Hi Li, the statement refers to operators with multiple inputs (two in this case). With the current implementation you will indeed block one of the inputs after receiving a checkpoint barrier n until you've received the corresponding checkpoint barrier n on the other input as well. This is what we call checkpoint barrier alignment. If the processing time on both input paths is similar and thus there is no back pressure on any of the inputs, the alignment should not take too long. In case where one of the inputs is considerably slower than the other, you should an additional delay.
For single input operators, you don't have to align the checkpoint barriers. The checkpoint barrier alignment is not strictly necessary, but it allows us to not having to store all in flight records from the second input which arrive between the checkpoint barrier on the first input and the corresponding barrier on the second input. We might change this implementation in the future, though. Cheers, Till On Tue, Nov 1, 2016 at 8:05 AM, Li Wang <wangli1...@gmail.com> wrote: > Hi all, > > I have a question regarding to the state checkpoint mechanism in Flink. I > find the statement "Once the last stream has received barrier n, the > operator emits all pending outgoing records, and then emits > snapshot n barriers itself” on the document https://ci.apache.org/ > projects/flink/flink-docs-master/internals/stream_ > checkpointing.html#exactly-once-vs-at-least-once. > > Does this mean that to achieve exactly-once semantic, instead of sending > tuples downstream immediately the operator buffers its outgoing tuples in a > pending queue until the current snapshot is committed? If yes, will this > introduce significant processing delay? > > Thanks, > Li > >
