Yes, the difficulty is to define that trigger. The existing triggers fire
at the end of window. (I could be mistaken, which will be good news)

B and C which are not mutually exclusive
More on my use case. Say a user visits http://foo at 1, http://foo/bar at 4
and back to http://foo at 5 all in a Session
we would want to emit

http://foo  when the watermark passes 1
http://foo -> http://foo/bar when the watermark passes 4
http://foo -> http://foo/bar -> http://foo when the watermark passes 5

On Fri, Dec 2, 2016 at 10:12 AM Ben Chambers <bchamb...@google.com> wrote:

> As a clarifying question:
> If you have three elements in the pane with timestamps [1, 4, 5], would
> you:
> A. want to emit that entire pane when the watermark passes 1
> B. want to emit that entire pane when the watermark passes 5
> C. emit a fragment of that pane containing only the first element when the
> watermark passes 1
> On Thu, Dec 1, 2016 at 6:01 PM Tyler Akidau <taki...@google.com> wrote:
> So what you want is essentially a trigger that fires when the watermark
> has passed the event time of the oldest un-emitted element in the current
> pane? You could them presumably wrap this in a repeat to get the overall
> desired semantics, right?
> -Tyler
> On Fri, Dec 2, 2016 at 7:32 AM Manu Zhang <owenzhang1...@gmail.com> wrote:
> My use case is to track user trajectory based on page view event when they
> visit a website.  The input would be like a list of PageView(userId, url,
> eventTimestamp) with watermarks (= eventTimestamp - duration). I'm trying
> Sessions with event time trigger. Note we can't wait for the end of session
> window due to latency requirement. Instead, we want to emit the user
> trajectories whenever a buffered PageView's event time is passed by
> watermark.
> On Fri, Dec 2, 2016 at 5:41 AM Lukasz Cwik <lc...@google.com> wrote:
> Can you provide more details about the problem your trying to solve with
> some examples showing input and the expected output?
> On Wed, Nov 30, 2016 at 11:08 PM, Manu Zhang <owenzhang1...@gmail.com>
> wrote:
> Hi,
> Recently I’m addressing a problem where users want to trigger after
> watermark past each element (i.e. in the middle of event-time window). I
> fail to find an existing trigger that does so. Any idea on model this
> problem with Beam ?
> Thanks,
> Manu Zhang

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