file and store the filtered records
in another
> file.
> > > The
> > > > > > >> nature
> > > > > > >> > of
> > > > > > >> > > > this app is to end once the entire file
is processed.
> > > > Following
> > > > > > >> things
> > > > > > >> > > are
> > > > > > >> > > > expected of the application:
> > > > > > >> > > >
> > > > > > >> > > >    1. Once the input data is over, finalize
the output
> > file
> > > > from
> > > > > > >> .tmp
> > > > > > >> > > >    files. - Responsibility of output
operator
> > > > > > >> > > >    2. End the application, once the
data is read and
> > > > processed -
> > > > > > >> > > >    Responsibility of input operator
> > > > > > >> > > >
> > > > > > >> > > > These functions are essential to allow
the user to do
> > higher
> > > > > level
> > > > > > >> > > > operations like scheduling or running
a workflow of
> batch
> > > > > > >> applications.
> > > > > > >> > > >
> > > > > > >> > > > I am not sure about intermediate (processing)
operators,
> > as
> > > > > there
> > > > > > >> is no
> > > > > > >> > > > change in their functionality for batch
use cases.
> > Perhaps,
> > > > > > allowing
> > > > > > >> > > > running multiple batches in a single
application may
> > require
> > > > > > similar
> > > > > > >> > > > changes in processing operators as well.
> > > > > > >> > > >
> > > > > > >> > > > ~ Bhupesh
> > > > > > >> > > >
> > > > > > >> > > > On Mon, Jan 16, 2017 at 2:19 PM, Priyanka
Gugale <
> > > > > > priyag@apache.org
> > > > > > >> >
> > > > > > >> > > > wrote:
> > > > > > >> > > >
> > > > > > >> > > > > Will it make an impression on user
that, if he has a
> > batch
> > > > > > >> usecase he
> > > > > > >> > > has
> > > > > > >> > > > > to use batch aware operators only?
If so, is that what
> > we
> > > > > > expect?
> > > > > > >> I
> > > > > > >> > am
> > > > > > >> > > > not
> > > > > > >> > > > > aware of how do we implement batch
scenario so this
> > might
> > > > be a
> > > > > > >> basic
> > > > > > >> > > > > question.
> > > > > > >> > > > >
> > > > > > >> > > > > -Priyanka
> > > > > > >> > > > >
> > > > > > >> > > > > On Mon, Jan 16, 2017 at 12:02 PM,
Bhupesh Chawda <
> > > > > > >> > > > bhupesh@datatorrent.com>
> > > > > > >> > > > > wrote:
> > > > > > >> > > > >
> > > > > > >> > > > > > Hi All,
> > > > > > >> > > > > >
> > > > > > >> > > > > > While design / implementation
for custom control
> > tuples
> > > is
> > > > > > >> > ongoing, I
> > > > > > >> > > > > > thought it would be a good
idea to consider its
> > > usefulness
> > > > > in
> > > > > > >> one
> > > > > > >> > of
> > > > > > >> > > > the
> > > > > > >> > > > > > use cases -  batch applications.
> > > > > > >> > > > > >
> > > > > > >> > > > > > This is a proposal to adapt
/ extend existing
> > operators
> > > in
> > > > > the
> > > > > > >> > Apache
> > > > > > >> > > > > Apex
> > > > > > >> > > > > > Malhar library so that it
is easy to use them in
> batch
> > > use
> > > > > > >> cases.
> > > > > > >> > > > > > Naturally, this would be applicable
for only a
> subset
> > of
> > > > > > >> operators
> > > > > > >> > > like
> > > > > > >> > > > > > File, JDBC and NoSQL databases.
> > > > > > >> > > > > > For example, for a file based
store, (say HDFS
> store),
> > > we
> > > > > > could
> > > > > > >> > have
> > > > > > >> > > > > > FileBatchInput and FileBatchOutput
operators which
> > allow
> > > > > easy
> > > > > > >> > > > integration
> > > > > > >> > > > > > into a batch application.
These operators would be
> > > > extended
> > > > > > from
> > > > > > >> > > their
> > > > > > >> > > > > > existing implementations and
would be "Batch Aware",
> > in
> > > > that
> > > > > > >> they
> > > > > > >> > may
> > > > > > >> > > > > > understand the meaning of
some specific control
> tuples
> > > > that
> > > > > > flow
> > > > > > >> > > > through
> > > > > > >> > > > > > the DAG. Start batch and end
batch seem to be the
> > > obvious
> > > > > > >> > candidates
> > > > > > >> > > > that
> > > > > > >> > > > > > come to mind. On receipt of
such control tuples,
> they
> > > may
> > > > > try
> > > > > > to
> > > > > > >> > > modify
> > > > > > >> > > > > the
> > > > > > >> > > > > > behavior of the operator -
to reinitialize some
> > metrics
> > > or
> > > > > > >> finalize
> > > > > > >> > > an
> > > > > > >> > > > > > output file for example.
> > > > > > >> > > > > >
> > > > > > >> > > > > > We can discuss the potential
control tuples and
> > actions
> > > in
> > > > > > >> detail,
> > > > > > >> > > but
> > > > > > >> > > > > > first I would like to understand
the views of the
> > > > community
> > > > > > for
> > > > > > >> > this
> > > > > > >> > > > > > proposal.
> > > > > > >> > > > > >
> > > > > > >> > > > > > ~ Bhupesh
> > > > > > >> > > > > >
> > > > > > >> > > > >
> > > > > > >> > > >
> > > > > > >> > >
> > > > > > >> >
> > > > > > >>
> > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
>

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