Blog of 2.4.0 release (apache#4677)

* 2.4.0 release blog.

* fix comments.

* fix comments.

* fix comments.

site2/website/blog/2019-07-05-Apache-Pulsar-2-4-0.md

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+---
+author: Penghui Li
+authorURL: https://twitter.com/lipenghui6
+title: Apache Pulsar 2.4.0
+---
+
+We are glad to publish Apache Pulsar 2.4.0. This is the result of a huge 
+effort from the community, with over 460 commits and a long list of new features, 
+general improvements and bug fixes.
+
+Check out the <b>[release notes](/release-notes/#2.4.0)</b> for a detailed list of 
+the changes, with links to the relevant pull requests, discussions and documentation.
+
+Regarding new features introduced, I just want to highlight here a tiny subset of them:
+
+<!--truncate-->
+
+### Delayed message delivery
+
+It's now possible to send a delayed message by Pulsar producer, and a delayed message will be
+available after a delay time.
+
+The Java code for a client using delayed messages delivery looks as follows:
+
+```java
+producer.newMessage().value("delayed message").deliverAfter(10, TimeUnit.SECONDS).send()
+```
+
+> Note:
+>
+> 1. Messages are only delayed on shared subscriptions, other subscriptions will deliver immediately.
+> 2. Delayed messages are sent individually even if you enable message batching on producer.
+
+### Go Functions
+
+Before 2.4.0 release, Java and Python are supported to write Pulsar Functions. Now, you can 
+use Go to write Pulsar Functions, the following is an example of 
+a Pulsar Function written in Go.
+
+```go
+import (
+    "fmt"
+    "context"
+
+    "github.com/apache/pulsar/pulsar-function-go/pf"
+)
+
+func HandleRequest(ctx context.Context, in []byte) error {
+    fmt.Println(string(in) + "!")
+    return nil
+}
+
+func main() {
+    pf.Start(HandleRequest)
+}
+```
+
+### Key_shared subscription
+
+A new subscription mode `Key_shared` is introduced in 2.4.0. In `Key_shared` subscription mode, 
+one partition could have several consumers to consume messages in parallelism and ensure messages 
+with the same key are distributed to a consumer in order. 
+Here is [architecture](http://pulsar.apache.org/docs/en/concepts-messaging/#key_shared) 
+for Key_Shared.
+
+The following is an example to use `Key_shared` subscription:
+
+```java
+client.newConsumer()
+        .topic("topic")
+        .subscriptionType(SubscriptionType.Key_Shared)
+        .subscriptionName("sub-1")
+        .subscribe();
+```
+
+### Schema versioning
+
+Before 2.4.0 release, Avro schema used one schema for both writer schema and reader schema. 
+Multiple schemas version is supported now.
+
+With multiple schemas, a producer can send messages with different schema versions and a consumer 
+can read messages with different schemas.
+
+In 2.4.0 release, `FORWARD_TRANSITIVE`, `BACKWARD_TRANSITIVE` and `FULL_TRANSITIVE` compatibility 
+strategies are added to check the compatibility with all existing schema version.
+
+### Replicated subscription
+
+In 2.4.0 release, a mechanism is added to keep subscription state in sync, within a sub-second timeframe, 
+in the context of a topic that is being asynchronously replicated across multiple geographical 
+regions. Here is [architecture](https://github.com/apache/pulsar/wiki/PIP-33%3A-Replicated-subscriptions) 
+for replicated subscription.
+
+The following is an example to use replicated subscription:
+
+```java
+Consumer<String> consumer = client.newConsumer(Schema.STRING)
+            .topic("my-topic")
+            .subscriptionName("my-subscription")
+            .replicateSubscriptionState(true)
+            .subscribe();
+```
+
+### New IO connectors
+
+A new batch of connectors is added, including Flume, Redis sink, Solr sink, RabbitMQ sink. 
+The following lists builtin [connectors](http://pulsar.apache.org/docs/en/io-connectors/) 
+that Pulsar supports.
+
+### Security
+
+In 2.4.0 release, Kerberos is supported in Apache Pulsar broker and client. 
+To enable Kerberos authentication, refer to the [document](http://pulsar.apache.org/docs/en/security-kerberos/).
+
+Also added role based Pulsar Function authentication and authorization.
+
+## Conclusion
+
+If you want to download Pulsar 2.4.0, click [here](/download). You can send any questions or suggestions 
+to our mailing lists, contribute to Pulsar on [GitHub](https://github.com/apache/pulsar) or join 
+the Apache Pulsar community on [Slack](https://apache-pulsar.herokuapp.com/).