> You're right. If you want to keep all data in Kafka without ever deleting > them, you'd need to add partitions dynamically (which is currently possible > with APIs that back the CLI). On the other hand, using Kafka this way is > the wrong approach IMO. If you really need to keep the full event history, > keep old events on HDFS or wherever and only the more recent ones in Kafka > (where a full replay must first read from HDFS and then from Kafka) or use > a journal plugin that is explicitly designed for long-term event storage. >
That was worrying me all the time with using Kafka in a situation where I would want to keep the events all the time (or at least unknown amount of time). The thing that seemed nice is that I would have journal/event store and pub-sub solution implemented in one technology - basically I want to go around current limitation of PersistentView. I wanted to use Kafka topic and replay all events from the topic to dynamically added read models in my cluster. Maybe in this situation I should stick to distributed publish-subscribe in cluster for current event-sending and Cassandra as journal/snapshot store. I did not read that much about Cassandra and the way it stores data so I do not know if reading all events would be easy. The main reason why I developed the Kafka plugin was to integrate my Akka > applications in unified log processing architectures as descibed in Jay > Kreps' excellent article > <http://engineering.linkedin.com/distributed-systems/log-what-every-software-engineer-should-know-about-real-time-datas-unifying>. > > Also mentioned in this article is a snapshotting strategy that fits typical > retention times in Kafka. > Thanks for the link. The most interesting next Kafka plugin feature for me to develop is an HDFS > integration for long-term event storage (and full event history replay). > WDYT? > That would be interesting feature - certainly would make akka + Kafka combination viable for more use cases. W dniu wtorek, 26 sierpnia 2014 19:44:05 UTC+2 użytkownik Martin Krasser napisał: > > > On 26.08.14 16:44, Andrzej Dębski wrote: > > My mind must have filtered out the possibility of making snapshots using > Views - thanks. > > About partitions: I suspected as much. The only thing that I am > wondering now is: if it is possible to dynamically create partitions in > Kafka? AFAIK the number of partitions is set during topic creation (be it > programmatically using API or CLI tools) and there is CLI tool you can use > to modify existing topic: > https://cwiki.apache.org/confluence/display/KAFKA/Replication+tools#Replicationtools-5.AddPartitionTool. > > To keep the invariant " PersistentActor is the only writer to a > partitioned journal topic" you would have to create those partitions > dynamically (usually you don't know up front how many PersistentActors your > system will have) on per-PersistentActor basis. > > > You're right. If you want to keep all data in Kafka without ever deleting > them, you'd need to add partitions dynamically (which is currently possible > with APIs that back the CLI). On the other hand, using Kafka this way is > the wrong approach IMO. If you really need to keep the full event history, > keep old events on HDFS or wherever and only the more recent ones in Kafka > (where a full replay must first read from HDFS and then from Kafka) or use > a journal plugin that is explicitly designed for long-term event storage. > > The main reason why I developed the Kafka plugin was to integrate my Akka > applications in unified log processing architectures as descibed in Jay > Kreps' excellent article > <http://engineering.linkedin.com/distributed-systems/log-what-every-software-engineer-should-know-about-real-time-datas-unifying>. > > Also mentioned in this article is a snapshotting strategy that fits typical > retention times in Kafka. > > > On the other hand maybe you are assuming that each actor is writing to > different topic > > > yes, and the Kafka plugin is currently implemented that way. > > - but I think this solution is not viable because information about > topics is limited by ZK and other factors: > http://grokbase.com/t/kafka/users/133v60ng6v/limit-on-number-of-kafka-topic > . > > > A more in-depth discussion about these limitations is given at > http://www.quora.com/How-many-topics-can-be-created-in-Apache-Kafka with > a detailed comment from Jay. I'd say that if you designed your application > to run more than a few hundred persistent actors, then the Kafka plugin is > the probably wrong choice. I tend to design my applications to have only a > small number of persistent actors (which is in contrast to many other > discussions on akka-user) which makes the Kafka plugin a good candidate. > > To recap, the Kafka plugin is a reasonable choice if > > - frequent snapshotting is done by persistent actors (every day or so) > - you don't have more than a few hundred persistent actors and > - your application is a component of a unified log processing architecture > (backed by Kafka) > > The most interesting next Kafka plugin feature for me to develop is an > HDFS integration for long-term event storage (and full event history > replay). WDYT? > > > W dniu wtorek, 26 sierpnia 2014 15:28:47 UTC+2 użytkownik Martin Krasser > napisał: >> >> Hi Andrzej, >> >> On 26.08.14 09:15, Andrzej Dębski wrote: >> >> Hello >> >> Lately I have been reading about a possibility of using Apache Kafka as >> journal/snapshot store for akka-persistence. >> >> I am aware of the plugin created by Martin Krasser: >> https://github.com/krasserm/akka-persistence-kafka/ and also I read >> other topic about Kafka as journal >> https://groups.google.com/forum/#!searchin/akka-user/kakfka/akka-user/iIHmvC6bVrI/zeZJtW0_6FwJ >> . >> >> In both sources I linked two ideas were presented: >> >> 1. Set log retention to 7 days, take snapshots every 3 days (example >> values) >> 2. Set log retention to unlimited. >> >> Here is the first question: in first case wouldn't it mean that >> persistent views would receive skewed view of the PersistentActor state >> (only events from 7 days) - is it really viable solution? As far as I know >> PersistentView can only receive events - it can't receive snapshots from >> corresponding PersistentActor (which is good in general case). >> >> >> PersistentViews can create their own snapshots which are isolated from >> the corresponding PersistentActor's snapshots. >> >> >> Second question (more directed to Martin): in the thread I linked you >> wrote: >> >> I don't go into Kafka partitioning details here but it is possible to >>> implement the journal driver in a way that both a single persistent actor's >>> data are partitioned *and* kept in order >>> >> >> I am very interested in this idea. AFAIK it is not yet implemented in >> current plugin but I was wondering if you could share high level idea how >> would you achieve that (one persistent actor, multiple partitions, ordering >> ensured)? >> >> >> The idea is to >> >> - first write events 1 to n to partition 1 >> - then write events n+1 to 2n to partition 2 >> - then write events 2n+1 to 3n to partition 3 >> - ... and so on >> >> This works because a PersistentActor is the only writer to a partitioned >> journal topic. During replay, you first replay partition 1, then partition >> 2 and so on. This should be rather easy to implement in the Kafka journal, >> just didn't have time so far; pull requests are welcome :) Btw, the >> Cassandra >> journal <https://github.com/krasserm/akka-persistence-cassandra> follows >> the very same strategy for scaling with data volume (by using different >> partition keys). >> >> Cheers, >> Martin >> >> -- >> >>>>>>>>>> Read the docs: http://akka.io/docs/ >> >>>>>>>>>> Check the FAQ: >> http://doc.akka.io/docs/akka/current/additional/faq.html >> >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user >> --- >> You received this message because you are subscribed to the Google Groups >> "Akka User List" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to akka-user+...@googlegroups.com. >> To post to this group, send email to akka...@googlegroups.com. >> Visit this group at http://groups.google.com/group/akka-user. >> For more options, visit https://groups.google.com/d/optout. >> >> >> -- >> Martin Krasser >> >> blog: http://krasserm.blogspot.com >> code: http://github.com/krasserm >> twitter: http://twitter.com/mrt1nz >> >> -- > >>>>>>>>>> Read the docs: http://akka.io/docs/ > >>>>>>>>>> Check the FAQ: > http://doc.akka.io/docs/akka/current/additional/faq.html > >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user > --- > You received this message because you are subscribed to the Google Groups > "Akka User List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to akka-user+...@googlegroups.com <javascript:>. > To post to this group, send email to akka...@googlegroups.com > <javascript:>. > Visit this group at http://groups.google.com/group/akka-user. > For more options, visit https://groups.google.com/d/optout. > > > -- > Martin Krasser > > blog: http://krasserm.blogspot.com > code: http://github.com/krasserm > twitter: http://twitter.com/mrt1nz > > -- >>>>>>>>>> Read the docs: http://akka.io/docs/ >>>>>>>>>> Check the FAQ: >>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user --- You received this message because you are subscribed to the Google Groups "Akka User List" group. 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