Pulsar geo-replication


Geo-replication is the replication of persistently stored message data across multiple clusters of a Pulsar instance.

How it works

The diagram below illustrates the process of geo-replication across Pulsar clusters:

Replication Diagram

In this diagram, whenever producers P1, P2, and P3 publish messages to the topic T1 on clusters Cluster-A, Cluster-B, and Cluster-C, respectively, those messages are instantly replicated across clusters. Once replicated, consumers C1 and C2 can consume those messages from their respective clusters.

Without geo-replication, consumers C1 and C2 wouldn’t be able to consume messages published by producer P3.

Geo-replication and Pulsar properties

Geo-replication must be enabled on a per-property basis in Pulsar. Geo-replication can be enabled between clusters only when a property has been created that allows access to both clusters.

Although geo-replication must be enabled between two clusters, it’s actually managed at the namespace level. You must do the following to enable geo-replication for a namespace:

Any message published on any topic in that namespace will then be replicated to all clusters in the specified set.

Local persistence and forwarding

When messages are produced on a Pulsar topic, they are first persisted in the local cluster and then forwarded asynchronously to the remote clusters.

In normal cases, when there are no connectivity issues, messages are replicated immediately, at the same time as they are dispatched to local consumers. Typically, end-to-end delivery latency is defined by the network round-trip time (RTT) between the remote regions.

Applications can create producers and consumers in any of the clusters, even when the remote clusters are not reachable (like during a network partition).

Subscriptions are local to a cluster

While producers and consumers can publish to and consume from any cluster in a Pulsar instance, subscriptions are local to the clusters in which they are created and cannot be transferred between clusters. If you do need to transfer a subscription, you’ll need to create a new subscription in the desired cluster.

In the example in the image above, the topic T1 is being replicated between 3 clusters, Cluster-A, Cluster-B, and Cluster-C.

All messages produced in any cluster will be delivered to all subscriptions in all the other clusters. In this case, consumers C1 and C2 will receive all messages published by producers P1, P2, and P3. Ordering is still guaranteed on a per-producer basis.

Configuring replication

As stated above, geo-replication in Pulsar is managed at the property level.

Granting permissions to properties

To establish replication to a cluster, the tenant needs permission to use that cluster. This permission can be granted when the property is created or later on.

At creation time, specify all the intended clusters:

$ bin/pulsar-admin properties create my-property \
  --admin-roles my-admin-role \
  --allowed-clusters us-west,us-east,us-cent

To update permissions of an existing property, use update instead of create.

Creating global namespaces

Replication must be used with global topics, meaning topics that belong to a global namespace and are thus not tied to any particular cluster.

Global namespaces need to be created in the global virtual cluster. For example:

$ bin/pulsar-admin namespaces create my-property/global/my-namespace

Initially, the namespace is not assigned to any cluster. You can assign the namespace to clusters using the set-clusters subcommand:

$ bin/pulsar-admin namespaces set-clusters my-property/global/my-namespace \
  --clusters us-west,us-east,us-cent

The replication clusters for a namespace can be changed at any time, with no disruption to ongoing traffic. Replication channels will be immediately set up or stopped in all the clusters as soon as the configuration changes.

Using global topics

Once you’ve created a global namespace, any topics that producers or consumers create within that namespace will be global. Typically, each application will use the serviceUrl for the local cluster.

Selective replication

By default, messages are replicated to all clusters configured for the namespace. You can restrict replication selectively by specifying a replication list for a message. That message will then be replicated only to the subset in the replication list.

Below is an example for the Java API. Note the use of the setReplicationClusters method when constructing the Message object:

List<String> restrictReplicationTo = Arrays.asList(
        "us-west",
        "us-east"
);

Producer producer = client.newProducer()
        .topic("some-topic")
        .create();

producer.newMessage()
        .value("my-payload".getBytes())
        .setReplicationClusters(restrictReplicationTo)
        .send();

Topic stats

Topic-specific statistics for global topics are available via the pulsar-admin tool and REST API:

$ bin/pulsar-admin persistent stats persistent://my-property/global/my-namespace/my-topic

Each cluster reports its own local stats, including incoming and outgoing replication rates and backlogs.

Deleting a global topic

Given that global topics exist in multiple regions, it’s not possible to directly delete a global topic. Instead, you should rely on automatic topic garbage collection.

In Pulsar, a topic is automatically deleted when it’s no longer used, that is to say, when no producers or consumers are connected and there are no subscriptions and no more messages are kept for retention. For global topics, each region will use a fault-tolerant mechanism to decide when it’s safe to delete the topic locally.

You can explicitly disable topic garbage collection by setting brokerDeleteInactiveTopicsEnabled to false in your broker configuration.

To delete a global topic, close all producers and consumers on the topic and delete all its local subscriptions in every replication cluster. When Pulsar determines that no valid subscription for the topic remains across the system, it will garbage collect the topic.