Top 25 Azure Service Bus Interview Questions And Answers

Question 1

What is the difference between `Receive` and `Peek` in the context of Azure Service Bus Topics?

Accepted Answer

- **Peek**: This method enables you to view messages without locking or receiving them.
- **Receive**: Can work in 2 modes: 
  - `PeekLock` (receives the message but keeps it peek-locked until the receiver abandons the message. The maximum timeout is 5 minutes before message expiration) and 
  - `ReceiveAndDelete` (deletes the message after it is received).

So this means you'll use the Peek when your goal is to look at the messages without actually consuming them (maybe you're building a "Queue browser", ... or your process needs to decided if it wants/can consume the message). And you'll use Receive when you're really planning to consume the message and do whatever you need to do with it.

Question 2

Explain why would you use _Scheduled_ messages in Azure Service Bus?

Accepted Answer

**Message scheduling** is helpful when you want to postpone non-critical yet essential workloads to a later point in time. Then consumers can pick them up after peak hours. Scheduled messages can be canceled before the enqueue time. To cancel a message, you need its sequence number.

For example, you can delay email-sending tasks until midnight to save the computational capacity of consumers.

Another case is a consumer writing to an _eventually consistent_ database. Adding a delay will help the message processor _catch up_ with database writes made by upstream components.

![https://miro.medium.com/v2/resize:fit:720/format:webp/1*6I5zP68kMCE5ZpeqXrKEgg.png](https://miro.medium.com/v2/resize:fit:720/format:webp/1*6I5zP68kMCE5ZpeqXrKEgg.png)

Question 3

Can Azure Service Bus message be _empty_?

Accepted Answer

**Yes**.

Messages carry a payload and _metadata_. The metadata is in the form of key-value pair properties and describes the payload, and gives handling instructions to Service Bus and applications. Occasionally, that _metadata alone is sufficient to carry the information that the sender wants to communicate to receivers_, and the payload remains empty.

Question 4

When would you use _Azure Storage Queues_ vs _Azure Service Bus_?

Accepted Answer

Choose **Storage Queues** if:

- You need a simple (REST-based GET/PUT/PEEK interface) queue with no particular additional requirements (point-to-point communication).
- You need an audit trail of all messages that pass through the queue.
- You expect the queue to exceed 80 GB in size.
- You want to track progress for processing a message inside of the queue.

Choose **Service Bus Queues** if:

- You need _duplicate_ detection.
- You need an _at-most-once_ delivery guarantee.
- You need a FIFO/ordering guarantee (using _sessions_).
- You need to group messages into _transactions_.
- You want to receive messages _without polling_ the queue.
- You need to provide _role-based access_ to the queues.
- You need to handle messages larger than 64 KB but smaller than 256 KB.
- Your queue size will _not_ grow larger than 80 GB.
- You need to have _dead-lettering_.
- You would like to be able to publish and consume _batches_ of messages.
- You want to use a publish-subscribe messaging pattern (using _topics_ and _subscribtions_).

Question 5

Compare different message receiving modes in queuing technology

Accepted Answer

**Receive Mode** is the way in which messages can be retrieved from a queuing technology by a consumer:

- **Peek & Lease**. Used by Azure Storage Queues. The lease duration is associated with the message, so is something that you specify when you send the message to the queue. Once a message is picked up by a consumer, it is not removed from the queue. Instead, a lease (i.e. time-based limit) is placed on the _message_ (on the message level), before it is available for another consumer to process. So in case a service that is holding a resource dies, the resource is freed after the set timeout, and the scenario of deadlock is avoided.
- **Peek & Lock**. Used by Azure Service Bus. The message is held in the _queue_, but a lock is put on the message which is held exclusively by the consumer. The lock can be renewed if needed. By default, this is set at 30 seconds but can be changed.
- **Receive & Delete**. Used by Azure Service Bus. The message is considered consumed as soon as the Service Bus has transferred the message (i.e. the consuming application won’t be able to guarantee that the message has been fully processed. So if there’s an issue mid-processing, then the message could be lost). This scenario makes sense if the messages are low-value, but makes little sense in the banking scenario that we outlined earlier.

Question 6

Explain the difference between Event vs Message Services in the context of Azure Services?

Accepted Answer

There's an important distinction between services that deliver an event and services that deliver a message.

* An **event** is a lightweight notification of a condition or a state change. The publisher of the event has no expectation about how the event is handled. The consumer of the event decides what to do with the notification. Events can be discrete units or part of a series. An event - SOMETHING HAS HAPPENED.

An example, an event notifies consumers that a_ file was created_. It may have general information about the file, but it doesn't have the file itself.

* A **message** is raw data produced by a service to be consumed or stored elsewhere. The message contains the data that triggered the message pipeline. The publisher of the message has an expectation about how the consumer handles the message. A contract exists between the two sides. A Message - DO SOMETHING or GIVE ME SOMETHING.

An example: the publisher sends a _message with the raw data_, and expects the consumer to create a file from that data and send a response when the work is done.

Question 7

Explain the difference between _At-most-once_ vs _At-least-once_ message processing in Azure Service Bus

Accepted Answer

* **At-most-once** message processing means that for each message sent, the side effects of its processing may either be _missing_ or _applied once_.

Consider a scenario in which the consumer issues the received request and then crashes before processing it. The Service Bus marks the message as consumed and the application begins consuming messages upon restart. And the app will miss processing the message that it consumed before the crash. The corresponding receiving mode in ASB is called **Receive and delete**.

* In contrast **at-least-once** message processing means that the side effects may be applied _multiple_ times but they may _never be skipped_.

If the application fails to process the message before the lock timeout expires, Service Bus unlocks the message and makes it available to be received again. If the application crashes after it processes the message, but before it requests the Service Bus service to _complete_ the message, Service Bus _redelivers_ the message to the application when it restarts.

* Finally, **exactly-once** message processing means that the side effects are _guaranteed_ to be _applied once_. Exactly once is the most difficult delivery semantic to implement. It is friendly to users, but it has a high cost for the system’s performance and complexity. Exactly once is especially important when duplication is not acceptable and the downstream service or third party doesn’t support idempotency.

Question 8

When would you use Azure Service Bus over Azure Event Grid and vice versa?

Accepted Answer

* Use **Service Bus (Messaging)** - For messaging. It’s about one application telling one or more apps to _DO SOMETHING or GIVE ME SOMETHING_. Receiving is done via polling (long polling) and usually, a namespace is accessed within/by a_ single organization_.
* Use **Event Grid (Eventing)** - For eventing when the applications are saying _SOMETHING HAS HAPPENED_. The service is designed to allow communication between multiple parties and can _span multiple organizations_ as published and/or subscribers.

The rule of thumb is: 
* If you can trust the subscribers to connect to the broker, great, use Azure Service Bus. 
* Otherwise, use Even Grid.

Question 9

Explain the use case for _At-most once_ delivery mode/message processing

Accepted Answer

As the name suggests, **at-most-once **means a message will be delivered not more than once. Messages may be lost but are not redelivered. This is how at-most-once delivery works at a high level.

Use cases: It is suitable for use cases like **monitoring metrics**, where a small amount of _data loss is acceptable_.

Question 10

What is the purpose of the dead-letter queue (DLQ) in Azure Service Bus?

Accepted Answer

The purpose of the dead-letter queue is to hold messages that can't be delivered to any receiver, or messages that couldn't be processed.

There are several activities in Service Bus that cause messages to get pushed to the DLQ from within the messaging engine itself. For example:
- The size quota for this stream has been exceeded.
- The message expired and was dead-lettered. 
- Session-enabled entity doesn't allow a message whose session identifier is null.
- The maximum number of allowed hops when forwarding between queues has been exceeded. This value is set to 4.
- Message couldn't be consumed after maximum delivery attempts.

An application can also explicitly move messages to the DLQ. They can include messages that can't be properly processed because of any sort of system issue, messages that hold malformed payloads, or messages that fail authentication when some message-level security scheme is used.

Once in DLQ messages can be removed from the DLQ and inspected. An application might, with the help of an operator, correct issues and resubmit the message, log the fact that there was an error, and take corrective action.

Top 25 Azure Service Bus Interview Questions

Can Azure Service Bus message be empty?

When would you use Azure Storage Queues vs Azure Service Bus?

What is the difference between `Receive` and `Peek` in the context of Azure Service Bus Topics?

Explain why would you use Scheduled messages in Azure Service Bus?

What are Transactions in Azure Service Bus?

Is it possible to send a message directly to a Subscription queue in Azure Service Bus?

Does Poison Queue exists in Azure Service Bus?

Explain the use case for At-most once delivery mode/message processing

Explain the difference between At-most-once vs At-least-once message processing in Azure Service Bus

What is the difference between Azure Queue Storage and Azure Service Bus with regards to dead letter queues & poison messages?

Compare different message receiving modes in queuing technology

When would you use Azure Event Grid vs Azure Service Bus and vice versa?

When would you use Azure Service Bus over Azure Event Grid and vice versa?

Explain the difference between Event vs Message Services in the context of Azure Services?

What is the purpose of the dead-letter queue (DLQ) in Azure Service Bus?

What are main considerations when creating Azure Service Bus queues/topics?

Explain the use of Express Entities in Azure Service Bus

Is Competing Consumers Patter possible with Azure Service Bus queues?

What is the best way to ensure exact-once delivery using Azure Services Bus?

Provide a use case for At-least once message receiving mode in Azure Service Bus

Explain the difference between Scheduled vs Deferred messages in Azure Service Bus

Explain the difference between Azure Service bus vs. Event Hub vs Event Grid

Do messages in dead-letter queues in Azure Service Bus expire?

Compare Message Sequencing (`SequenceNumber`) vs Sessions in Azure Service Bus. When would you use one vs another?

Explain how to abandon ASB message so it becomes visible again in a way you can control?

Top 25 Azure Service Bus Interview Questions

Can Azure Service Bus message be empty?

When would you use Azure Storage Queues vs Azure Service Bus?

What is the difference between Receive and Peek in the context of Azure Service Bus Topics?

Explain why would you use Scheduled messages in Azure Service Bus?

What are Transactions in Azure Service Bus?

Is it possible to send a message directly to a Subscription queue in Azure Service Bus?

Does Poison Queue exists in Azure Service Bus?

Explain the use case for At-most once delivery mode/message processing

Explain the difference between At-most-once vs At-least-once message processing in Azure Service Bus

What is the difference between Azure Queue Storage and Azure Service Bus with regards to dead letter queues & poison messages?

Compare different message receiving modes in queuing technology

When would you use Azure Event Grid vs Azure Service Bus and vice versa?

When would you use Azure Service Bus over Azure Event Grid and vice versa?

Explain the difference between Event vs Message Services in the context of Azure Services?

What is the purpose of the dead-letter queue (DLQ) in Azure Service Bus?

What are main considerations when creating Azure Service Bus queues/topics?

Explain the use of Express Entities in Azure Service Bus

Is Competing Consumers Patter possible with Azure Service Bus queues?

What is the best way to ensure exact-once delivery using Azure Services Bus?

Provide a use case for At-least once message receiving mode in Azure Service Bus

Explain the difference between Scheduled vs Deferred messages in Azure Service Bus

Explain the difference between Azure Service bus vs. Event Hub vs Event Grid

Do messages in dead-letter queues in Azure Service Bus expire?

Compare Message Sequencing (SequenceNumber) vs Sessions in Azure Service Bus. When would you use one vs another?

Explain how to abandon ASB message so it becomes visible again in a way you can control?

What is the difference between `Receive` and `Peek` in the context of Azure Service Bus Topics?

Compare Message Sequencing (`SequenceNumber`) vs Sessions in Azure Service Bus. When would you use one vs another?