Provability not probability (iPad helping) On Wednesday, August 27, 2014, Greg Young <gregoryyou...@gmail.com> wrote:
> I have used 1:n it's a fairly common pattern I just wanted to point out > it's not a panacea that works everywhere and had some rather large > downsides if applied in the wrong place. I agree in the discussion being > around when which is applicable. My original point was that you can't > really do 1:1 with many of the backends as they don't support millions of > streams. > > Btw for performance what many do is an identity map and caching in memory > (assuming the whole set does not fit in memory) > > There is a side bit to this as well in terms of probability. If you use > one actor for n it's next to impossible to show that transactions do not > interfere with each other (while this is rather trivial with 1:1 as they > would need messages between each other) > > Cheers, > > Greg > > On Wednesday, August 27, 2014, Martin Krasser <krass...@googlemail.com > <javascript:_e(%7B%7D,'cvml','krass...@googlemail.com');>> wrote: > >> Whether to go for a 1:1 approach or a 1:n approach (or a partitioned m:n >> approach where m << n) really depends on the concrete use case and >> non-functional requirements. Your example might be a good candidate for a >> 1:1 approach (see also further comments inline) but there are also examples >> for which a 1:n or m:n approach is a better choice. Here are some general >> influencing factors: >> >> - length of event history required to recover state: bank accounts need >> the full event history to be recovered but order management is an example >> where this is often not the case. Orders (trade orders in finance, lab >> orders during medical treatments, ...) usually have a limited validity so >> that you can recover active orders from a limited event history (last 10 >> days, for example) which should make migrations after code changes rather >> painless. BTW, having only a single persistent actor (or a few) that >> maintains state is comparable to role of a "Business Logic Processor" in >> the LMAX architecture which originated from the high frequency trading >> domain. >> >> - latency requirements: creating a new persistent actor has some >> overhead, not only memory but also bootstrap as its creation requires a >> roundtrip to the backend store. Re-activation of passivated actors that >> have been designed around a 1:1 approach, may also be in conflict with low >> latency requirements. Good compromises can often be found by following an >> m:n approach in this case. >> >> - write throughput: high write throughput can only be achieved by >> batching writes and batching is currently implemented on a per persistent >> actor basis. Throughput therefore scales better when having a small(er) >> number of actors. A large number of actors will create more but smaller >> batches, reducing throughput. This is however more a limitation of the >> current implementation of akka-persistence. Maybe a switch to batching on >> journal level is a good idea, so that a single write batch can contain >> events from several actors. >> >> - ... >> >> Even if you need to replay a long event history (for example after a code >> change), you can always do that in the background on a separate node until >> the new version of the persistent actor caught up and switch the >> application to it when done. You could even have both versions running at >> the same time for A/B testing for example. With a replay rate of 100k/sec >> you can replay a billion events within a few hours. >> >> Further comments inline ... >> >> On 26.08.14 20:34, Greg Young wrote: >> >> OK for bank accounts there is some amount of state needed to verify a >> transaction. Let's propose that for now its the branch you opened your >> account at, your current balance,your address and a risk classification as >> well as a customer profitability/loyalty score (these are all reasonable >> things to track in terms of deciding if a transaction should be accepted or >> not) >> >> >> When validating commands, you only need to keep that part of application >> state within persistent actors for which you have strict consistency >> requirements. In context of bank accounts, this is for sure the case for >> the balance, but not necessarily for customer profitability, loyality score >> or whatever. These metrics may be calculated in the background, hence, >> having eventual read consistency for them should be sufficient. >> Consequently this state can be maintained elsewhere (as part of a separate >> read model) and requested from persistent actors during transaction >> validation. If you need further metrics in the future, new read models can >> be added and included into the validation workflow initiated by a >> persistent actor. >> >> >> I could keep millions of these inside of a single actor. >> >> A few problems come up though: >> >> Replaying this actor from events is very painful (millions possibly >> hundreds of millions of events and they must be processes serially) >> solution->snapshots? >> Snapshots have all the same versioning issues people are used to with >> keeping state around. What happens when the state I am keeping changes say >> now I also need to keep avg+stddev of transaction amount or we found a bug >> in how we were maintaining the loyalty score (back to #1) this will >> invalidate my snapshot >> >> >> See above, there's no need to keep all of that inside the persistent >> actor for strict read consistency. Allowing eventual consistency during >> command validation where possible not only makes the validation process >> more flexible (by just including new read models if required) but also >> reduces snapshot migration efforts (by simplifying the state structure >> inside persistent actors). >> >> Furthermore, ensuring strict consistency for persistent actor state >> requires usage of persist() instead of persistAsync() which reduces >> throughput at least by a factor of 10. That may again be in conflict with >> write throughput requirements. >> >> To conclude, I think there are use cases where a 1:1 approach makes sense >> but this shouldn't be a general recommendation IMO. It really depends on >> the specific functional and non-functional requirements for finding the >> best compromise. >> >> (requiring a full replay or else you run into another whole series of >> hokey problems trying to do "from here forward" type things (imagine a new >> feature that relies on a 6 month moving average) >> >> >> >> >> >> >> On Tue, Aug 26, 2014 at 2:15 PM, Martin Krasser <krass...@googlemail.com> >> wrote: >> >>> >>> On 26.08.14 20:12, Greg Young wrote: >>> >>> In particular I am interested in the associated state thats needed, I >>> can see keeping it in a single actor but this does not turn out well at all >>> for most production systems in particular as changes happen over time. >>> >>> >>> I don't get your point. Please elaborate. >>> >>> >>> >>> On Tue, Aug 26, 2014 at 2:08 PM, Martin Krasser <krass...@googlemail.com >>> > wrote: >>> >>>> See my eventsourced example(s), that I published 1-2 years ago, others >>>> are closed source >>>> >>>> >>>> On 26.08.14 20:06, Greg Young wrote: >>>> >>>> Love to see an example >>>> >>>> On Tuesday, August 26, 2014, Martin Krasser <krass...@googlemail.com> >>>> wrote: >>>> >>>>> >>>>> On 26.08.14 19:56, Greg Young wrote: >>>>> >>>>> I'm curious how you would model say bank accounts with only a few >>>>> hundred actors can you go into a bit of detail >>>>> >>>>> >>>>> persistent-actor : bank-account = 1:n (instead of 1:1) >>>>> >>>>> >>>>> On Tuesday, August 26, 2014, Martin Krasser <krass...@googlemail.com> >>>>> wrote: >>>>> >>>>>> >>>>>> 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+unsubscr...@googlegroups.com. >>>>>> To post to this group, send email to akka-user@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 a topic in >>>>>> the Google Groups "Akka User List" group. >>>>>> To unsubscribe from this topic, visit >>>>>> https://groups.google.com/d/topic/akka-user/Bz9pWyK7V7g/unsubscribe. >>>>>> To unsubscribe from this group and all its topics, send an email to >>>>>> akka-user+unsubscr...@googlegroups.com. >>>>>> To post to this group, send email to akka-user@googlegroups.com. >>>>>> Visit this group at http://groups.google.com/group/akka-user. >>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>> >>>>> >>>>> >>>>> -- >>>>> Studying for the Turing test >>>>> >>>>> -- >>>>> >>>>>>>>>> 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+unsubscr...@googlegroups.com. >>>>> To post to this group, send email to akka-user@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 a topic in the >>>>> Google Groups "Akka User List" group. >>>>> To unsubscribe from this topic, visit >>>>> https://groups.google.com/d/topic/akka-user/Bz9pWyK7V7g/unsubscribe. >>>>> To unsubscribe from this group and all its topics, send an email to >>>>> akka-user+unsubscr...@googlegroups.com. >>>>> To post to this group, send email to akka-user@googlegroups.com. >>>>> Visit this group at http://groups.google.com/group/akka-user. >>>>> For more options, visit https://groups.google.com/d/optout. >>>>> >>>> >>>> >>>> -- >>>> Studying for the Turing test >>>> >>>> -- >>>> >>>>>>>>>> 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+unsubscr...@googlegroups.com. >>>> To post to this group, send email to akka-user@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 a topic in the >>>> Google Groups "Akka User List" group. >>>> To unsubscribe from this topic, visit >>>> https://groups.google.com/d/topic/akka-user/Bz9pWyK7V7g/unsubscribe. >>>> To unsubscribe from this group and all its topics, send an email to >>>> akka-user+unsubscr...@googlegroups.com. >>>> To post to this group, send email to akka-user@googlegroups.com. >>>> Visit this group at http://groups.google.com/group/akka-user. >>>> For more options, visit https://groups.google.com/d/optout. >>>> >>> >>> >>> >>> -- >>> Studying for the Turing test >>> -- >>> >>>>>>>>>> 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+unsubscr...@googlegroups.com. >>> To post to this group, send email to akka-user@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 a topic in the >>> Google Groups "Akka User List" group. >>> To unsubscribe from this topic, visit >>> https://groups.google.com/d/topic/akka-user/Bz9pWyK7V7g/unsubscribe. >>> To unsubscribe from this group and all its topics, send an email to >>> akka-user+unsubscr...@googlegroups.com. >>> To post to this group, send email to akka-user@googlegroups.com. >>> Visit this group at http://groups.google.com/group/akka-user. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> >> >> -- >> Studying for the Turing test >> -- >> >>>>>>>>>> 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+unsubscr...@googlegroups.com. >> To post to this group, send email to akka-user@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 a topic in the >> Google Groups "Akka User List" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/akka-user/Bz9pWyK7V7g/unsubscribe. >> To unsubscribe from this group and all its topics, send an email to >> akka-user+unsubscr...@googlegroups.com. >> To post to this group, send email to akka-user@googlegroups.com. >> Visit this group at http://groups.google.com/group/akka-user. >> For more options, visit https://groups.google.com/d/optout. >> > > > -- > Studying for the Turing test > > -- Studying for the Turing test -- >>>>>>>>>> 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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