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# Pulsar Geo-Replication | ||
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<!-- TOC depthFrom:2 depthTo:3 withLinks:1 updateOnSave:1 orderedList:0 --> | ||
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- [Replication model](#replication-model) | ||
- [Configuring replication](#configuring-replication) | ||
- [Granting permissions to properties](#granting-permissions-to-properties) | ||
- [Creating the namespace](#creating-the-namespace) | ||
- [Using global topics](#using-global-topics) | ||
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<!-- /TOC --> | ||
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## Replication model | ||
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Pulsar properties provisioned on more than one cluster can enable replication | ||
among those clusters. Replication is managed directly by the users, at the | ||
namespace level. Any topic in a global namespace is replicated. A user creates a | ||
global namespace, and configures that namespace to replicate between two or more | ||
of the provisioned clusters. Any message published on any topic in that namespace | ||
will then be replicated to all clusters in that set. | ||
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Messages are first persisted in the local cluster, and then forwarded | ||
asynchronously to the remote clusters. | ||
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In normal case, with no connectivity problems, 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 *RTT* between the remote regions. | ||
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Applications can create producers and consumers in any of the clusters, | ||
even when the remote clusters are not reachable (eg: during a network | ||
partition). | ||
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Subscriptions are local to the clusters in which they are created and they | ||
cannot be transferred between clusters. | ||
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![Replication Diagram](img/GeoReplication.png) | ||
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In the above example, the topic is being replicated between 3 clusters, | ||
***Cluster-A***, ***Cluster-B*** and ***Cluster-C***. | ||
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All messages produced in any cluster will be delivered to all subscriptions | ||
in all the clusters. In this case, both ***C1*** and ***C2*** will receive all | ||
messages published by ***P1***, ***P2*** and ***P3***. Ordering is still | ||
guaranteed per-producer. | ||
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## Configuring replication | ||
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### Granting permissions to properties | ||
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To replicate to a cluster, the tenant needs permission to use that cluster. | ||
This permission can be granted when the property is created, or it can be | ||
updated later. | ||
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At creation time, specify all the intended clusters: | ||
```shell | ||
$ bin/pulsar-admin properties create my-property \ | ||
--admin-roles my-admin-role \ | ||
--allowed-clusters us-west,us-east,us-cent | ||
``` | ||
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To update permissions of an existing property, use `update` instead of `create`. | ||
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### Creating global namespaces | ||
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To use replication, we need a *global* topic, that is to say, a topic | ||
that belongs to a *global* namespace, not tied to any particular cluster. | ||
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Global namespaces need to be created in the ***global*** virtual cluster. | ||
For example: | ||
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```shell | ||
$ bin/pulsar admin namespaces create my-property/global/my-namespace | ||
``` | ||
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Initially, the namespace is not assigned to any cluster. | ||
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```shell | ||
$ bin/pulsar admin namespaces set-clusters | ||
my-property/global/my-namespace \ | ||
--clusters us-west,us-east,us-cent | ||
``` | ||
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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. | ||
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### Using global topics | ||
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At this point you can just create a producer or consumers and automatically | ||
create a global topic. | ||
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Typically, each application will use the `serviceUrl` for the local cluster. | ||
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#### Selective replication | ||
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By default, messages are replicated to all configured clusters for the namespace. | ||
You can restrict replication selectively, by specifying a replication list for a | ||
message. The message will then be replicated only to the subset in the | ||
replication list. | ||
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In the Java API, this is done on the message builder: | ||
```java | ||
... | ||
List<String> restrictReplicationTo = new ArrayList<>(); | ||
restrictReplicationTo.add("us-west"); | ||
restrictReplicationTo.add("us-east"); | ||
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Message message = MessageBuilder.create() | ||
.setContent("my-payload".getBytes()) | ||
.setReplicationClusters(restrictReplicationTo) | ||
.build(); | ||
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producer.send(message); | ||
``` | ||
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#### Topic stats | ||
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Topic stats for global topics are available through the same API and tools: | ||
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```shell | ||
$ bin/pulsar-admin persistent stats persistent://my-property/global/my-namespace/my-topic | ||
``` | ||
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Each cluster will report its own local stats, including incoming and outgoing | ||
replication rates and backlogs. | ||
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#### Deleting a global topic | ||
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Given that the topic exist in multiple regions, it's not possible to directly | ||
delete a global topic. The procedure is to rely on the automatic topic garbage | ||
collection. | ||
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In Pulsar, a topic is automatically deleted when it's not used anymore, that is | ||
to say, when no producers or consumers are connected ***and*** there are no | ||
subscriptions. | ||
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For global topics, each region will use a fault-tolerant mechanism to decide | ||
when it's safe to delete the topic locally. | ||
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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. |
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