Hello Pushkar, Yes, that’s correct. The operation you describe is currently not supported. If you want to keep the structure you described in place, I’d suggest using an external database for the admin objects. I’ll give another idea below.
With your current architecture, I’m a little concerned about data races. From what I saw, nothing would prevent processing stream records with agent 10 before you process the admin record with agent 10. This problem will persist no matter where you locate the cache. GlobalKTable would no doubt make it worse, since it increases the latency before admin record 10 is queriable everywhere. I think you’ll want to make a call between architecture simplicity (remote cache or global KTable) vs the probability of missed joins. I think the “best” way to solve this problem (that comes to mind anyway) might be to 1. Repartition the stream to be co-partitioned with the admin records. 2. Do a local (not global) stream-table join 3. Enable task idling You can do the repartition today with a ‘map’ or ‘selectKey’ to make the agent Id the new key of the stream, and then use ‘through’, (where the intermediate topic has the same number of partitions as the admin topic) to do the repartitioning. In 2.6, there is a “repartition” operator that will make this easier. The repartition ensures that all stream records with agent id 10 will be processed by the same thread that processes the admin records with agent id 10, hence it will be able to find agent 10 in the local KTable store. Task idling will minimize your chances of missing any enrichments. When a task has two inputs (E.g., your repartitioned stream joining with the admin table), it makes Streams wait until both inputs are buffered before processing, so it can do a better job of processing in timestamp order. I hope this helps! -John On Mon, May 4, 2020, at 05:30, Pushkar Deole wrote: > If i understand correctly, Kafka is not designed to provide replicated > caching mechanism wherein the updates to cache will be synchronous across > multiple cache instances. > > On Sun, May 3, 2020 at 10:49 PM Pushkar Deole <pdeole2...@gmail.com> wrote: > > > Thanks John. > > > > Actually, this is a normal consumer-producer application wherein there are > > 2 consumers (admin consumer and main consumer) consuming messages from 2 > > different topics. > > One of the consumers consumes messages from a admin topic and populates > > data in a cache e.g. lets say agent with agent id 10 for which the first > > name and last name is received is populated in cache. When the other > > consumer consumes message and it has agent id 10 then it reads the cache, > > appends the first name and last name and then sends enriched event to > > producer. > > In this case, each application instance consumes all the events from admin > > topic (unique consumer id) and keeps them in the cache in memory. > > Now the requirement is to persist the cache and make is shared between the > > application instances, so each instance would consume partitions of admin > > topic and write to admin cache. > > > > If we want to use kafka streams, the application is so much evolved that > > it is difficult to migrate to streams at this stage. Secondly, from past > > mail chains, streams also won't serve the requirement since local state > > stores would just hold the local state of admin data and the cache written > > by each instance won't be shared with other instances. > > > > Global state stores may help but again it requires writing to the topic > > which is then synced with the state stores in the instances and the > > instances may not be in sync with each. > > I am not sure if this would cause any inconsistencies since i don't know > > how the events would flow from source e.g. if admin data is consumed by one > > instance which then modified the topic but it is not yet synced to all the > > global state stores and the next event arrived on the main consumer on a > > different instance and it tried to read from store cache then it doesn't > > get the data, so the event passed on without enriched data. > > That's pretty much about the use case. > > > > > > On Sun, May 3, 2020 at 9:42 PM John Roesler <vvcep...@apache.org> wrote: > > > >> Hi Pushkar, > >> > >> I’ve been wondering if we should add writable tables to the Streams api. > >> Can you explain more about your use case and how it would integrate with > >> your application? > >> > >> Incidentally, this would also help us provide more concrete advice. > >> > >> Thanks! > >> John > >> > >> On Fri, May 1, 2020, at 15:28, Matthias J. Sax wrote: > >> > Both stores sever a different purpose. > >> > > >> > Regular stores allow you to store state the application computes. > >> > Writing into the changelog is a fault-tolerance mechanism. > >> > > >> > Global store hold "axially" data that is provided from "outside" of the > >> > app. There is no changelog topic, but only the input topic (that is used > >> > to re-create the global state). > >> > > >> > Local stores are sharded and updates are "sync" as they don't need to be > >> > shared with anybody else. > >> > > >> > For global stores, as all instances need to be updated, updates are > >> > async (we don't know when which instance will update it's own global > >> > store replica). > >> > > >> > >> Say one stream thread updates the topic for global store and starts > >> > >> processing next event wherein the processor tries to read the global > >> store > >> > >> which may not have been synced with the topic? > >> > > >> > Correct. There is no guarantee when the update to the global store will > >> > be applied. As said, global stores are not designed to hold data the > >> > application computes. > >> > > >> > > >> > -Matthias > >> > > >> > > >> > On 4/30/20 11:11 PM, Pushkar Deole wrote: > >> > > thanks... will try with GlobalKTable. > >> > > As a side question, I didn't really understand the significance of > >> global > >> > > state store which kind of works in a reverse way to local state store > >> i.e. > >> > > local state store is updated and then saved to changelog topic > >> whereas in > >> > > case of global state store the topic is updated first and then synced > >> to > >> > > global state store. Do these two work in sync i.e. the update to > >> topic and > >> > > global state store ? > >> > > > >> > > Say one stream thread updates the topic for global store and starts > >> > > processing next event wherein the processor tries to read the global > >> store > >> > > which may not have been synced with the topic? > >> > > > >> > > On Fri, May 1, 2020 at 3:35 AM Matthias J. Sax <mj...@apache.org> > >> wrote: > >> > > > >> > >> Yes. > >> > >> > >> > >> A `GlobalKTable` uses a global store internally. > >> > >> > >> > >> You can also use `StreamsBuilder.addGlobalStore()` or > >> > >> `Topology.addGlobalStore()` to add a global store "manually". > >> > >> > >> > >> > >> > >> -Matthias > >> > >> > >> > >> > >> > >> On 4/30/20 7:42 AM, Pushkar Deole wrote: > >> > >>> Thanks Matthias. > >> > >>> Can you elaborate on the replicated caching layer part? > >> > >>> When you say global stores, do you mean GlobalKTable created from a > >> topic > >> > >>> e.g. using StreamsBuilder.globalTable(String topic) method ? > >> > >>> > >> > >>> On Thu, Apr 30, 2020 at 12:44 PM Matthias J. Sax <mj...@apache.org> > >> > >> wrote: > >> > >>> > >> > >>>> It's not possible to modify state store from "outside". > >> > >>>> > >> > >>>> If you want to build a "replicated caching layer", you could use > >> global > >> > >>>> stores and write into the corresponding topics to update all > >> stores. Of > >> > >>>> course, those updates would be async. > >> > >>>> > >> > >>>> > >> > >>>> -Matthias > >> > >>>> > >> > >>>> On 4/29/20 10:52 PM, Pushkar Deole wrote: > >> > >>>>> Hi All, > >> > >>>>> > >> > >>>>> I am wondering if this is possible: i have been asked to use state > >> > >> stores > >> > >>>>> as a general replicated cache among multiple instances of service > >> > >>>> instances > >> > >>>>> however the state store is created through streambuilder but is > >> not > >> > >>>>> actually modified through stream processor topology however it is > >> to be > >> > >>>>> modified from outside the stream topology. So, essentially, the > >> state > >> > >>>> store > >> > >>>>> is just to be created from streambuilder and then to be used as an > >> > >>>>> application level cache that will get replicated between > >> application > >> > >>>>> instances. Is this possible using state stores? > >> > >>>>> > >> > >>>>> Secondly, if possible, is this a good design approach? > >> > >>>>> > >> > >>>>> Appreciate your response since I don't know the internals of state > >> > >>>> stores. > >> > >>>>> > >> > >>>> > >> > >>>> > >> > >>> > >> > >> > >> > >> > >> > > > >> > > >> > > >> > Attachments: > >> > * signature.asc > >> > > >
