Very useful feature. I would like the proposers to think just beyond scaling consumers. If done right, this has the potential to open up a lot of use cases in ML, where you need to reprocess old/archived data. Being able to spin up a read-only broker, ( dedicated brokers that read from tiered storage, without interfering with current flow of the stream) is extremely valuable. With small tweaks to this PIP, about data access boundaries, and without lot of additional complexity to this proposal, that can be achieved
On Tue, May 12, 2020 at 5:37 AM Jia Zhai <zhaiji...@gmail.com> wrote: > 👍 > > Best Regards. > > > Jia Zhai > > Beijing, China > > Mobile: +86 15810491983 > > > > > On Fri, May 8, 2020 at 4:29 AM Sijie Guo <guosi...@gmail.com> wrote: > > > Dezhi, thank you for sharing the proposal! > > > > It is great to see Tencent started contributing this great feature back > to > > Pulsar! This feature will unlock a lot of new capabilities of Pulsar. > > > > I have moved the proposal to > > > > > https://github.com/apache/pulsar/wiki/PIP-63:-Readonly-Topic-Ownership-Support > > > > - Sijie > > > > > > On Thu, May 7, 2020 at 5:23 AM dezhi liu <liudezhi2...@gmail.com> wrote: > > > > > Hi all, > > > Here is a suggest (PIP) ReadOnly Topic Ownership Support > > > ------------ > > > # PIP-63: ReadOnly Topic Ownership Support > > > > > > * Author: Penghui LI, Jia Zhai, Sijie Guo, Dezhi Liu > > > > > > ## Motivation > > > People usually use Pulsar as an event-bus or event center to unify all > > > their message data or event data. > > > One same set of event data will usually be shared across multiple > > > applications. Problems occur when the number of subscriptions of same > > topic > > > increased. > > > > > > - The bandwidth of a broker limits the number of subscriptions for a > > single > > > topic. > > > - Subscriptions are competing the network bandwidth on brokers. > Different > > > subscription might have different level of severity. > > > - When synchronizing cross-city message reading, cross-city access > needs > > to > > > be minimized. > > > > > > This proposal is proposing adding readonly topic ownership support. If > > > Pulsar supports readonly ownership, users can then use it to setup a > > (few) > > > separated broker clusters for readonly, to segregate the consumption > > > traffic by their service severity. And this would also allow Pulsar > > > supporting large number of subscriptions. > > > > > > ## Changes > > > There are a few key changes for supporting readonly topic ownership. > > > > > > - how does readonly topic owner read data > > > - how does readonly topic owner keep metadata in-sync > > > - how does readonly topic owner handle acknowledges > > > > > > The first two problems have been well addressed in DistributedLog. We > can > > > just add similar features in managed ledger. > > > > > > ### How readonly topic owner read data > > > > > > In order for a readonly topic owner keep reading data in a streaming > way, > > > the managed ledger should be able to refresh its LAC. The easiest > change > > > is to call `readLastAddConfirmedAsync` when a cursor requests entries > > > beyond existing LAC. A more advanced approach is to switch the regular > > read > > > entries request to bookkeeper’s long poll read requests. However long > > poll > > > read requests are not support in the bookkeeper v2 protocol. > > > > > > Required Changes: > > > > > > - Refresh LastAddConfirmed when a managed cursor requests entries > beyond > > > known LAC. > > > - Enable `explicitLac` at managed ledger. So the topic writable owner > > will > > > periodically advance LAC, which will make sure readonly owner will be > > able > > > to catch with the latest data. > > > > > > ### How readonly topic owner keep metadata in-sync > > > > > > Ledgers are rolled at a given interval. Readonly topic owner should > find > > a > > > way to know the ledgers has been rolled. There are a couple of options. > > > These options are categorized into two approaches : notification vs > > > polling. > > > > > > *Notification* > > > > > > A) use zookeeper watcher. Readonly topic owner will set a watcher at > the > > > managed ledger’s metadata. So it will be notified when a ledger is > > rolled. > > > B) similar as A), introduce a “notification” request between readonly > > topic > > > owner and writable topic owner. Writable topic owner notifies readonly > > > topic owner with metadata changes. > > > > > > *Polling* > > > > > > C) Readonly Broker polling zookeeper to see if there is new metadata, > > > *only* when LAC in the last ledger has not been advanced for a given > > > interval. Readonly Broker checks zookeeper to see if there is a new > > ledger > > > rolled. > > > D）Readonly Broker polling new metadata by read events from system topic > > of > > > write broker cluster, write broker add the ledger meta change events to > > the > > > system topic when mledger metadata update. > > > > > > Solution C) will be the simplest solution to start with > > > > > > ### How does readonly topic owner handle acknowledges > > > > > > Currently Pulsar deploys a centralized solution for managing cursors > and > > > use cursors for managing data retention. This PIP will not change this > > > solution. Instead, readonly topic owner will only maintains a cursor > > cache, > > > all the actual cursor updates will be sent back to the writable topic > > > owner. > > > > > > This requires introducing a set of “cursor” related RPCs between > writable > > > topic owner and readonly topic owners. > > > > > > - Read `Cursor` of a Subscription > > > > > > So readonly topic owner will handle following requests using these new > > > cursor RPCs > > > > > > - Subscribe : forward the subscribe request to writable topic owner. > Upon > > > successfully subscribe, readonly topic owner caches the corresponding > > > cursor. > > > - Unsubscribe: remove cursor from cursor cache, and forward the > > unsubscribe > > > request to writable topic owner. > > > - Consume: when a consumer is connected, it will then `read` the cursor > > > from writable topic owner and cache it locally. > > > - Ack: forward the ack request to the writable topic owner, and update > > the > > > cursor locally in the cache. > > > > > > ## Compatibility, Deprecation and Migration Plan > > > Since most of the changes are internally changes to managed ledger, and > > it > > > is a new feature which doesn’t change pulsar’s wire protocol and public > > > api. There is no backward compatibility issue. > > > > > > It is a newly added feature. So there is nothing to deprecate or > migrate. > > > > > > ## Test Plan > > > - Unit tests for each individual change > > > - Integration tests for end-to-end pipeline > > > - Chaos testing to ensure correctness > > > - Load testing for ensuring performance > > > > > > ## Rejected Alternatives > > > ### Use Geo Replication to replicate data between clusters > > > > > > A simplest alternative solution would be using Pulsar’s built-in > > > geo-replication mechanism to replicate data from one cluster to the > other > > > cluster. > > > > > > #### Two completely separated clusters > > > > > > The idea is pretty straightforward - You created two separated > clusters, > > > one cluster is for your online services - `Cluster-A`, while the other > > > cluster is for your analytical workloads - `Cluster-B`. `ClusterA` is > > used > > > for serving both write (produce) and read (consume) traffic, while > > > `ClusterB` is used for serving readonly (consume) traffic. Both > > `Cluster-A` > > > and `Cluster-B` have their own zookeeper cluster, bookkeeper cluster, > and > > > brokers. In order to make sure a topic’s data can be replicated between > > > `Cluster-A` and `Cluster-B`, we need do make sure `Cluster-A` and > > > `Cluster-B` sharing same configuration storage. There are two > approaches > > to > > > do so: > > > > > > a) a completely separated zookeeper cluster as configuration storage. > > > > > > In this approach, everything is completely separated. So you can treat > > > these two clusters just as two different regions, and follow the > > > instructions in [Pulsar geo-replication · Apache Pulsar]( > > > http://pulsar.apache.org/docs/en/administration-geo/) to setup data > > > replication between these two clusters. > > > > > > b) `ClusterB` and `ClusterA` share same configuration storage. > > > > > > The approach in a) requires setting up a separate zookeeper cluster as > > > configuration storage. But since `ClusterA` and `ClusterB` already have > > > their own zookeeper clusters, you don’t want to setup another zookeeper > > > cluster. You can let both `ClusterA` and `ClusterB` use `ClusterA`’s > > > zookeeper cluster as the configuration store. You can achieve it using > > > zookeeper’s chroot mechanism to put configuration data in a separate > root > > > in `ClusterA`’s zookeeper cluster. > > > > > > For example: > > > > > > - Command to initialize `ClusterA`’s metadata > > > > > > ``` > > > $ bin/pulsar initialize-cluster-metadata \ > > > --cluster ClusterA \ > > > --zookeeper zookeeper.cluster-a.example.com:2181 \ > > > --configuration-store > > > zookeeper.cluster-a.example.com:2181/configuration-store \ > > > --web-service-url http://broker.cluster-a.example.com:8080/ \ > > > --broker-service-url pulsar://broker.cluster-a.example.com:6650/ > > > ``` > > > > > > - Command to initialize `ClusterB`’s metadata > > > ``` > > > $ bin/pulsar initialize-cluster-metadata \ > > > --cluster ClusterB \ > > > --zookeeper zookeeper.cluster-b.example.com:2181 \ > > > --configuration-store > > > zookeeper.cluster-a.example.com:2181/configuration-store \ > > > --web-service-url http://broker.cluster-b.example.com:8080/ \ > > > --broker-service-url pulsar://broker.cluster-b.example.com:6650/ > > > ``` > > > > > > #### Shared bookkeeper and zookeeper cluster, but separated brokers > > > > > > Sometimes it is unaffordable to have two completely separated clusters. > > You > > > might want to share the existing infrastructures, such as data storage > > > (bookkeeper) and metadata storage (zookeeper). Similar as the b) > solution > > > described above, you can use zookeeper chroot to achieve that. > > > > > > Let’s assume there is only one zookeeper cluster and one bookkeeper > > > cluster. The zookeeper cluster is `zookeeper.shared.example.com:2181`. > > > You have two clusters of brokers, one cluster of broker is ` > > > broker-a.example.com`, and the other broker cluster is ` > > > broker-b.example.com`. > > > So when you create the clusters, you can use ` > > > zookeeper.shared.example.com:2181/configuration-store` > <http://zookeeper.shared.example.com:2181/configuration-store> > > <http://zookeeper.shared.example.com:2181/configuration-store> > > > <http://zookeeper.shared.example.com:2181/configuration-store> as the > > > shared > > > configuration storage, and use ` > > > zookeeper.shared.example.com:2181/cluster-a`for > <http://zookeeper.shared.example.com:2181/cluster-afor> > > <http://zookeeper.shared.example.com:2181/cluster-afor> > > > <http://zookeeper.shared.example.com:2181/cluster-afor> `ClusterA`’s > > > local metadata > > > storage, and use `zookeeper.shared.example.com:2181/cluster-b` > <http://zookeeper.shared.example.com:2181/cluster-b> > > <http://zookeeper.shared.example.com:2181/cluster-b> > > > <http://zookeeper.shared.example.com:2181/cluster-b> for > > > `ClusterB`’s local metadata storage. > > > > > > This would allows you have two “broker-separated” clusters sharing same > > > storage cluster (both zookeeper and bookkeeper). > > > > > > No matter how the physical clusters are setup, there is a downside of > > using > > > geo-replications for isolating the online workloads and analytics > > workloads > > > - data has to be replicated at least twice, if you have configured > pulsar > > > topics to store data in 3 replicas, you will end up have at least 6 > > copies > > > of data. So “geo-replication” might not be ideal for addressing this > use > > > case. > > > > > > ------------ > > > > > > > > > Thanks, > > > > > > Dezhi > > > > > >
