Azure Service Bus: Queues vs Topics

Azure Service Bus is Microsoft’s enterprise messaging service. But should you use queues or topics? Here’s the difference and when to use each.

Queues: Point-to-Point

Queues deliver messages to a single receiver. First-in, first-out. Perfect for work distribution:

// Send to queue
var client = new QueueClient(connectionString, "orders");
var message = new Message(Encoding.UTF8.GetBytes(JsonSerializer.Serialize(order)));
await client.SendAsync(message);

// Receive from queue (competing consumers)
client.RegisterMessageHandler(async (message, token) =>
{
    var order = JsonSerializer.Deserialize<Order>(message.Body);
    await ProcessOrder(order);
    await client.CompleteAsync(message.SystemProperties.LockToken);
},
new MessageHandlerOptions(ExceptionHandler));

Topics: Publish-Subscribe

Topics deliver messages to multiple subscribers. Each subscriber has its own subscription:

// Publish to topic
var topicClient = new TopicClient(connectionString, "orders");
await topicClient.SendAsync(message);

// Subscribe
var subClient = new SubscriptionClient(connectionString, "orders", "email-notifications");
subClient.RegisterMessageHandler(async (msg, token) =>
{
    await SendEmailNotification(msg);
    await subClient.CompleteAsync(msg.SystemProperties.LockToken);
}, options);

When to Use Which

• Queue: Work items processed by one consumer (order processing, job queue)
• Topic: Events consumed by multiple systems (order placed → notify, invoice, inventory)

Subscription Filters

Topics support filtering—subscribers only get messages they care about:

// SQL filter
var rule = new RuleDescription("HighValueOrders", new SqlFilter("Amount > 1000"));
await subscriptionClient.AddRuleAsync(rule);

Key Features

• Dead-letter queue: Failed messages go here for inspection
• Sessions: Ordered processing for related messages
• Scheduled messages: Deliver at a future time
• Duplicate detection: Automatic deduplication

References

• Azure Service Bus Documentation