Hi, Flink advances watermarks based on all parallel source tasks. If one of the source tasks lags behind the others, the event time progresses as determined by the "slowest" source task. Hence, records ingested from a faster task might have a higher processing latency.
Best, Fabian 2018-03-14 1:36 GMT+01:00 Yan Zhou [FDS Science] <yz...@coupang.com>: > Hi, > > I am using flink sql in my application. It simply reads records from kafka > source, converts to table, then runs an query to have over window > aggregation for each record. Time lag watermark assigner with 10ms time lag > is used. > > The performance is not ideal. the end-to-end latency, which is the > difference between the time an record arrives in flink source and the time > the record arrives in flink sink, is around 250ms (median). Please note > that my query, which is over window aggregation, will generate one result > for each input record. I was expecting it to be less then 100ms. I increase > the number of query to 100 times and still have same median end-to-end > latency with plenty of CPU and memory available. It seems to me that > something is holding my application back. > > However, When I use process time as time attribute without changing > anything else, the latency is reduced to 50ms. I understand that in general > using process time should be faster. But for my test using event time, the > time lag is set to only 10ms, which should mean the operators will almost > immediately process the events after they arrives. And the classes which > calculate over window aggregation(ProcTimeBoundedRangeOve, > RowTimeBoundedRowsOver and etc...) basically have same logic. Why does > using process_time or event_time could bring such big difference in > end-to-end latency? And what is hold my application back if time attribute > is set event time? > > Below is my cluster and application setup and thank you for your time. > > > *The cluster:* > > *The cluster runs in standalone mode with 7 servers. Each server has 24 > cores, 240 GB memory. There are 1 job manager and 6 task managers. Each > task manager is allocated with 12 cores, 120 GB memory and 6 taskmanager > slots. Running hdfs over ssd on these servers as well. * > > > *The application:* > > *When the event arrives flink from kafka, an ingestionTs is set for the > event by the application. When the event arrives sink, the process latency > is calculated as System.currentTimeMillis() - ingestionTs. The value is > consider the end-to-end latency and recorded with histogram metric and can > be view in flink web portal. RocksDB state backend is used. Time lag water > assigner with time lag of 10ms is used. * > > > *Custom Source * > *-> Flat Map * > *-> Timestamps/Watermarks * > *-> (from: (id, ip, type, ingestionTs, eventTs) -> select: (id, ip, type, > ingestionTs, eventTs))* > *--HASH-->* > *over:( PARTITION BY: ip,* > > *ORDER BY: eventTs, * > > *RANGEBETWEEN 86400000 PRECEDING AND CURRENT ROW, * > > *select: (id, ip, eventTs, COUNT(*) AS w0$o0), ingestionTs) * > > *-> select: (id, eventTs, w0$o0 AS CNT), ingestionTs) * > *-> to: Tuple2 * > *-> Sink: Unnamed* > > *select id, eventTs, count(*) over (partition by id order by eventTs > ranges between interval '24' hour preceding and current row) as cnt1 from > myTable.* > > > >
