[#7] Added Geo-Replication documentation (#10)

docs/Documentation.md

@@ -8,5 +8,6 @@
 * Features
   - [Authentication](Authentication.md)
   - [Authorization](Authorization.md)
+  - [Geo-Replication](GeoReplication.md)
   - [WebSocket API](WebSocket.md)
   - [Apache Storm adaptor](PulsarStorm.md)

docs/GeoReplication.md

+
+# Pulsar Geo-Replication
+
+<!-- TOC depthFrom:2 depthTo:3 withLinks:1 updateOnSave:1 orderedList:0 -->
+
+- [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)
+
+<!-- /TOC -->
+
+## Replication model
+
+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.
+
+Messages are first persisted in the local cluster, and then forwarded
+asynchronously to the remote clusters.
+
+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.
+
+Applications can create producers and consumers in any of the clusters,
+even when the remote clusters are not reachable (eg: during a network
+partition).
+
+Subscriptions are local to the clusters in which they are created and they
+cannot be transferred between clusters.
+
+![Replication Diagram](img/GeoReplication.png)
+
+In the above example, the topic 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 clusters. In this case, both ***C1*** and ***C2*** will receive all
+messages published by ***P1***, ***P2*** and ***P3***. Ordering is still
+guaranteed per-producer.
+
+## Configuring replication
+
+### Granting permissions to properties
+
+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.
+
+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
+```
+
+To update permissions of an existing property, use `update` instead of `create`.
+
+### Creating global namespaces
+
+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.
+
+Global namespaces need to be created in the ***global*** virtual cluster.
+For example:
+
+```shell
+$ bin/pulsar admin namespaces create my-property/global/my-namespace
+```
+
+Initially, the namespace is not assigned to any cluster.
+
+```shell
+$ 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
+
+At this point you can just create a producer or consumers and automatically
+create a global topic.
+
+Typically, each application will use the `serviceUrl` for the local cluster.
+
+#### Selective replication
+
+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.
+
+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");
+
+Message message = MessageBuilder.create()
+              .setContent("my-payload".getBytes())
+              .setReplicationClusters(restrictReplicationTo)
+              .build();
+
+producer.send(message);
+```
+
+#### Topic stats
+
+Topic stats for global topics are available through the same API and tools:
+
+```shell
+$ bin/pulsar-admin persistent stats persistent://my-property/global/my-namespace/my-topic
+```
+
+Each cluster will report its own local stats, including incoming and outgoing
+replication rates and backlogs.
+
+#### Deleting a global topic
+
+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.
+
+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.
+
+For global topics, each region will use a fault-tolerant mechanism to decide
+when it's safe to delete the topic locally.
+
+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.