# You picked the topic. Then it filled up.

> Five Service Bus traps that only show themselves six months later.

- **Format:** short video, 10 steps, ~32 seconds
- **Topic:** Service Bus queue versus topic — the five things that go wrong months after you correctly chose the topic.
- **Author:** Suthahar Jegatheesan (MSDEVBUILD)
- **Category:** Azure · Azure
- **Tags:** azure, azureservicebus, servicebus, pubsub, messagequeue, systemdesign, microsoftazure, azurecloud, cloudarchitecture, dotnet, sre, az305, msdevbuild
- **Canonical URL:** https://blog.msdevbuild.com/shorts/azure-queue-vs-topic-traps/

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## What you'll learn

- How the $Default rule keeps matching everything you filtered
- Why one idle subscription fills the topic and pages the publisher
- The five settings that prevent all of it, none of them code

## Understand it one step at a time

### 1. You picked a topic. Good.

Four teams needed the same event, so you created a topic and four subscriptions. Correct call.

### 2. Six months later

The publisher starts throwing QuotaExceededException. Nothing about the publisher changed.

### 3. The $Default rule

You added a filter to billing. You did not remove the rule Azure created for you, so it still matches everything.

### 4. One idle subscription fills it

A topic keeps a message until every subscription has completed it. legacy-export stopped reading in March.

### 5. N subs, N dead-letter queues

Every subscription has its own DLQ. You set up one alert, years ago, on the queue this replaced.

### 6. Ordering caps your fan-out

Need FIFO? Enable sessions — per subscription. Now it is one consumer per session, and throughput drops.

### 7. You can't convert a queue

There is no toggle. A queue becomes a topic by creating a new entity and repointing every producer.

### 8. Five settings prevent all of it

None of these are code. Remove the default rule, set TTLs, auto-delete idle subs, alert on every DLQ.

### 9. Queue or topic, honestly

A queue is less to run. A topic is still the right default — as long as somebody owns the subscriptions.

### 10. A topic is a queue with homework

The fan-out is free. The subscriptions are not — every one of them is a thing somebody has to own.

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## The takeaway

**Every subscription is a thing somebody must own**

The one nobody owns is the one that fills the topic and pages the publisher.