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From Paris Carbone <par...@kth.se>
Subject Re: flink snapshotting fault-tolerance
Date Thu, 19 May 2016 18:48:52 GMT
Sure, in practice you can set a threshold of retries since an operator implementation could
cause this indefinitely or any other reason can make snapshotting generally infeasible. If
I recall correctly that threshold exists in the Flink configuration.

On 19 May 2016, at 20:42, Stavros Kontopoulos <st.kontopoulos@gmail.com<mailto:st.kontopoulos@gmail.com>>

The problem here is different though if something is keep failing (permanently) in practice
someone needs to be notified. If the user loses snapshotting he must know.

On Thu, May 19, 2016 at 9:36 PM, Abhishek R. Singh <abhishsi@tetrationanalytics.com<mailto:abhishsi@tetrationanalytics.com>>
I was wondering how checkpoints can be async? Because your state is constantly mutating. You
probably need versioned state, or immutable data structs?


On May 19, 2016, at 11:14 AM, Paris Carbone <parisc@kth.se<mailto:parisc@kth.se>>

Hi Stavros,

Currently, rollback failure recovery in Flink works in the pipeline level, not in the task
level (see Millwheel [1]). It further builds on repayable stream logs (i.e. Kafka), thus,
there is no need for 3pc or backup in the pipeline sources. You can also check this presentation
[2] which explains the basic concepts more in detail I hope. Mind that many upcoming optimisation
opportunities are going to be addressed in the not so long-term Flink roadmap.


[1] http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/41378.pdf
[2] http://www.slideshare.net/ParisCarbone/tech-talk-google-on-flink-fault-tolerance-and-ha


On 19 May 2016, at 19:43, Stavros Kontopoulos <st.kontopoulos@gmail.com<mailto:st.kontopoulos@gmail.com>>

Cool thnx. So if a checkpoint expires the pipeline will block or fail in total or only the
specific task related to the operator (running along with the checkpoint task) or nothing

On Tue, May 17, 2016 at 3:49 PM, Robert Metzger <rmetzger@apache.org<mailto:rmetzger@apache.org>>
Hi Stravos,

I haven't implemented our checkpointing mechanism and I didn't participate in the design decisions
while implementing it, so I can not compare it in detail to other approaches.

>From a "does it work perspective": Checkpoints are only confirmed if all parallel subtasks
successfully created a valid snapshot of the state. So if there is a failure in the checkpointing
mechanism, no valid checkpoint will be created. The system will recover from the last valid
There is a timeout for checkpoints. So if a barrier doesn't pass through the system for a
certain period of time, the checkpoint is cancelled. The default timeout is 10 minutes.


On Mon, May 16, 2016 at 1:22 PM, Stavros Kontopoulos <st.kontopoulos@gmail.com<mailto:st.kontopoulos@gmail.com>>

I was looking into the flink snapshotting algorithm details also mentioned here:

>From other sources i understand that it assumes no failures to work for message delivery
or for example a process hanging for ever:

So my understanding (maybe wrong) is that this is a solution which seems not to address the
fault tolerance issue in a strong manner like for example if it was to use a 3pc protocol
for local state propagation and global agreement. I know the latter is not efficient just
mentioning it for comparison.

How the algorithm behaves in practical terms under the presence of its own failures (this
is a background process collecting partial states)? Are there timeouts for reaching a barrier?

PS. have not looked deep into the code details yet, planning to.


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