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PIP 63: Readonly Topic Ownership Support

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Sijie Guo edited this page May 7, 2020 · 1 revision

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 for the network bandwidth on brokers. Different subscriptions might have different levels of severity.
  • When synchronizing cross-city message reading, cross-city access needs to be minimized.

This proposal is proposing adding read-only topic ownership support. If Pulsar supports read-only ownership, users can then use it to set up a (few) separated broker clusters for read-only, to segregate the consumption traffic by their service severity. And this would also allow Pulsar supporting a large number of subscriptions.

Changes

There are a few key changes for supporting read-only topic ownership.

  • how does readonly topic owner read data
  • how does readonly topic owner keep metadata in-sync
  • how does readonly topic owner handle acknowledgment

The first two problems have been well addressed in DistributedLog. We can just add similar features in the 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 supported in the bookkeeper v2 protocol.

Required Changes:

  • Refresh LastAddConfirmed when a managed cursor requests entries beyond known LAC.
  • Enable explicitLac at the 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 to 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 reading 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 acknowledgment

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 maintain 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 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 as the shared configuration storage, and use zookeeper.shared.example.com:2181/cluster-afor ClusterA’s local metadata storage, and use 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.
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