Hi folks, Thanks for the discussion. I'd like to share my two cents on this topic.
Firstly, I'd like to clarify my understanding of the concepts "native k8s integration" and "active resource management". - Native k8s integration means Flink's master interacts with k8s' api server directly. It acts like embedding an operator inside Flink's master, which manages the resources (pod, deployment, configmap, etc.) and watches / reacts to related events. - Active resource management means Flink can actively start / terminate workers as needed. Its key characteristic is that the resource a Flink deployment uses is decided by the job's execution plan, unlike the opposite reactive mode (resource available to the deployment decides the execution plan) or the standalone mode (both execution plan and deployment resources are predefined). Currently, we have the yarn and native k8s deployments (and the recently removed mesos deployment) in active mode, due to their ability to request / release worker resources from the underlying cluster. And all the existing operators, AFAIK, work with a Flink standalone deployment, where Flink cannot request / release resources by itself. >From this perspective, I think a large part of the native k8s integration advantages come from the active mode: being able to better understand the job's resource requirements and adjust the deployment resource accordingly. Both fine-grained resource management (customizing TM resources for different tasks / operators) and adaptive batch scheduler (rescale the deployment w.r.t. different stages) fall into this category. I'm wondering if we can have an operator that also works with the active mode. Instead of talking to the api server directly for adding / deleting resources, Flink's active resource manager can talk to the operator (via CR) about the resources the deployment needs, and let the operator to actually add / remove the resources. The operator should be able to work with (active) or without (standalone) the information of deployment's resource requirements. In this way, users are free to choose between active and reactive (e.g., HPA) rescaling, while always benefiting from the beyond-deployment lifecycle (upgrades, savepoint management, etc.) and alignment with the K8s ecosystem (Flink client free, operating via kubectl, etc.). Thank you~ Xintong Song On Thu, Jan 6, 2022 at 1:06 PM Thomas Weise <t...@apache.org> wrote: > Hi David, > > Thank you for the reply and context! > > As for workload types and where native integration might fit: I think > that any k8s native solution that satisfies category 3) can also take > care of 1) and 2) while the native integration by itself can't achieve > that. Existence of [1] might serve as further indication. > > The k8s operator pattern would be an essential building block for a > k8s native solution that is interoperable with k8s ecosystem tooling > like kubectl, which is why [2] and subsequent derived art were > created. Specifically the CRD allows us to directly express the > concept of a Flink application consisting of job manager and task > manager pods along with associated create/update/delete operations. > > Would it make sense to gauge interest to have such an operator as part > of Flink? It appears so from discussions like [3]. I think such > addition would significantly lower the barrier to adoption, since like > you mentioned one cannot really run mission critical streaming > workloads with just the Apache Flink release binaries alone. While it > is great to have multiple k8s operators to choose from that are > managed outside Flink, it is unfortunately also evident that today's > hot operator turns into tomorrow's tech debt. I think such fate would > be less likely within the project, when multiple parties can join > forces and benefit from each other's contributions. There were similar > considerations and discussions around Docker images in the past. > > Out of the features that you listed it is particularly the application > upgrade that needs to be solved through an external process like > operator. The good thing is that many folks have already thought hard > about this and in existing implementations we see different strategies > that have their merit and production mileage (certainly applies to > [2]). We could combine the best of these ideas into a unified > implementation as part of Flink itself as starting point. > > Cheers, > Thomas > > > [1] https://github.com/wangyang0918/flink-native-k8s-operator > [2] https://github.com/lyft/flinkk8soperator > [3] https://lists.apache.org/thread/fhcr5gj1txcr0fo4s821jkp6d4tk6080 > > > On Tue, Jan 4, 2022 at 4:04 AM David Morávek <d...@apache.org> wrote: > > > > Hi Thomas, > > > > AFAIK there are no specific plans in this direction with the native > integration, but I'd like to share some thoughts on the topic > > > > In my understanding there are three major groups of workloads in Flink: > > > > 1) Batch workloads > > 2) Interactive workloads (Both Batch and Streaming; eg. SQL Gateway / > Zeppelin Notebooks / VVP ...) > > 3) "Mission Critical" Streaming workloads > > > > I think the native integration fits really well in the first two > categories. Let's talk about these first: > > > > 1) Batch workloads > > > > You don't really need to address the upgrade story here. The interesting > topic is how to "dynamically" adjust parallelism as the workload can change > between stages. This is where the Adaptive Batch Scheduler [1] comes into > play. To leverage the scheduler to the full extend, it needs to be deployed > with the remote shuffle service in place [2], so the Flink's Resource > Manager can free TaskManagers that are no longer needed. > > > > This can IMO work really well with the native integration as there is > really clear approach on how the Resource Manager should decide on what > resources should be allocated. > > > > 2) Interactive workloads > > > > Again, the upgrade story is not really interesting in this scenario. For > batch workloads, it's basically the same as the above. For streaming one > this gets tricky. The main initiative that we current have in terms of auto > scaling / re-scaling of the streaming workloads is the reactive mode > (adaptive scheduler) [3]. > > > > I can totally see how the reactive mode could be integrated in the > native integration, but with the application mode, which is not really > suitable for the interactive workloads. For integration with session > cluster, we'd first need to address the "scheduling" problem of how to > distribute newly available resources between multiple jobs. > > > > What's pretty neat here is that the interpreter (zeppelin, sql gw, ...) > have a really convenient way of spinning up a new cluster inside k8s. > > > > 3) "Mission Critical" Streaming workloads > > > > This one is IMO the primary reason why one would consider building a new > operator these days as this needs a careful lifecycle management of the > pipeline. I assume this is also the use case that you're investigating, am > I correct? > > > > I'd second the requirements that you've already stated: > > a) Resource efficiency - being able to re-scale based on the workload, > in order to keep up with the input / not waste resources > > b) Fast recovery > > c) Application upgrades > > > > I personally don't think that the native integration is really suitable > here. The direction that we're headed is with the standalone deployment on > Kubernetes + the reactive mode (adaptive scheduler). > > > > In theory, if we want to build a really cloud (Kubernetes) native stream > processor, deploying the pipeline should be as simple as deploying any > other application. It should be also simple to integrate with CI & CD > environment and the fast / frequent deploy philosophy. > > > > Let's see where we stand and where we can expand from there: > > > > a) Resource efficiency > > > > We already have the reactive mode in place. This allows you to add / > remove task managers by adjusting the TM deployment (`kubectl scale ...`) > and Flink will automatically react to the available resources. This is > currently only supported with the Application Mode, that is limited to a > single job (which should be enough for this kind of workload). > > > > The re-scaling logic is left completely up to the user and can be as > simple as setting up a HPA (Horizontal Pod Autoscaler). I tend to think in > the direction, that we might want to provide a custom k8s metrics server, > that allows HPA to query the metrics from JM, to make this more flexible > and easy to use. > > > > As this looks really great in theory, there are still some shortcomings > that we're actively working on addressing. For this feature to be really > widely adopted, we need to make the re-scaling experience as fast as > possible, so we can re-scale often to react to the input rate. This could > be currently a problem with large RocksDB states as this involves full > re-balance of the state (splitting / merging RocksDB instances). The k8s > operator approach has the same / even worse limitation as it involves > taking a savepoint a re-building the state from it. > > > > b) Fast recovery > > > > This is IMO not as different from the native mode (although I'd have to > check whether RM failover can reuse task managers). This involves frequent > and fast checkpointing, local recovery (which is still not supported in > reactive mode, but this will be hopefully addressed soon) and working > directory efforts [4]. > > > > c) Application upgrades > > > > This is the topic I'm still struggling with a little. Historically this > involves external lifecycle management (savepoint + submitting a new job). > I think at the end of the day, with application mode on standalone k8s, it > could be as simple as updating the docker image of the JM deployment. > > > > If I think about the simplest upgrade scenario, simple in-place restore > from the latest checkpoint, it may be fairly simple to implement. What I'm > struggling with are the more complex upgrade scenarios such as dual, blue / > green deployment. > > > > > > To sum this up, I'd really love if Flink could provide great out-of the > box experience with standalone mode on k8s, that makes the experience as > close to running / operating any other application as possible. > > > > I'd really appreciate to hear your thoughts on this topic. > > > > [1] > https://cwiki.apache.org/confluence/display/FLINK/FLIP-187%3A+Adaptive+Batch+Job+Scheduler > > [2] https://github.com/flink-extended/flink-remote-shuffle > > [3] > https://nightlies.apache.org/flink/flink-docs-release-1.14/docs/deployment/elastic_scaling/ > > [4] > https://cwiki.apache.org/confluence/display/FLINK/FLIP-198%3A+Working+directory+for+Flink+processes > > > > Best, > > D. > > > > On Tue, Jan 4, 2022 at 12:44 AM Thomas Weise <t...@apache.org> wrote: > >> > >> Hi, > >> > >> I was recently looking at the Flink native Kubernetes integration [1] > >> to get an idea how it relates to existing operator based solutions > >> [2], [3]. > >> > >> Part of the native integration's motivations was simplicity (no extra > >> component to install), but arguably that is also a shortcoming. The > >> k8s operator model can offer support for application lifecycle like > >> upgrade and rescaling, as well as job submission without a Flink > >> client. > >> > >> When using the Flink native integration it would still be necessary to > >> provide that controller functionality. Is the idea to use the native > >> integration for task manager resource allocation in tandem with an > >> operator that provides the external controller functionality? If > >> anyone using the Flink native integration can share experience, I > >> would be curious to learn more about the specific setup and if there > >> are plans to expand the k8s native integration capabilities. > >> > >> For example: > >> > >> * Application upgrade with features such as [4]. Since the job manager > >> is part of the deployment it cannot orchestrate the deployment. It > >> needs to be the responsibility of an external process. Has anyone > >> contemplated adding such a component to Flink itself? > >> > >> * Rescaling: Theoretically a parallelism change could be performed w/o > >> restart of the job manager pod. Hence, building blocks to trigger and > >> apply rescaling could be part of Flink itself. Has this been explored > >> further? > >> > >> Yang kindly pointed me to [5]. Is the recommendation/conclusion that > >> when a k8s operator is already used, then let it be in charge of the > >> task manager resource allocation? If so, what scenario was the native > >> k8s integration originally intended for? > >> > >> Thanks, > >> Thomas > >> > >> [1] > https://nightlies.apache.org/flink/flink-docs-release-1.13/docs/deployment/resource-providers/native_kubernetes/#deployment-modes > >> [2] https://github.com/lyft/flinkk8soperator > >> [3] https://github.com/spotify/flink-on-k8s-operator > >> [4] > https://github.com/lyft/flinkk8soperator/blob/master/docs/state_machine.md > >> [5] https://lists.apache.org/thread/8cn99f6n8nhr07n5vqfo880tpm624s5d >
