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Azure Event Hubs client library for Java

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

Getting started

Prerequisites

Adding the package to your product

<dependency>
    <groupId>com.azure</groupId>
    <artifactId>azure-messaging-eventhubs</artifactId>
    <version>5.0.0-preview.5</version>
</dependency>

Methods to authorize with Event Hubs

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 client 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 clients can be created using EventHubClientBuilder. Invoking .buildAsyncClient() will create an EventHubAsyncClient and invoking .buildClient() will create an EventHubClient.

The snippet below creates an asynchronous Event Hub client.

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

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

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

<dependency>
    <groupId>com.azure</groupId>
    <artifactId>azure-identity</artifactId>
    <version>1.0.0</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 AAD application client secret

Follow the instructions in Creating a service principal using Azure Portal to create a service principal and a client secret. The corresponding clientId and tenantId for the service principal can be obtained from the App registration page.

ClientSecretCredential credential = new ClientSecretCredentialBuilder()
    .clientId("<< APPLICATION (CLIENT) ID >>")
    .clientSecret("<< APPLICATION SECRET >>")
    .tenantId("<< DIRECTORY (TENANT) ID >>")
    .build();

// The fully qualified domain name (FQDN) for the Event Hubs namespace. This is likely to be similar to:
// {your-namespace}.servicebus.windows.net
String host = "<< EVENT HUBS HOST >>"
String eventHubName = "<< NAME OF THE EVENT HUB >>";
EventHubAsyncClient client = new EventHubClientBuilder()
    .credential(host, eventHubName, credential)
    .buildAsyncClient();

Key concepts

  • An Event Hub client is the primary interface for developers interacting with the Event Hubs client library, allowing for inspection of Event Hub metadata and providing a guided experience towards specific Event Hub operations such as the creation of producers and consumers.

  • 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 web site.

  • 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

In order to publish events, you'll need to create an asynchronous EventHubProducerAsyncClient or a synchronous EventHubProducerClient. Producers may be dedicated to 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. In the our example, we will take advantage of automatic routing.

Event Hub producer creation

With an existing EventHubAsyncClient or EventHubClient, developers can create a producer by calling createProducer() or createProducer(EventHubProducerOptions). If EventHubClient is used, a synchronous EventHubProducerClient is created. If EventHubAsyncClient is used, an asynchronous EventHubProducerAsyncClient is returned.

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

EventHubAsyncClient client = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .buildAsyncClient();
EventHubProducerAsyncClient producer = client.createProducer();

To send events to a particular partition, set the optional parameter partitionId on SendOptions or BatchOptions.

Partition identifier

Many Event Hub operations take place within the scope of a specific partition. Because partitions are owned by the Event Hub, their names are assigned at the time of creation. To understand what partitions are available, You can use the getPartitionIds function to get the ids of all available partitions in your Event Hub instance.

Flux<String> firstPartition = client.getPartitionIds();

Partition key

When an Event Hub producer is 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.

SendOptions sendOptions = new SendOptions().partitionKey("grouping-key");
producer.send(dataList, sendOptions).subscribe(
    ...
);

Consume events from an Event Hub partition

In order to consume events, you'll need to create an EventHubAsyncConsumer or EventHubConsumer for a specific partition and consumer group combination. When an Event Hub is created, it starts with a default consumer group that can be used to get started. A consumer also needs to specify where in the event stream to begin receiving events; in our example, we will focus on reading new events as they are published.

Consume events with EventHubAsyncConsumer

In the snippet below, we are creating 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 by calling .receive() and subscribing to the stream.

EventHubAsyncClient client = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .buildAsyncClient();

String partitionId = "<< EVENT HUB PARTITION ID >>"
EventHubAsyncConsumer consumer = client.createConsumer(EventHubAsyncClient.DEFAULT_CONSUMER_GROUP_NAME, partitionId,
    EventPosition.earliest());

consumer.receive().subscribe(event -> {
    // Process each event as it arrives.
});

Consume events with EventHubConsumer

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

EventHubClient client = new EventHubClientBuilder()
    .connectionString("<< CONNECTION STRING FOR SPECIFIC EVENT HUB INSTANCE >>")
    .buildClient();

String partitionId = "<< EVENT HUB PARTITION ID >>";
EventHubConsumer consumer = client.createConsumer(EventHubAsyncClient.DEFAULT_CONSUMER_GROUP_NAME, partitionId,
    EventPosition.earliest());

// Get the first 15 events in the stream, or as many events as can be received within 40 seconds.
IterableStream<EventData> events = consumer.receive(15, Duration.ofSeconds(40));
for (EventData event : events) {
    // Process each event
}

// Calling receive again returns the next 15 events in the stream, or as many as possible in 40 seconds.
IterableStream<EventData> nextEvents = consumer.receive(15, Duration.ofSeconds(40));

Consume events using an Event Processor

To consume events for all partitions of an Event Hub, you'll create an EventProcessor for a specific consumer group. When an Event Hub is created, it provides a default consumer group that can be used to get started.

The EventProcessor will delegate processing of events to a PartitionProcessor implementation that you provide, allowing your logic 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 EventProcessor, use the built-in InMemoryPartitionManager, and a PartitionProcessor implementation that logs events received to console.

class Program {
    public static void main(String[] args) {
        EventHubAsyncClient eventHubAsyncClient = new EventHubClientBuilder()
            .connectionString("<< CONNECTION STRING FOR THE EVENT HUB INSTANCE >>")
            .buildAsyncClient();

        EventProcessor eventProcessor = new EventProcessorBuilder()
            .consumerGroup("<< CONSUMER GROUP NAME>>")
            .eventHubClient(eventHubAsyncClient)
            .partitionProcessorFactory((SimplePartitionProcessor::new))
            .partitionManager(new InMemoryPartitionManager())
            .buildEventProcessor();

        // This will start the processor. It will start processing events from all partitions.
        eventProcessor.start();
        
        // (for demo purposes only - adding sleep to wait for receiving events)
        TimeUnit.SECONDS.sleep(2); 

        // When the user wishes to stop processing events, they can call `stop()`.
        eventProcessor.stop();
    }
}

class SimplePartitionProcessor extends PartitionProcessor {
    /**
     * Processes the event data.
     */
    // @Override
    public Mono<Void> processEvent(PartitionContext partitionContext, EventData eventData) {
        System.out.println("Processing event with sequence number " + eventData.sequenceNumber());
        return partitionContext.updateCheckpoint(eventData);
    }
}

Troubleshooting

Enable client logging

You can set the AZURE_LOG_LEVEL environment variable to view logging statements made in the client library. For example, setting AZURE_LOG_LEVEL=2 would show all informational, warning, and error log messages. The log levels can be found here: log levels.

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.

Sample "logging.config" 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

Common 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 ErrorContext. 'isTransient' is a boolean indicating if the exception is a transient error or not. If true, then the request can be retried; otherwise not.

The ErrorCondition 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 ErrorContext 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.

Operation cancelled exception

It occurs when the underlying AMQP layer encounters an abnormal link abort or the connection is disconnected in an unexpected fashion. It is recommended to attempt to verify the current state and retry if necessary.

Message size exceeded

Event data, both individual and in batches, have a maximum size allowed. This includes the data of the event, as well as any associated metadata and system overhead. The best approach for resolving this error is to reduce the number of events being sent in a batch or the size of data included in the message. Because size limits are subject to change, please refer to Azure Event Hubs quotas and limits for specifics.

Other exceptions

For detailed information about these and other exceptions that may occur, please refer to Event Hubs Messaging Exceptions.

Next steps

Beyond those discussed, the Azure Event Hubs client library offers support for many additional scenarios to help take advantage of the full feature set of the Azure Event Hubs service. In order to help explore some of the these scenarios, the following set of sample is available:

Contributing

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

Impressions