I forgot to say that timers are fault-tolerant. You set them, and Flink takes care of checkpointing and
restoring them after failure. The flag will also be fault-tolerant as, i suppose, you will use Flink’s keyed state.

For more info, you can check the ProcessFunction documentation that Konstantin provided.
There, the example uses a value state to hold the counter, you can do sth similar to keep the flag.
Keep in mind that the state will already be scoped by key so you do not have to worry about that
either.

Kostas

On Apr 18, 2017, at 11:11 PM, Kostas Kloudas <k.kloudas@data-artisans.com> wrote:

No problem! Glad I could help!

Kostas

On Apr 18, 2017, at 11:01 PM, Ryan Conway <ryanmackenzieconway@gmail.com> wrote:

Hi Kostas,

Re restarting: I missed that ProcessFunction.OnTimerContext extends ProcessFunction.Context! Until now my thought was that OnTimerContext did not provide a means of restarting a timer.

Re initial timer, you're right, I'll just need to track a boolean in a state variable that notes whether or not the timer has been initialized. What I am not confident about is how to manage timer recovery after a node failure; I imagine it will make sense to not track this variable. I will do more research and cross that bridge when I get there.

So I think a process function will work just fine, here. Thank you again for your time, Kostas and Konstantin.

Ryan

On Tue, Apr 18, 2017 at 12:07 PM, Kostas Kloudas <k.kloudas@data-artisans.com> wrote:
Hi Ryan,

“A periodic window like this requires the ability to start a timer without an element and to restart a timer when fired.”

For the second part, i.e. “to restart a timer when fired”, you can re-register the timer in the onTimer() method (set a 
new timer for “now + T"), so that the next one fires after T time units, where T is your period.

For the first part, where you set the initial timer for a window, this needs to have a first element right? If not, how
do you know the key for which to set the timer? Are all the keys known in advance?

Kostas



On Apr 18, 2017, at 8:35 PM, Ryan Conway <ryanmackenzieconway@gmail.com> wrote:

A periodic window like this requires the ability to start a timer without an element and to restart a timer when fired.