Hi Yi! I understand the overall motivation but I think we need concise and direct guidance, not only when we recommend using the RpcOperator but when we dont.
Features like "fault isolation and flexible scaling" sound too powerful and users will tend to gravitate to these features if they promise a silver bullet against Flink's current limitations. Especially with the possibility of RPC-only jobs the whole datastream api may be questionable for users for stateless jobs. We have to clearly enumerate the cases when we recommend the rpc operator but also when we dont (this is very important) We have to explicitly and in the primary position call out what it doesn't provide: such as state. Are there any difference in processing semantics / guarantees? Can it even provide at least once guarantees for an RPC only job? Based on these we should aim to completely clarify the scope (not the features) to the users. Cheers Gyula On Fri, Jun 12, 2026 at 4:30 AM Yi Zhang <[email protected]> wrote: > Hi Guowei, > > > Thank you for your patience — I'm sorry for the delayed reply. I really > appreciate your > follow-up questions; they point to two key design choices that deserve > thorough > consideration. > > > 1. Auto-scaling & parallelism API > I believe we are in agreement that auto-scaling logic will not be included > in this FLIP. > Given that, I wonder whether introducing `min` and `max` parameters at > this stage > might actually confuse users — they would see an API that accepts a range > but offers > no runtime behavior to act on it. > I'd like to clarify how I think about the `parallelism` parameter: it does > not necessarily > represent a "fixed" parallelism, but rather an **initial parallelism** — > the service starts > with this level of concurrency and may be adjusted during subsequent > execution. Of > course, such adjustment would indeed require bounds. Would it be > acceptable to > introduce them when we actually bring in the auto-scaling capability? This > way, the > initial API remains simple and intuitive. > > > 2. Lifecycle semantics and API expression > This is a great question, and it actually connects closely to the **Shared > RPC Service** > task I mentioned in the Follow-up section. After further discussion and > reflection, I now > believe it would be better to include the shared service capability in > this FLIP as well. > The idea is to introduce a separate API: > `env.startSharedRpcOperatorService(...)`, which > is clearly distinct from `env.addRpcOperatorService(...)`. The semantics > would be: > a. `addRpcOperatorService` retains its **per-job** semantics, similar to > how > transformations are cleared after `execute()`, services added via this API > are also scoped > to a single job execution and cleaned up afterward. > b. `startSharedRpcOperatorService` starts a service that lives > independently of any > particular job, accessible by multiple jobs within the same application. > > > Regarding your suggestion of `ds.query(new RPCOperatorService)` — it's a > nice idea > for expressing per-job ownership, but I see one limitation: since the > service is bound to > a specific DataStream operation, every `query` call would instantiate a > new, independent > service. This makes it difficult to share a single service across multiple > operators within > the same job, which is a key use case we want to support. > > > With the `addRpcOperatorService` / `startSharedRpcOperatorService` pair, > both per-job > and shared scenarios are covered through a unified and explicit API. Do > you think this > approach works? > > > If you are aligned with this approach, I will update the FLIP accordingly. > Thank you again > for raising these important points! > > > Best, > Yi > > > > At 2026-06-02 14:27:26, "Guowei Ma" <[email protected]> wrote: > >Hi, YI > >Thanks a lot for the explanation and the update. I still have two small > >questions I'd like to get your thoughts on: > > > >*1. On auto-scaling* > > > >I think auto-scaling should be a first-class citizen in Flink as a whole — > >and RpcOperator was proposed precisely to make this achievable for GPU > >workloads. For that reason, baking a fixed parallelism into the API as the > >first step doesn't feel right to me; put simply, I don't think we should > >introduce an API that locks parallelism to a single value. > > > >That said, I fully agree that a complete auto-scaling implementation is > >complex and isn't the focus of this FLIP. So my suggestion is that the > >first version could just support the degenerate case — e.g. min = max — > >without committing the API itself to fixed parallelism. > > > >*2. On the lifecycle question* > > > >a. This is actually tied to how we express the API. What I've had in mind > >is something like ds.query(new RPCOperatorService), since it leaves no > room > >for ambiguity: the service clearly belongs to a single job, and every job > >has its own. > > > >b. Consider this scenario: a Flink application-mode deployment containing > >two jobs, where each job needs its own independent RPC service. With your > >current design, would I have to call addRpcService twice? > > > >c. If it's StreamExecutionEnvironment that interacts with the RPC > operator, > >that feels more like something shared *across* jobs — analogous to > >registering a UDF in the Table API, where multiple jobs can use it. That > >seems to sit in tension with the per-job semantics above. > >Best, > >Guowei > > > > > >On Tue, Jun 2, 2026 at 11:16 AM Gen Luo <[email protected]> wrote: > > > >> Hi Yi, > >> > >> Thanks for this impressive proposal. Adding an RPC service deployment > >> primitive can greatly expand Flink's capability scope and flexibility, > with > >> clear benefits for AI workloads and agent applications. It can also > play a > >> role in scenarios like state management or data lake compaction — > relevant > >> to but decoupled from the data pipeline. This is a very promising > >> direction. That said, I still have some questions. > >> > >> 1. My biggest concern is heterogeneous resources, echoing Hongshun's Q4. > >> Flink today uses homogeneous TM resources sliced for different task > >> requirements, but decoupling GPU/CPU resources and enabling flexible > >> independent scaling is one of the core values of this FLIP — and the > >> current resource allocation approach cannot support this. We likely need > >> new implementations for the resource allocation component. > >> > >> 2. Some components are described only at the idea level without > >> implementation details. Hopefully these can be supplemented later or > >> discussed in dedicated threads. A few worth discussing: > >> > >> - RpcTask: It's currently described as a simple single-threaded loop, > >> with no plan to use Mailbox or OperatorChain. But if we want to support > >> async, checkpoint, etc. later, introducing Mailbox would be valuable — > >> handle the input queue as defaultAction, with async/CP/system messages > as > >> mails. This keeps semantics consistent with StreamTask and reduces > >> maintenance burden. System events like FO-triggered request cancellation > >> can also be prioritized via mailbox. OperatorChain is what > >> SubtaskCheckpointCoordinator directly contacts, so having RpcOperator as > >> the only node in the chain wouldn't add much complexity and lowers the > cost > >> when CP support comes. > >> > >> - Communication implementation: Will it reuse the Pekko + proxy > >> mechanism? Pekko has risks with large message bodies (OOM, silently drop > >> oversized frame, etc.). Since ROS is user-facing, we can't fully avoid > >> this, and large-model scenarios often carry sizable context — needs > careful > >> handling. Also, the current design doesn't include table-function-style > >> interfaces for unfolding data, so a large unfolded result has to be > >> returned as a single response, inflating per-request payload size and > >> amplifying the same risk. > >> > >> - Client-side implementation: Only the interface is described, no > >> implementation details. Checkpoint is the prominent issue — since ROS is > >> stateless by design, state recording falls to the client. Do we go with > >> drain or record? Drain may take a long time; record on a keyed stream > may > >> cause request distribution confusion after parallelism changes. > >> > >> 3. ROS as container, not service: Is this allowed? If the system itself > has > >> service capabilities — especially non-Java stacks (agents, LLMs, also > >> Python remote functions) — forwarding requests through the Flink Java > layer > >> adds unnecessary overhead. In that case RpcOperator would only handle > >> lifecycle, while the framework manages resource scheduling and service > >> discovery without carrying business traffic. > >> > >> nit: addRpcService is inconsistent with other places that use > RpcOperator / > >> the ros keyword. Worth unifying. > >> > >> Best, > >> Gen > >> > >> On Mon, Jun 1, 2026 at 2:02 PM Yi Zhang <[email protected]> wrote: > >> > >> > Hi Guowei, > >> > > >> > > >> > Thanks for the thoughtful review and valuable feedback! Your questions > >> are > >> > insightful > >> > and have directly driven refinements to the proposal. Here are my > >> > responses to each > >> > point: > >> > > >> > > >> > 1. Autoscaling > >> > Supporting a (min, max) concurrency range is a great idea. However, > >> > auto-scaling > >> > involves much more than just the API surface — it requires discussion > >> > around > >> > scaling policies, coordination with the scheduler, metric-driven > >> triggers, > >> > and more. > >> > To keep this FLIP focused, I'd like to defer that to a follow-up. > Fixed > >> > parallelism is > >> > a common and necessary capability on its own, and the first version > will > >> > support > >> > that. The interfaces can be extended later to include auto-scaling > >> support > >> > without > >> > breaking changes. > >> > > >> > > >> > 2. Async invocation & end-to-end examples > >> > This is a good point. The `RosClient` already provides an async > >> interface, > >> > so using > >> > it with `AsyncDataStream` is indeed more natural and idiomatic. I will > >> add > >> > an > >> > asynchronous invocation example in the proposal. Regarding a complete > >> > end-to-end > >> > inference example: at the Java API level, it will still be primarily > >> > conceptual, since > >> > practical model inference examples really need Python API support > (e.g., > >> > loading a > >> > PyTorch or ONNX model). I will refine the existing example to make the > >> > overall > >> > invocation chain as close to a real-world scenario as possible. > >> > > >> > > >> > 3. GPU as a resource > >> > This is indeed a key concern. The `extendedResource` API in the > original > >> > proposal > >> > comes from Flink's existing resource model (introduced by FLIP-108). > >> > However, I > >> > agree that GPU deserves first-class treatment rather than being > accessed > >> > through a > >> > generic extension mechanism. Promoting GPU to a first-class resource > will > >> > involve > >> > changes to both the API and the scheduling layer, which we plan to > >> address > >> > in a > >> > dedicated FLIP. For now, I will remove the `extendedResource` API from > >> > `RosDescriptor` to keep this FLIP focused on the RPC service primitive > >> > itself. The > >> > resource declaration API can be extended once the GPU-specific FLIP > is in > >> > place. > >> > > >> > > >> > 4. Serial method invocation > >> > Agreed. I will make the serial invocation semantics more prominent — > both > >> > in the > >> > FLIP documentation and in the code-level JavaDoc for `RpcOperator`. > >> > > >> > > >> > 5. Why synchronous methods? > >> > This is a key question. The initial design chose synchronous methods > for > >> > simplicity > >> > — users don't need to worry about thread safety, and concurrency is > >> > achieved by > >> > scaling out instances (parallelism). However, I recognize that > >> > asynchronous > >> > processing is a critical requirement for certain scenarios (e.g., when > >> the > >> > RPC service > >> > performs expensive operations that can be parallelized internally). I > am > >> > planning to > >> > extend the current design to support asynchronous method execution: > if a > >> > user's > >> > method returns a `CompletableFuture`, the framework will recognize it > and > >> > wait for > >> > the future to complete before sending the response, without blocking > the > >> > invoke > >> > thread. This allows the invoke thread to continue dispatching > subsequent > >> > requests, > >> > while the user controls internal concurrency. Importantly, > >> > `processRequest` calls > >> > remain serial — the async behavior is purely opt-in. > >> > > >> > > >> > 6. SQL UDF usage > >> > The current design is primarily oriented toward the DataStream API, > as it > >> > provides > >> > the foundational building blocks. I agree that SQL and Table API > support > >> > is an > >> > important aspect to consider as well. On the invocation side, both > Table > >> > API and > >> > SQL UDFs share the same runtime path through `FunctionContext`, so > >> > enabling > >> > RPC service access from UDFs is straightforward — I will add a > >> > `getRosClient` > >> > method to `FunctionContext` and include that change in this FLIP. > >> > The real challenge for SQL lies on the definition side: how to define > and > >> > register > >> > an RPC service using pure SQL syntax, which would require SQL grammar > >> > extensions and planner changes. I think that part would be better > >> > addressed in > >> > a dedicated follow-up FLIP to give it the attention it deserves. > >> > > >> > > >> > 7. Service sharing across jobs & lifecycle > >> > In this FLIP, the RPC service is defined as part of the job (via > >> > `env.addRpcService()` > >> > before `env.execute()`). It cannot be accessed across jobs, and it is > >> > released when > >> > the job reaches a terminal state. I believe your question is more > >> relevant > >> > to the > >> > "Shared RPC Service" mentioned in the Follow-up Tasks section. While > it's > >> > out of > >> > scope for this FLIP, let me briefly describe the envisioned design: a > >> > shared RPC > >> > service would run as an independent job within the same Application, > >> > accessible by > >> > all jobs in that Application. It would terminate when the Application > >> > ends, or could be > >> > stopped manually via a user-facing API. > >> > The embedded (per-job) service model discussed in this FLIP is > simpler to > >> > manage > >> > and well-suited for many streaming scenarios, which is why we focus > on it > >> > first. > >> > Importantly, both the embedded and shared modes will reuse the same > set > >> of > >> > deployment primitives (`RpcOperator`, `RosClient`, `RosDescriptor`, > etc.) > >> > — the > >> > difference is only in lifecycle management and visibility scope. > >> > > >> > > >> > > >> > > >> > I hope the above addresses your questions. I will update the proposal > to > >> > reflect the > >> > changes mentioned above. Please feel free to follow up if you have any > >> > further > >> > questions or suggestions. > >> > > >> > > >> > Best, > >> > Yi > >> > > >> > > >> > > >> > At 2026-05-28 16:39:49, "Guowei Ma" <[email protected]> wrote: > >> > >HI，YI > >> > > > >> > >One more question I'd like to raise is around how the service behaves > >> when > >> > >it's referenced by more than one job. If two jobs are submitted > within > >> the > >> > >same application and both reference the same RpcOperator service, is > >> that > >> > >service shared across them, and what is the expected behavior in that > >> > case? > >> > >I'm also curious how this plays out for bounded (batch) jobs in > >> > particular: > >> > >if one job completes while the other is still running, what does the > >> > >service lifecycle look like — is the service torn down as soon as the > >> > first > >> > >job finishes, and if so, how does that affect the job that's still > >> > running? > >> > > > >> > >Best, > >> > >Guowei > >> > > > >> > > > >> > >On Thu, May 28, 2026 at 2:55 PM Guowei Ma <[email protected]> > wrote: > >> > > > >> > >> Hi,YI > >> > >> > >> > >> Thanks for putting together this FLIP — it's a well-thought-out > >> > proposal, > >> > >> and the RpcOperator Service is a really valuable addition for > >> > AI/inference > >> > >> workloads on Flink. I have a few questions and suggestions: > >> > >> > >> > >> > >> > >> 1. Autoscaling > >> > >> How is scale-up/scale-down expressed in this design? Could you > >> consider > >> > >> allowing concurrency to be configured as a (min, max) range? > Currently > >> > it > >> > >> appears only min = max (i.e. fixed parallelism) is supported. > >> > >> > >> > >> > >> > >> 2. Async invocation & end-to-end examples > >> > >> The example shows a synchronous invocation style. I'd suggest also > >> > >> providing an asynchronous invocation example — or even just a > scaffold > >> > that > >> > >> exposes only the RpcOperator's handle and method. In addition, it > >> would > >> > be > >> > >> best to include a complete end-to-end example of invoking a model > (CPU > >> > >> inference would be fine). > >> > >> > >> > >> > >> > >> 3. GPU as a resource > >> > >> GPU is surely a common/first-class resource. Accessing it here > through > >> > an > >> > >> extend mechanism — wouldn't that mislead users? It would seem more > >> > >> natural to declare GPU as a standard resource rather than going > >> through > >> > an > >> > >> extension. > >> > >> > >> > >> > >> > >> 4. Serial method invocation > >> > >> Each method of an RpcOperator is invoked serially. I'd suggest > stating > >> > >> this explicitly in both the documentation and the code comments, so > >> that > >> > >> users — or an AI — can be clearly aware of it. > >> > >> > >> > >> > >> > >> 5. Why synchronous methods? > >> > >> Why is every method in RpcOperator a synchronous call? Does this > imply > >> > >> that no model inference supports concurrent invocation? > >> > >> > >> > >> > >> > >> 6. SQL UDF usage > >> > >> How would SQL UDF users invoke this capability? I'd suggest > providing > >> a > >> > >> complete example for this as well. > >> > >> > >> > >> > >> > >> Thanks again for the great work! > >> > >> > >> > >> Best, > >> > >> Guowei > >> > >> > >> > >> > >> > >> On Wed, May 27, 2026 at 8:36 PM Charles Zhang < > [email protected] > >> > > >> > >> wrote: > >> > >> > >> > >>> Hi Yi, and Flink Community, > >> > >>> > >> > >>> Thanks for bringing up this excellent proposal. I am fully in > favor > >> of > >> > >>> FLIP-582. > >> > >>> > >> > >>> In our production workloads, especially regarding large-scale AI > >> > >>> inference, > >> > >>> the tight coupling of the data plane and compute units has always > >> been > >> > a > >> > >>> major pain point. If an inference subtask fails due to a GPU OOM > or > >> > driver > >> > >>> issue, triggering a global rollback is incredibly expensive since > >> model > >> > >>> reloading takes minutes. > >> > >>> > >> > >>> The introduction of the RpcOperator Service as a first-class > >> primitive > >> > >>> masterfully decouples the heavy inference tasks from the > mainstream > >> > >>> topology. The fault isolation, independent scaling, and stateless > >> > design > >> > >>> perfectly match the requirements of modern AI-oriented data > >> processing. > >> > >>> > >> > >>> This is a clean and robust architecture. Looking forward to seeing > >> this > >> > >>> merged! > >> > >>> > >> > >>> > >> > >>> Best wishes, > >> > >>> Charles Zhang > >> > >>> from Apache InLong > >> > >>> > >> > >>> > >> > >>> Yi Zhang <[email protected]> 于2026年5月27日周三 14:12写道： > >> > >>> > >> > >>> > Hi everyone, > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > I would like to start a discussion on FLIP-582: Support > RpcOperator > >> > >>> > Service [1]. > >> > >>> > > >> > >>> > > >> > >>> > AI-oriented workloads like multimodal data processing and model > >> > >>> inference > >> > >>> > are > >> > >>> > growing rapidly in recent years. These workloads are > characterized > >> by > >> > >>> > expensive > >> > >>> > resources (GPUs) and high initialization costs (seconds to > minutes > >> > for > >> > >>> > model > >> > >>> > loading). In today's Flink, embedding them in the data plane > >> couples > >> > >>> their > >> > >>> > parallelism and failover with surrounding operators; deploying > them > >> > as > >> > >>> > external > >> > >>> > services disconnects their lifecycle from the job and doubles > >> > >>> operational > >> > >>> > overhead. > >> > >>> > > >> > >>> > > >> > >>> > This FLIP introduces RpcOperator Service — a framework-level > >> > primitive > >> > >>> > that runs > >> > >>> > user-defined compute as RPC services in an independent Pipelined > >> > Region > >> > >>> > within > >> > >>> > the Flink job. Because the service is isolated at the scheduling > >> > level, > >> > >>> it > >> > >>> > can achieve > >> > >>> > fault isolation, independent scaling, and dedicated resource > >> > allocation. > >> > >>> > As a native > >> > >>> > Flink primitive, it also lays the foundation for automatic flow > >> > control, > >> > >>> > flexible load > >> > >>> > balancing, and coordinated auto-scaling — all without > introducing > >> > >>> external > >> > >>> > infrastructure or additional operational burden. > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > Looking forward to your feedback and suggestions! > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > [1] > >> > >>> > > >> > >>> > > >> > >>> > >> > > >> > https://cwiki.apache.org/confluence/display/FLINK/FLIP-582%3A+Support+RpcOperator+Service > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > > >> > >>> > Best Regards, > >> > >>> > Yi Zhang > >> > >>> > >> > >> > >> > > >> >
