Hi Alex, Thanks for writing down the proposal for this! As I had previously suggested this when implementing the Persistence of Polaris Events <https://github.com/apache/polaris/pull/1844>, I am obviously very much in favor of doing this :)
A few questions I have regarding your vision of how we should implement this: * Are you envisioning anything for being able to make dependencies between event listeners? Or are we taking a set direction that Event Listeners should be independent of each other? * In some listeners we have the ability to make events emission synchronous [example <https://github.com/apache/polaris/blob/main/runtime/service/src/main/java/org/apache/polaris/service/events/jsonEventListener/aws/cloudwatch/AwsCloudWatchEventListener.java#L186>]. How do we plan to support/advise (or not...) that with the introduction of @Blocking annotations. Best, Adnan Hemani On Mon, Nov 10, 2025 at 11:29 AM Yufei Gu <[email protected]> wrote: > Thanks for the reply. It's overall a good idea to have async event > listeners so that they are not blocking each other. > > One downside of the async ones is that event order isn't deterministic. > For example, event listeners of Spark need the order to understand the > execution semantics. I think Polaris is fine with that, given the ts of > each event is generated by Polaris. The downstream can still figure out the > order. > > Thanks Pierre for sharing, I think any I/O-bound or potentially slow > listener should be annotated with @Blocking. That ensures we keep the event > loop responsive and avoid impacting REST latency. > > Yufei > > > On Mon, Nov 10, 2025 at 9:43 AM Alexandre Dutra <[email protected]> wrote: > > > Hi all, > > > > Answering the questions above: > > > > > However, we can easily make sure that we use Quarkus's SmallRye Fault > > Tolerance > > > > Yes, that was my idea. It's not so much the bus itself that needs to > > be fault tolerant, but the receiving end, that is, the listeners. A > > listener can fail for a variety of reasons (e.g. remote broker > > unavailable), it would be nice to be able to backoff and retry > > automatically. > > > > > Since the Vert.x event bus runs on event-loop threads [...] could > > blocking or slow event listeners potentially stall REST requests and > impact > > latency? > > > > What Pierre said: this could indeed happen, but it's possible to > > annotate the receiving end with @Blocking, in which case, the listener > > will be invoked in a separate pool. > > > > > With asynchronous event listeners, is there a guarantee of delivery to > > all listeners for a given event? > > > > If I understand the question correctly: with asynchronous delivery, a > > slow or failing listener wouldn't impact the delivery of the same > > event to other listeners. > > > > Thanks, > > Alex > > > > On Mon, Nov 10, 2025 at 10:12 AM Pierre Laporte <[email protected]> > > wrote: > > > > > > Thanks for the proposal, Alex. This sounds like a great improvement. > > > > > > @Yufei As per Quarkus documentation, slow event listeners should be > > marked > > > with @Blocking so that they are not run on the event loop threads. > > > -- > > > > > > Pierre > > > > > > > > > On Sat, Nov 8, 2025 at 2:14 AM Michael Collado <[email protected] > > > > > wrote: > > > > > > > With asynchronous event listeners, is there a guarantee of delivery > to > > all > > > > listeners for a given event? The downside of synchronous listeners is > > that > > > > everything is serial, but also if something fails, processing stops. > > This > > > > feels important for auditing purposes, though less important for > other > > > > cases. > > > > > > > > Mike > > > > > > > > On Fri, Nov 7, 2025 at 2:28 PM Yufei Gu <[email protected]> > wrote: > > > > > > > > > Thanks, Alex and Adam. One concern I have is about the shared > runtime > > > > > thread pool. > > > > > Since the Vert.x event bus runs on event-loop threads that are also > > used > > > > by > > > > > Quarkus’ reactive REST endpoints, could blocking or slow event > > listeners > > > > > potentially stall REST requests and impact latency? > > > > > > > > > > Yufei > > > > > > > > > > > > > > > On Fri, Nov 7, 2025 at 11:25 AM Adam Christian < > > > > > [email protected]> wrote: > > > > > > > > > > > I think that this would be a great enhancement. Thanks for > > proposing > > > > it! > > > > > > > > > > > > The only concern I would have is around fault-tolerance. From > what > > I > > > > can > > > > > > tell, from the Quarkus documentation, the Quarkus event bus uses > > Vert.x > > > > > > EventBus which does not guarantee message delivery if failure of > > part > > > > of > > > > > > the event bus occurs [1]. However, we can easily make sure that > we > > use > > > > > > Quarkus's SmallRye Fault Tolerance [2]. Is my rough understanding > > > > inline > > > > > > with your proposal? > > > > > > > > > > > > Go community, > > > > > > > > > > > > Adam > > > > > > > > > > > > [1]: > > > > https://vertx.io/docs/apidocs/io/vertx/core/eventbus/EventBus.html > > > > > > [2]: https://quarkus.io/guides/smallrye-fault-tolerance > > > > > > > > > > > > On Fri, Nov 7, 2025 at 11:49 AM Alexandre Dutra < > [email protected] > > > > > > > > wrote: > > > > > > > > > > > > > Hi all, > > > > > > > > > > > > > > I'd like to propose an enhancement to our existing events > > feature: > > > > the > > > > > > > ability to support multiple listeners. > > > > > > > > > > > > > > Currently, only a single listener can be active at a time, > which > > is > > > > > > > quite limiting. For example, we might need to persist events > for > > > > audit > > > > > > > purposes and simultaneously send them to a message queue for > > > > > > > optimization. With the current setup, this isn't easily > > achievable. > > > > > > > > > > > > > > While a composite listener could be created, it feels like a > less > > > > > > > elegant solution, and the delivery would be strictly serial, > > > > > > > processing one listener after another. > > > > > > > > > > > > > > My suggestion is to leverage Quarkus internal event bus [1]: > > > > > > > > > > > > > > 1) There will be one central event emitter responsible for > > publishing > > > > > > > events to the bus. > > > > > > > > > > > > > > 2) We will have zero to N listeners, each independently > watching > > the > > > > > > > event bus for relevant events. They will be discovered by CDI. > > > > > > > > > > > > > > 3) We could apply filters to each listener, e.g. listener A > > listens > > > > > > > for event types X and Y, listener B only listens to event type > Y. > > > > > > > > > > > > > > 4) This approach would ensure fully asynchronous delivery of > > events > > > > to > > > > > > > all interested listeners. > > > > > > > > > > > > > > 5) Fault-tolerance could also be easily implemented (event > > delivery > > > > > > > retries, timeouts, etc.). > > > > > > > > > > > > > > What do you all think? > > > > > > > > > > > > > > Thanks, > > > > > > > Alex > > > > > > > > > > > > > > [1]: https://quarkus.io/guides/reactive-event-bus > > > > > > > > > > > > > > > > > > > > > > > > >
