Azure Service Bus Topics and Subscriptions

While queues deliver each message to a single consumer, Service Bus topics enable publish-subscribe patterns where a single message can be delivered to many consumers. Topics are essential for broadcasting events, notifications, and updates across multiple independent services. This lesson covers how topics and subscriptions work, how to use filters to route messages, and practical patterns for real-world systems.

---

How Topics and Subscriptions Work

A topic is similar to a queue in that it receives messages from senders. However, instead of delivering messages directly to consumers, the topic distributes copies of each message to one or more subscriptions. Each subscription behaves like a virtual queue that can have its own set of consumers.

                         +---> [ Subscription: billing ] ---> Billing Service
                         |
Producer --> [ Topic ] --+---> [ Subscription: audit ] ---> Audit Service
                         |
                         +---> [ Subscription: email ] ---> Email Service

Key characteristics:

• Decoupling — the producer publishes once; it does not know who is listening
• Fan-out — each subscription receives a copy of every matching message
• Independent consumption — each subscription maintains its own read position and can process at its own pace
• Filtering — subscriptions can define rules to receive only relevant messages

---

Creating Topics and Subscriptions

Using the Azure CLI

# Create a topic
az servicebus topic create \
  --name order-events \
  --namespace-name sb-demo-ns \
  --resource-group rg-messaging

# Create subscriptions
az servicebus topic subscription create \
  --name billing \
  --topic-name order-events \
  --namespace-name sb-demo-ns \
  --resource-group rg-messaging

az servicebus topic subscription create \
  --name audit \
  --topic-name order-events \
  --namespace-name sb-demo-ns \
  --resource-group rg-messaging

Key Topic Properties

Property	Description	Default
maxSizeInMegabytes	Maximum topic size	1024 MB
defaultMessageTimeToLive	Default TTL for messages	14 days
requiresDuplicateDetection	Enable duplicate detection	false
enablePartitioning	Partition across multiple message brokers	false

Key Subscription Properties

Property	Description	Default
lockDuration	PeekLock duration	30 seconds
maxDeliveryCount	Attempts before dead-lettering	10
deadLetteringOnFilterEvaluationExceptions	Dead-letter on filter errors	false
autoDeleteOnIdle	Delete subscription after idle period	disabled

---

Subscription Filters

By default, a subscription receives all messages sent to the topic. You can add filter rules so that a subscription only receives messages matching specific criteria.

Filter Types

Service Bus supports three types of filters:

1. SQL Filter

A SQL-like expression evaluated against message properties:

az servicebus topic subscription rule create \
  --name high-value-filter \
  --subscription-name billing \
  --topic-name order-events \
  --namespace-name sb-demo-ns \
  --resource-group rg-messaging \
  --filter-sql-expression "amount > 100"