azure-docs-sdk-java/docs-ref-services/archive/messaging-eventhubs-readme.md

title	keywords	ms.date	ms.topic	ms.devlang	ms.service
Azure Event Hubs client library for Java	Azure, java, SDK, API, azure-messaging-eventhubs, eventhubs	10/31/2020	reference	java	eventhubs

Azure Event Hubs client library for Java - version 5.3.1

Azure Event Hubs is a highly scalable publish-subscribe service that can ingest millions of events per second and stream them to multiple consumers. This lets you process and analyze the massive amounts of data produced by your connected devices and applications. Once Event Hubs has collected the data, you can retrieve, transform, and store it by using any real-time analytics provider or with batching/storage adapters. If you would like to know more about Azure Event Hubs, you may wish to review: What is Event Hubs?

The Azure Event Hubs client library allows for publishing and consuming of Azure Event Hubs events and may be used to:

• Emit telemetry about your application for business intelligence and diagnostic purposes.
• Publish facts about the state of your application which interested parties may observe and use as a trigger for taking action.
• Observe interesting operations and interactions happening within your business or other ecosystem, allowing loosely coupled systems to interact without the need to bind them together.
• Receive events from one or more publishers, transform them to better meet the needs of your ecosystem, then publish the transformed events to a new stream for consumers to observe.

Source code | API reference documentation | Product documentation | Samples

Table of contents

• Table of contents
• Getting started
  • Prerequisites
  • Include the package
  • Authenticate the client
    • Create an Event Hub producer using a connection string
    • Create an Event Hub client using Microsoft identity platform (formerly Azure Active Directory)
• Key concepts
• Examples
  • Publish events to an Event Hub
  • Consume events from an Event Hub partition
  • Consume events using an EventProcessorClient
• Troubleshooting
  • Enable client logging
  • Enable AMQP transport logging
  • Exceptions
• Next steps
• Contributing

Getting started

Prerequisites

• A Java Development Kit (JDK), version 8 or later.
• Maven
• Microsoft Azure subscription
  • You can create a free account at: https://azure.microsoft.com
• Azure Event Hubs instance
  • Step-by-step guide for creating an Event Hub using the Azure Portal

Include the package

<dependency>
    <groupId>com.azure</groupId>
    <artifactId>azure-messaging-eventhubs</artifactId>
    <version>5.3.1</version>
</dependency>

Authenticate the client

For the Event Hubs client library to interact with an Event Hub, it will need to understand how to connect and authorize with it.

Create an Event Hub producer using a connection string

The easiest means for doing so is to use a connection string, which is created automatically when creating an Event Hubs namespace. If you aren't familiar with shared access policies in Azure, you may wish to follow the step-by-step guide to get an Event Hubs connection string.

Both the asynchronous and synchronous Event Hub producer and consumer clients can be created using EventHubClientBuilder. Invoking build*Client() creates a synchronous producer or consumer while build*AsyncClient() creates its asynchronous counterpart.

The snippet below creates a synchronous Event Hub producer.

String connectionString = "<< CONNECTION STRING FOR THE EVENT HUBS NAMESPACE >>";
String eventHubName = "<< NAME OF THE EVENT HUB >>";
EventHubProducerClient producer = new EventHubClientBuilder()
    .connectionString(connectionString, eventHubName)
    .buildProducerClient();

Create an Event Hub client using Microsoft identity platform (formerly Azure Active Directory)

Azure SDK for Java supports an Azure Identity package, making it easy to get credentials from Microsoft identity platform. First, add the package:

<dependency>
    <groupId>com.azure</groupId>
    <artifactId>azure-identity</artifactId>
    <version>1.1.2</version>
</dependency>

All the implemented ways to request a credential can be found under the com.azure.identity.credential package. The sample below shows how to use an Azure Active Directory (AAD) application client secret to authorize with Azure Event Hubs.

Authorizing with DefaultAzureCredential

Authorization is easiest using DefaultAzureCredential. It finds the best credential to use in its running environment. For more information about using Azure Active Directory authorization with Event Hubs, please refer to the associated documentation.

TokenCredential credential = new DefaultAzureCredentialBuilder()
    .build();

// The fully qualified namespace for the Event Hubs instance. This is likely to be similar to:
// {your-namespace}.servicebus.windows.net
String fullyQualifiedNamespace = "my-test-eventhubs.servicebus.windows.net";
String eventHubName = "<< NAME OF THE EVENT HUB >>";
EventHubProducerClient client = new EventHubClientBuilder()
    .credential(fullyQualifiedNamespace, eventHubName, credential)
    .buildProducerClient();

Key concepts

• An Event Hub producer is a source of telemetry data, diagnostics information, usage logs, or other log data, as part of an embedded device solution, a mobile device application, a game title running on a console or other device, some client or server based business solution, or a website.

• An Event Hub consumer picks up such information from the Event Hub and processes it. Processing may involve aggregation, complex computation, and filtering. Processing may also involve distribution or storage of the information in a raw or transformed fashion. Event Hub consumers are often robust and high-scale platform infrastructure parts with built-in analytics capabilities, like Azure Stream Analytics, Apache Spark, or Apache Storm.

• A partition is an ordered sequence of events that is held in an Event Hub. Azure Event Hubs provides message streaming through a partitioned consumer pattern in which each consumer only reads a specific subset, or partition, of the message stream. As newer events arrive, they are added to the end of this sequence. The number of partitions is specified at the time an Event Hub is created and cannot be changed.

• A consumer group is a view of an entire Event Hub. Consumer groups enable multiple consuming applications to each have a separate view of the event stream, and to read the stream independently at their own pace and from their own position. There can be at most 5 concurrent readers on a partition per consumer group; however it is recommended that there is only one active consumer for a given partition and consumer group pairing. Each active reader receives all of the events from its partition; if there are multiple readers on the same partition, then they will receive duplicate events.

For more concepts and deeper discussion, see: Event Hubs Features. Also, the concepts for AMQP are well documented in OASIS Advanced Messaging Queuing Protocol (AMQP) Version 1.0.

Examples

Publish events to an Event Hub

To publish events, you'll need to create an asynchronous EventHubProducerAsyncClient or a synchronous EventHubProducerClient. Each producer can send events to either, a specific partition, or allow the Event Hubs service to decide which partition events should be published to. It is recommended to use automatic routing when the publishing of events needs to be highly available or when event data should be distributed evenly among the partitions.

Create an Event Hub producer and publish events

Developers can create a producer using EventHubClientBuilder and calling buildProducer*Client(). Specifying CreateBatchOptions.setPartitionId(String) will send events to a specific partition. If partitionId is not specified, events are automatically routed to a partition. Specifying CreateBatchOptions.setPartitionKey(String) will tell Event Hubs service to hash the events and send them to the same partition.

The snippet below creates a synchronous producer and sends events to any partition, allowing Event Hubs service to route the event to an available partition.

EventHubProducerClient producer = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .buildProducerClient();

List<EventData> allEvents = Arrays.asList(new EventData("Foo"), new EventData("Bar"));
EventDataBatch eventDataBatch = producer.createBatch();

for (EventData eventData : allEvents) {
    if (!eventDataBatch.tryAdd(eventData)) {
        producer.send(eventDataBatch);
        eventDataBatch = producer.createBatch();

        // Try to add that event that couldn't fit before.
        if (!eventDataBatch.tryAdd(eventData)) {
            throw new IllegalArgumentException("Event is too large for an empty batch. Max size: "
                + eventDataBatch.getMaxSizeInBytes());
        }
    }
}
// send the last batch of remaining events
if (eventDataBatch.getCount() > 0) {
    producer.send(eventDataBatch);
}

Publish events using partition identifier

Many Event Hub operations take place within the scope of a specific partition. Any client can call getPartitionIds() or getEventHubProperties() to get the partition ids and metadata about in their Event Hub instance.

EventHubProducerClient producer = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .buildProducerClient();

CreateBatchOptions options = new CreateBatchOptions().setPartitionId("0");
EventDataBatch batch = producer.createBatch(options);

// Add events to batch and when you want to send the batch, send it using the producer.
producer.send(batch);

Publish events using partition key

When a set of events are not associated with any specific partition, it may be desirable to request that the Event Hubs service keep different events or batches of events together on the same partition. This can be accomplished by setting a partition key when publishing the events.

EventHubProducerClient producer = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .buildProducerClient();

CreateBatchOptions batchOptions = new CreateBatchOptions().setPartitionKey("grouping-key");
EventDataBatch eventDataBatch = producer.createBatch(batchOptions);

// Add events to batch and when you want to send the batch, send it using the producer.
producer.send(eventDataBatch);

Consume events from an Event Hub partition

To consume events, create an EventHubConsumerAsyncClient or EventHubConsumerClient for a specific consumer group. In addition, a consumer needs to specify where in the event stream to begin receiving events.

Consume events with EventHubConsumerAsyncClient

In the snippet below, we create an asynchronous consumer that receives events from partitionId and only listens to newest events that get pushed to the partition. Developers can begin receiving events from multiple partitions using the same EventHubConsumerAsyncClient by calling receiveFromPartition(String, EventPosition) with another partition id.

EventHubConsumerAsyncClient consumer = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .consumerGroup(EventHubClientBuilder.DEFAULT_CONSUMER_GROUP_NAME)
    .buildAsyncConsumerClient();

// Receive newly added events from partition with id "0". EventPosition specifies the position
// within the Event Hub partition to begin consuming events.
consumer.receiveFromPartition("0", EventPosition.latest()).subscribe(event -> {
    // Process each event as it arrives.
});
// add sleep or System.in.read() to receive events before exiting the process.

Consume events with EventHubConsumerClient

Developers can create a synchronous consumer that returns events in batches using an EventHubConsumerClient. In the snippet below, a consumer is created that starts reading events from the beginning of the partition's event stream.

EventHubConsumerClient consumer = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .consumerGroup(EventHubClientBuilder.DEFAULT_CONSUMER_GROUP_NAME)
    .buildConsumerClient();

String partitionId = "<< EVENT HUB PARTITION ID >>";

// Get the first 15 events in the stream, or as many events as can be received within 40 seconds.
IterableStream<PartitionEvent> events = consumer.receiveFromPartition(partitionId, 15,
    EventPosition.earliest(), Duration.ofSeconds(40));
for (PartitionEvent event : events) {
    System.out.println("Event: " + event.getData().getBodyAsString());
}

Consume events using an EventProcessorClient

To consume events for all partitions of an Event Hub, you can create an EventProcessorClient for a specific consumer group.

The EventProcessorClient will delegate processing of events to a callback function that you provide, allowing you to focus on the logic needed to provide value while the processor holds responsibility for managing the underlying consumer operations.

In our example, we will focus on building the EventProcessorClient, use the SampleCheckpointStore available in samples, and a callback function that processes events received from the Event Hub and writes to console. For production applications, it's recommended to use a durable store like Checkpoint Store with Azure Storage Blobs.

EventProcessorClient eventProcessorClient = new EventProcessorClientBuilder()
    .consumerGroup("<< CONSUMER GROUP NAME >>")
    .connectionString("<< EVENT HUB CONNECTION STRING >>")
    .checkpointStore(new SampleCheckpointStore())
    .processEvent(eventContext -> {
        System.out.println("Partition id = " + eventContext.getPartitionContext().getPartitionId() + " and "
            + "sequence number of event = " + eventContext.getEventData().getSequenceNumber());
    })
    .processError(errorContext -> {
        System.out
            .println("Error occurred while processing events " + errorContext.getThrowable().getMessage());
    })
    .buildEventProcessorClient();

// This will start the processor. It will start processing events from all partitions.
eventProcessorClient.start();

// (for demo purposes only - adding sleep to wait for receiving events)
TimeUnit.SECONDS.sleep(2);

// This will stop processing events.
eventProcessorClient.stop();

Troubleshooting

Enable client logging

Azure SDK for Java offers a consistent logging story to help aid in troubleshooting application errors and expedite their resolution. The logs produced will capture the flow of an application before reaching the terminal state to help locate the root issue. View the logging wiki for guidance about enabling logging.

Enable AMQP transport logging

If enabling client logging is not enough to diagnose your issues. You can enable logging to a file in the underlying AMQP library, Qpid Proton-J. Qpid Proton-J uses java.util.logging. You can enable logging by create a configuration file with the contents below. Or set proton.trace.level=ALL and whichever configuration options you want for the java.util.logging.Handler implementation. Implementation classes and their options can be found in Java 8 SDK javadoc.

To trace the AMQP transport frames, set the environment variable: PN_TRACE_FRM=1.

Sample "logging.properties" file

The configuration file below logs trace output from proton-j to the file "proton-trace.log".

handlers=java.util.logging.FileHandler
.level=OFF
proton.trace.level=ALL
java.util.logging.FileHandler.level=ALL
java.util.logging.FileHandler.pattern=proton-trace.log
java.util.logging.FileHandler.formatter=java.util.logging.SimpleFormatter
java.util.logging.SimpleFormatter.format=[%1$tF %1$tr] %3$s %4$s: %5$s %n

Exceptions

AMQP exception

This is a general exception for AMQP related failures, which includes the AMQP errors as ErrorCondition and the context that caused this exception as AmqpErrorContext. isTransient is a boolean indicating if the exception is a transient error or not. If true, then the request can be retried according to retry options set; otherwise not.

AmqpErrorCondition contains error conditions common to the AMQP protocol and used by Azure services. When an AMQP exception is thrown, examining the error condition field can inform developers as to why the AMQP exception occurred and if possible, how to mitigate this exception. A list of all the AMQP exceptions can be found in OASIS AMQP Version 1.0 Transport Errors.

The AmqpErrorContext in the AmqpException provides information about the AMQP session, link, or connection that the exception occurred in. This is useful to diagnose which level in the transport this exception occurred at and whether it was an issue in one of the producers or consumers.

The recommended way to solve the specific exception the AMQP exception represents is to follow the Event Hubs Messaging Exceptions guidance.

Next steps

Beyond those discussed, the Azure Event Hubs client library offers support for many other scenarios to take advantage of the full feature set of the Azure Event Hubs service. To explore some of these scenarios, check out the samples README.

Contributing

If you would like to become an active contributor to this project please refer to our Contribution Guidelines for more information.