What is Azure Service Bus?

Microsoft Azure Service Bus is a fully managed enterprise message broker with message queues and public-subscribe topics. Service Bus is used to decouple applications and services from each other, providing the following benefits:

  • Load-balancing work across competing workers
  • Safely routing and transferring data and control across service and application boundaries
  • Coordinating transactional work that requires a high-degree of reliability

Overview

Data is transferred between different applications and services using messages. A message is a container decorated with metadata, and contains data. The data can be any kind of information, including structured data encoded with the common formats such as the following ones: JSON, XML, Apache Avro, Plain Text.

Some common messaging scenarios are:

  • Messaging. Transfer business data, such as sales or purchase orders, journals, or inventory movements.

  • Decouple applications. Improve reliability and scalability of applications and services. Producer and consumer don't have to be online or readily available at the same time. The load is leveled such that traffic spikes don't overtax a service.

  • Load Balancing. Allow for multiple competing consumers to read from a queue at the same time, each safely obtaining exclusive ownership to specific messages.

  • Topics and subscriptions. Enable 1:n relationships between publishers and subscribers, allowing subscribers to select particular messages from a published message stream.

  • Transactions. Allows you to do several operations, all in the scope of an atomic transaction. For example, the following operations can be done in the scope of a transaction.

    1. Obtain a message from one queue.
    2. Post results of processing to one or more different queues.
    3. Move the input message from the original queue.

    The results become visible to downstream consumers only upon success, including the successful settlement of input message, allowing for once-only processing semantics. This transaction model is a robust foundation for the compensating transactions pattern in the greater solution context.

  • Message sessions. Implement high-scale coordination of workflows and multiplexed transfers that require strict message ordering or message deferral.

If you're familiar with other message brokers like Apache ActiveMQ, Service Bus concepts are similar to what you know. As Service Bus is a platform-as-a-service (PaaS) offering, a key difference is that you don't need to worry about the following actions. Azure takes care of those chores for you.

  • Placing logs and managing disk space
  • Handling backups
  • Keeping the operating systems or the products patched
  • Worrying about hardware failures
  • Failing over to a reserve machine

Compliance with standards and protocols

The primary wire protocol for Service Bus is Advanced Messaging Queueing Protocol (AMQP) 1.0, an open ISO/IEC standard. It allows customers to write applications that work against Service Bus and on-premises brokers such as ActiveMQ or RabbitMQ. The AMQP protocol guide provides detailed information in case you want to build such an abstraction.

Service Bus Premium is fully compliant with the Java/Jakarta EE Java Message Service (JMS) 2.0 API. And, Service Bus Standard supports the JMS 1.1 subset focused on queues. JMS is a common abstraction for message brokers and integrates with many applications and frameworks, including the popular Spring framework. To switch from other brokers to Azure Service Bus, you just need to recreate the topology of queues and topics, and change the client provider dependencies and configuration. For an example, see the ActiveMQ migration guide.

Concepts and terminology

This section discusses concepts and terminology of Service Bus.

Namespaces

A namespace is a container for all messaging components. Multiple queues and topics can be in a single namespace, and namespaces often serve as application containers.

A namespace can be compared to a "server" in the terminology of other brokers, but the concepts aren't directly equivalent. A Service Bus namespace is your own capacity slice of a large cluster made up of dozens of all-active virtual machines. It may optionally span three Azure availability zones. So, you get all the availability and robustness benefits of running the message broker at enormous scale. And, you don't need to worry about underlying complexities. Service Bus is "serverless" messaging.

Queues

Messages are sent to and received from queues. Queues store messages until the receiving application is available to receive and process them.

Queue

Messages in queues are ordered and timestamped on arrival. Once accepted by the broker, the message is always held durably in triple-redundant storage, spread across availability zones if the namespace is zone-enabled. Service Bus never leaves messages in memory or volatile storage after they've been reported to the client as accepted.

Messages are delivered in pull mode, only delivering messages when requested. Unlike the busy-polling model of some other cloud queues, the pull operation can be long-lived and only complete once a message is available.

Topics

You can also use topics to send and receive messages. While a queue is often used for point-to-point communication, topics are useful in publish/subscribe scenarios.

Topic

Topics can have multiple, independent subscriptions, which attach to the topic and otherwise work exactly like queues from the receiver side. A subscriber to a topic can receive a copy of each message sent to that topic. Subscriptions are named entities. Subscriptions are durable by default, but can be configured to expire and then be automatically deleted. Via the JMS API, Service Bus Premium also allows you to create volatile subscriptions that exist for the duration of the connection.

You can define rules on a subscription. A subscription rule has a filter to define a condition for the message to be copied into the subscription and an optional action that can modify message metadata. For more information, see Topic filters and actions. This feature is useful in the following scenarios:

  • You don't want a subscription to receive all messages sent to a topic.
  • You want to mark up messages with extra metadata when they pass through a subscription.

Advanced features

Service Bus includes advanced features that enable you to solve more complex messaging problems. The following sections describe several of these features.

Message sessions

To create a first-in, first-out (FIFO) guarantee in Service Bus, use sessions. Message sessions enable exclusive, ordered handling of unbounded sequences of related messages. To allow for handling sessions in high-scale, high-availability systems, the session feature also allows for storing session state, which allows sessions to safely move between handlers. For more information, see Message sessions: first in, first out (FIFO).

Autoforwarding

The autoforwarding feature chains a queue or subscription to another queue or topic inside the same namespace. When you use this feature, Service Bus automatically moves messages from a queue or subscription to a target queue or topic. All such moves are done transactionally. For more information, see Chaining Service Bus entities with autoforwarding.

Dead-letter queue

All Service Bus queues and topic subscriptions have an associated dead-letter queue (DLQ). A DLQ holds messages that meet the following criteria:

  • They can't be delivered successfully to any receiver.
  • They timed out.
  • They're explicitly sidelined by the receiving application.

Messages in the dead-letter queue are annotated with the reason why they've been placed there. The dead-letter queue has a special endpoint, but otherwise acts like any regular queue. An application or tool can browse a DLQ or dequeue from it. You can also autoforward out of a dead-letter queue. For more information, see Overview of Service Bus dead-letter queues.

Scheduled delivery

You can submit messages to a queue or topic for delayed processing, setting a time when the message will become available for consumption. Scheduled messages can also be canceled. For more information, see Scheduled messages.

Message deferral

A queue or subscription client can defer retrieval of a received message until a later time. The message may have been posted out of an expected order and the client wants to wait until it receives another message. Deferred messages remain in the queue or subscription and must be reactivated explicitly using their service-assigned sequence number. For more information, see Message deferral.

Batching

Client-side batching enables a queue or topic client to accumulate a set of messages and transfer them together. It's often done to either save bandwidth or to increase throughput. For more information, see Client-side batching.

Transactions

A transaction groups two or more operations together into an execution scope. Service Bus allows you to group operations against multiple messaging entities within the scope of a single transaction. A message entity can be a queue, topic, or subscription. For more information, see Overview of Service Bus transaction processing.

Autodelete on idle

Autodelete on idle enables you to specify an idle interval after which a queue or topic subscription is automatically deleted. The minimum duration is 5 minutes. For more information, see the QueueDescription.AutoDeleteOnIdle Property.

Duplicate detection

The duplicate detection feature enables the sender to resend the same message again and for the broker to drop a potential duplicate. The duplicate detection is based on tracking the message-id property of a message, meaning the application needs to take care to use the same value when resending the message, which might be directly derived from some application-specific context. For more information, see Duplicate detection.

Geo-disaster recovery

When an Azure region experiences downtime, the disaster recovery feature enables data processing to continue operating in a different region or data center. The feature keeps a structural mirror of a namespace available in the secondary region and allows the namespace identity to switch to the secondary namespace. Already posted messages remain in the former primary namespace for recovery once the availability episode subsides. For more information, see Azure Service Bus Geo-disaster recovery.

Security

Service Bus supports standard AMQP 1.0 and HTTP or REST protocols and their respective security facilities, including transport-level security (TLS). Clients can be authorized for access using Shared Access Signature or Azure Active Directory role-based security.

For protection against unwanted traffic, Service Bus provides security features such as IP firewall and integration with virtual networks.

Client libraries

Fully supported Service Bus client libraries are available via the Azure SDK.

Azure Service Bus' primary protocol is AMQP 1.0 and it can be used from any AMQP 1.0 compliant protocol client. Several open-source AMQP clients have samples that explicitly demonstrate Service Bus interoperability. Review the AMQP 1.0 protocol guide to understand how to use Service Bus' features with AMQP 1.0 clients directly.

Language Library
Java Apache Qpid Proton-J
C/C++ Azure uAMQP C, Apache Qpid Proton-C
Python Azure uAMQP for Python, Apache Qpid Proton Python
PHP Azure uAMQP for PHP
Ruby Apache Qpid Proton Ruby
Go Azure Go AMQP, Apache Qpid Proton Go
C#/F#/VB AMQP .NET Lite, Apache NMS AMQP
JavaScript/Node Rhea

Integration

Service Bus fully integrates with many Microsoft and Azure services, for instance:

Next steps

To get started using Service Bus messaging, see the following articles: