---
layout: page
title: Connecting to Eventhubs
---
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You can configure your Samza jobs to process data from [Azure Eventhubs](https://docs.microsoft.com/en-us/azure/event-hubs/event-hubs-features), Microsoft's data streaming service. An `event hub` is similar to a Kafka topic and can have multiple partitions with producers and consumers. Each message produced or consumed from an event hub is an instance of [EventData](https://docs.microsoft.com/en-us/java/api/com.microsoft.azure.eventhubs._event_data). 

### Consuming from EventHubs:

Samza's [EventHubSystemConsumer](https://github.com/apache/samza/blob/master/samza-azure/src/main/java/org/apache/samza/system/eventhub/consumer/EventHubSystemConsumer.java) wraps the EventData into an [EventHubIncomingMessageEnvelope](https://github.com/apache/samza/blob/master/samza-azure/src/main/java/org/apache/samza/system/eventhub/consumer/EventHubIncomingMessageEnvelope.java). The key of the message is set to the partition key of the EventData. The message is obtained from the EventData body. 

To configure Samza to configure from EventHub streams: 

```
# define an event hub system factory with your identifier. eg: eh-system
systems.eh-system.samza.factory=org.apache.samza.system.eventhub.EventHubSystemFactory

# define your streams
systems.eh-system.stream.list=input0, output0

# define required properties for your streams
systems.eh-system.streams.input0.eventhubs.namespace=YOUR-STREAM-NAMESPACE
systems.eh-system.streams.input0.eventhubs.entitypath=YOUR-ENTITY-NAME
systems.eh-system.streams.input0.eventhubs.sas.keyname=YOUR-SAS-KEY-NAME
systems.eh-system.streams.input0.eventhubs.sas.token=YOUR-SAS-KEY-TOKEN

systems.eh-system.streams.output0.eventhubs.namespace=YOUR-STREAM-NAMESPACE
systems.eh-system.streams.output0.eventhubs.entitypath=YOUR-ENTITY-NAME
systems.eh-system.streams.output0.eventhubs.sas.keyname=YOUR-SAS-KEY-NAME
systems.eh-system.streams.output0.eventhubs.sas.token=YOUR-SAS-KEY-TOKEN
```

The tuple required to access the Eventhubs entity per stream must be provided, namely the fields `YOUR-STREAM-NAMESPACE`, `YOUR-ENTITY-NAME`, `YOUR-SAS-KEY-NAME`, `YOUR-SAS-KEY-TOKEN`.

### Producing to EventHubs:

Similarly, you can also configure your Samza job to write to EventHubs.  
```
OutgoingMessageEnvelope envelope = new OutgoingMessageEnvelope(new SystemStream("eh-system", "output0"), key, message);
collector.send(envelope);
```

Each [OutgoingMessageEnvelope](https://samza.apache.org/learn/documentation/latest/api/javadocs/org/apache/samza/system/OutgoingMessageEnvelope.html) is converted into an [EventData](https://docs.microsoft.com/en-us/java/api/com.microsoft.azure.eventhubs._event_data) instance whose body is set to the `message` in the envelope. Additionally, the `key` and the `produce timestamp` are set as properties in the EventData before sending it to EventHubs.

#### Size limit of partition key:

Note that EventHubs has a limit on the length of partition key (128 characters). In [EventHubSystemProducer](https://github.com/apache/samza/blob/master/samza-azure/src/main/java/org/apache/samza/system/eventhub/producer/EventHubSystemProducer.java) we truncate the partition key if the size of the key exceeds the limit.

### Advanced configuration:

##### Producer partitioning: 

The `partition.method` property determines how outgoing messages are partitioned. Valid values for this config are `EVENT_HUB_HASHING`, `PARTITION_KEY_AS_PARTITION` or `ROUND_ROBIN`. 

`EVENT_HUB_HASHING`: By default, Samza computes the partition for an outgoing message based on the hash of its partition-key. This ensures that events with the same key are sent to the same partition. If this option is chosen, the partition key should be a string. If the partition key is not set, the key in the message is used for partitioning.

`PARTITION_KEY_AS_PARTITION`: In this method, each message is sent to the partition specified by its partition key. This requires the partition key to be an integer. If the key is greater than the number of partitions, a modulo operation will be performed on the key. Similar to EVENT_HUB_HASHING, the key in the message is used if the partition key is not specified.

`ROUND_ROBIN`: In this method, outgoing messages are distributed in a round-robin across all partitions. The key and the partition key in the message are ignored.

![diagram-medium](/img/{{site.version}}/learn/documentation/azure/eventhub_send_methods.png)

```
systems.eh-system.partition.method = EVENT_HUB_HASHING
```

##### Consumer groups: 

Eventhub supports a notion of [consumer groups](https://docs.microsoft.com/en-us/azure/event-hubs/event-hubs-features#consumer-groups) which enable multiple applications have their own view of the event stream. Each event hub stream has a pre-defined consumer group named `$Default`. You can define your own consumer group for your job and configure a `eventhubs.consumer.group`  

```
systems.eh-system.streams.eh-input0.eventhubs.consumer.group = my-group
```

##### Serde: 

By default, the messages from EventHubs are sent and received as byte arrays. You can configure a serializer and deserializer for your message by setting a value for `msg.serde` for your stream. 

```
streams.input0.samza.msg.serde = json
streams.output0.samza.msg.serde = json
```

##### Consumer buffer size: 

When the consumer reads a message from event hubs, it appends them to a shared producer-consumer buffer corresponding to its partition. This config determines the per-partition queue size. Setting a higher value for this config typically achieves a higher throughput at the expense of increased on-heap memory.

```
systems.eh-system.eventhubs.receive.queue.size = 10
```

For the list of all configs, check out the configuration table page [here](../jobs/configuration-table.html)

### Azure Eventhubs Hello-Samza Example

The [hello-samza](https://github.com/apache/samza-hello-samza) project contains an example of a high level job that consumes and produces to Eventhub using the Zookeeper deployment model.

#### Get the Code

Let's get started by cloning the hello-samza project

```
git clone https://git.apache.org/samza-hello-samza.git hello-samza
cd hello-samza
git checkout latest
```

The project comes up with numerous examples and for this tutorial, we will pick the Azure application.

#### Setting up the Deployment Environment

For our Azure application, we require [ZooKeeper](http://zookeeper.apache.org/). The hello-samza project comes with a script called "grid" to help with the environment setup

```
./bin/grid standalone
```

This command will download, install, and start ZooKeeper and Kafka. It will also check out the latest version of Samza and build it. All package files will be put in a sub-directory called "deploy" inside hello-samza's root folder.

If you get a complaint that JAVA_HOME is not set, then you'll need to set it to the path where Java is installed on your system.


#### Configuring the Azure application

Here are the configs you must set before building the project. Configure these in the `src/main/config/azure-application-local-runner.properties` file.

```
# Add your EventHubs input stream credentials here
systems.eventhubs.streams.input-stream.eventhubs.namespace=YOUR-STREAM-NAMESPACE
systems.eventhubs.streams.input-stream.eventhubs.entitypath=YOUR-ENTITY-NAME
systems.eventhubs.streams.input-stream.eventhubs.sas.keyname=YOUR-SAS-KEY-NAME
systems.eventhubs.streams.input-stream.eventhubs.sas.token=YOUR-SAS-KEY-TOKEN

# Add your EventHubs output stream credentials here
systems.eventhubs.streams.output-stream.eventhubs.namespace=YOUR-STREAM-NAMESPACE
systems.eventhubs.streams.output-stream.eventhubs.entitypath=YOUR-ENTITY-NAME
systems.eventhubs.streams.output-stream.eventhubs.sas.keyname=YOUR-SAS-KEY-NAME
systems.eventhubs.streams.output-stream.eventhubs.sas.token=YOUR-SAS-KEY-TOKEN
```

Optionally, you may also use the Azure Checkpoint Manager. Otherwise, comment out both these lines.

```
# Azure Table Checkpoint Manager
task.checkpoint.factory=org.apache.samza.checkpoint.azure.AzureCheckpointManagerFactory
azure.storage.connect=YOUR-STORAGE-ACCOUNT-CONNECTION-STRING
```

#### Building the Hello Samza Project

With the environment setup complete, let us move on to building the hello-samza project. Execute the following commands:

```
mvn clean package
mkdir -p deploy/samza
tar -xvf ./target/hello-samza-0.14.1-dist.tar.gz -C deploy/samza
```

We are now all set to deploy the application locally.

#### Running the Azure application

In order to run the application, we will use the *run-azure-application* script.

```
./deploy/samza/bin/run-azure-application.sh
```

The above command executes the helper script which invokes the *AzureZKLocalApplication* main class, which starts the *AzureApplication*. This application filters out the messages consumed without keys, prints them out and send them the configured output stream.

The messages consumed should be printed in the following format:
```
Sending: 
Received Key: <KEY>
Received Message: <VALUE>
```

#### Shutdown

This application can be shutdown by terminating the *run-azure-application* script.
We can use the *grid* script to tear down the local environment ([Kafka](http://kafka.apache.org/) and [Zookeeper](http://zookeeper.apache.org/)).

```
bin/grid stop all
```