Hi Otis, Yes this is a limitation in the old consumer. i.e., a number of per-topic/partition mbeans remain even on a rebalance. Those need to be de-registered. So if you stop consuming from some partition after a rebalance that lag mbean currently remain which is why it remains flat. This is a known issue.
On the restart, the lag goes down to zero because - well the mbeans get recreated and the consumer starts fetching. If the fetch request reads up to the end of the log then the mbean will report zero. Your actual committed offset may be behind though which is why your true lag is > 0. The lag mbeans are useful, but have a number of limitations - it depends on active fetches in progress; it also does not exactly correspond with your actual processed (and committed) offset. The most reliable way to monitor application lag is to use the committed offsets and the current log end offsets. Todd has been doing a lot of interesting work in making lag monitoring less painful and can comment more. Joel On Thu, Jun 04, 2015 at 04:55:44PM -0400, Otis Gospodnetić wrote: > Hi, > > On Thu, Jun 4, 2015 at 4:26 PM, Scott Reynolds <sreyno...@twilio.com> wrote: > > > I believe the JMX metrics reflect the consumer PRIOR to committing offsets > > to Kafka / Zookeeper. But when you query from the command line using the > > kafka tools, you are just getting the committed offsets. > > > > Even if that were the case, and maybe it is, it doesn't explain why the > ConsumerLag in JMX often remains *completely constant*.....forever... until > the consumer is restarted. You see what I mean? > > Otis > -- > Monitoring * Alerting * Anomaly Detection * Centralized Log Management > Solr & Elasticsearch Support * http://sematext.com/ > > > > > On Thu, Jun 4, 2015 at 1:23 PM, Otis Gospodnetic < > > otis.gospodne...@gmail.com > > > wrote: > > > > > Hi, > > > > > > Here's something potentially useful. > > > > > > 1) Before: https://apps.sematext.com/spm-reports/s/eQ9WhLegW9 - the > > "flat > > > Lag situation" > > > > > > 2) I restarted the consumer whose lag is shown in the above graph > > > > > > 3) After restart: https://apps.sematext.com/spm-reports/s/4YGkcUP9ms - > > NO > > > lag at all!? > > > > > > So that 81560 Lag value that was stuck in JMX is gone. Went down to 0. > > > Kind of makes sense - the whole consumer was restarted, consumer/java > > > process was restarted, everything that was in JMX got reset, and if there > > > is truly no consumer lag it makes sense that the values in JMX are 0. > > > > > > HOWEVER, is the Lag *really* always *exactly* 0? No way. Look what > > Offset > > > Checker shows for this one consumer: > > > > > > af_servers spm_cluster_topic-new-cdh 18 220551962 > > > 220586078 34116 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > af_servers spm_cluster_topic-new-cdh 19 161936440 > > > 161960377 23937 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > af_servers spm_cluster_topic-new-cdh 20 248308642 > > > 248340350 31708 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > af_servers spm_cluster_topic-new-cdh 21 259901355 > > > 259934911 33556 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > af_servers spm_cluster_topic-new-cdh 22 205274547 > > > 205296950 22403 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > af_servers spm_cluster_topic-new-cdh 23 167756654 > > > 167780028 23374 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > af_servers spm_cluster_topic-new-cdh 24 357517989 > > > 357574627 56638 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > af_servers spm_cluster_topic-new-cdh 25 194313232 > > > 194338154 24922 > > > af_servers_spm-afs-6.prod.sematext-1433447997963-c40182c8-0 > > > > > > You can see Lag is NOT == 0 for any of the partitions! > > > > > > And yet, look what JMX is showing now -- all Lag for all partitions is > > > supposedly 0. Always: > > > > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=17 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=18 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=18 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=18 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=18 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=19 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=19 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=19 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=19 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=20 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=20 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=20 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=20 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=21 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=21 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=21 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=21 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=22 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=22 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=22 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=22 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=23 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=23 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=23 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=23 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=24 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=24 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=24 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=24 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-2-new-cdh,partition=25 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-3-new-cdh,partition=25 > > > Value java.lang.Object = 0 > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=25 > > > Value java.lang.Object = 0 > > > > > > Somebody's lying here. :) > > > > > > Any ideas? > > > > > > Thanks, > > > Otis > > > -- > > > Monitoring * Alerting * Anomaly Detection * Centralized Log Management > > > Solr & Elasticsearch Support * http://sematext.com/ > > > > > > > > > > > > On Thu, Jun 4, 2015 at 3:54 PM, Otis Gospodnetic < > > > otis.gospodne...@gmail.com > > > > wrote: > > > > > > > Hi Jason, > > > > > > > > (note: Kafka 0.8.2. GA) > > > > Got some new info below! Could be a Kafka metrics bug.... > > > > > > > > On Thu, Jun 4, 2015 at 2:11 PM, Jason Rosenberg <j...@squareup.com> > > > wrote: > > > > > > > >> I assume you are looking at a 'MaxLag' metric, which reports the worst > > > >> case > > > >> lag over a set of partitions. > > > > > > > > > > > > No, we're looking at MBeans that look like this one: > > > > > > > > > > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_free_system_topic-new-cdh,partition=10 > > > > Value java.lang.Object = 0 > > > > > > > > > > > >> Are you consuming multiple partitions, and maybe one of them is stuck? > > > >> > > > > > > > > Don't think so... Maybe what we are seeing is a Kafka bug. > > > > > > > > Here is what we just discovered: > > > > > > > > Dumped JMX on the consumer and we see this: > > > > > > > > > > > > > > > > > kafka.server:type=FetcherLagMetrics,name=ConsumerLag,clientId=af_servers,topic=spm_cluster_topic-new-cdh,partition=24 > > > > Value java.lang.Object = 81560 > > > > > > > > This 81560 is also what we see in SPM - see the flat line here: > > > > https://apps.sematext.com/spm-reports/s/eQ9WhLegW9 > > > > (you can hover over a datapoint on that 81K line to see server name, > > > > topic, and partition) > > > > > > > > This 81560 is just not going down. If I look at JMX in 5 minutes, it > > > will > > > > show the same value - the ConsumerLag of 81560! > > > > > > > > BUT, this gives different numbers: > > > > > > > > /usr/lib/kafka_2.8.0-0.8.1.1/bin/kafka-run-class.sh > > > > kafka.tools.ConsumerOffsetChecker --zkconnect localhost:2181 --group > > > > af_servers | grep spm_cluster_topic-new-cdh > > > > > > > > af_servers spm_cluster_topic-new-cdh 24 *355209634* > > > > *355249858* 40224 > > > > af_servers_spm-afs-6.prod.sematext-1433430424202-e366dfdf-0 > > > > > > > > The delta between the bolded numbers is NOT 81560. And if I run this > > > > command N times the delta keeps going down, because the consumer is > > > > catching up. Just like you'd expect. > > > > > > > > But the JMX number remains constant <== could this be a Kafka > > metrics/JMX > > > > bug? > > > > > > > > Thanks, > > > > Otis > > > > -- > > > > Monitoring * Alerting * Anomaly Detection * Centralized Log Management > > > > Solr & Elasticsearch Support * http://sematext.com/ > > > > > > > > > > > > > > > >> > > > >> On Tue, Jun 2, 2015 at 4:00 PM, Otis Gospodnetic < > > > >> otis.gospodne...@gmail.com > > > >> > wrote: > > > >> > > > >> > Hi, > > > >> > > > > >> > I've noticed that when we restart our Kafka consumers our consumer > > lag > > > >> > metric sometimes looks "weird". > > > >> > > > > >> > Here's an example: > > https://apps.sematext.com/spm-reports/s/0Hq5zNb4hH > > > >> > > > > >> > You can see lag go up around 15:00, when some consumers were > > > restarted. > > > >> > The "weird" thing is that the lag remains flat! > > > >> > How could it remain flat if consumers are running? (they have enough > > > >> juice > > > >> > to catch up!) > > > >> > > > > >> > What I think is happening is this: > > > >> > 1) consumers are initially not really lagging > > > >> > 2) consumers get stopped > > > >> > 3) lag grows > > > >> > 4) consumers get started again > > > >> > 5) something shifts around...not sure what... > > > >> > 6) consumers start consuming, and there is actually no lag, but the > > > >> offsets > > > >> > written to ZK sometime during 3) don't get updated because after > > > restart > > > >> > consumers are reading from somewhere else, not from partition(s) > > whose > > > >> lag > > > >> > and offset delta jumped during 3) > > > >> > > > > >> > Oh, and: > > > >> > 7) Kafka JMX still exposes all offsets, event those for partitions > > > that > > > >> are > > > >> > no longer being read, so the consumer lag metric remains > > > constant/flat, > > > >> > even though consumers are actually not lagging on partitions from > > > which > > > >> > they are now consuming. > > > >> > > > > >> > What bugs me is 7), because reading lag info from JMX looks like > > it's > > > >> > "lying". > > > >> > > > > >> > Does this sound crazy or reasonable? > > > >> > > > > >> > If anyone has any comments/advice/suggestions for what one can do > > > about > > > >> > this, I'm all ears! > > > >> > > > > >> > Thanks, > > > >> > Otis > > > >> > -- > > > >> > Monitoring * Alerting * Anomaly Detection * Centralized Log > > Management > > > >> > Solr & Elasticsearch Support * http://sematext.com/ > > > >> > > > > >> > > > > > > > > > > > > >
