Azure service bus topics vs Queues and example of which one should be used and when ?

Azure Service Bus provides two types of messaging entities: queues and topics. Both queues and topics can be used to implement messaging patterns like point-to-point and publish-subscribe, but they have different characteristics and are best suited for different scenarios.

Azure Service Bus Queues:

• Queues provide a one-to-one messaging pattern, where a message is sent to a single recipient (receiver) that retrieves and processes the message from the queue.
• The messages in the queue are processed in the order they were received.
• Queues guarantee message delivery to a single recipient in a first-in, first-out (FIFO) order.
• Queues can be used for load leveling, as multiple receivers can be set up to retrieve and process messages from the same queue.

Azure Service Bus Topics:

• Topics provide a one-to-many messaging pattern, where a message is sent to a topic and all subscribers receive a copy of the message.
• Subscribers can filter the messages they receive based on message properties.
• Topics are used for publish-subscribe scenarios, where multiple subscribers need to receive the same message.
• Topics provide a way to decouple the sender and the receivers, as the sender doesn't need to know who the subscribers are or how many there are.

Here's an example of when to use a queue versus a topic:

Suppose you have an e-commerce website that needs to process orders. Each order consists of multiple items, and each item needs to be processed by a different component in your system. You have two options to implement the message passing mechanism:

1. Use a queue: In this case, you can have a single queue for all the orders, and each item in the order is sent as a separate message to the queue. Each component can retrieve and process the messages from the queue in a FIFO order, and you can use multiple instances of the component to process messages in parallel.

2. Use a topic: In this case, you can have a topic for each component, and each item in the order is sent as a message to the corresponding topic. Each component subscribes to its own topic and receives only the messages that match its subscription filter. This way, each component processes only the messages it needs to process, and you can add or remove components without affecting the other components.

In summary, queues are best suited for point-to-point scenarios where messages need to be processed in a specific order, while topics are best suited for publish-subscribe scenarios where multiple subscribers need to receive the same message.