A message generally contains the data itself, not just a reference (like an ID or URL) to data. Sending the data as part of the datagram is less brittle than sending a reference. The messaging architecture guarantees delivery of the message, and because no additional lookups are required, the message is reliably handled. However, the sending application needs to know exactly what data to include to avoid sending too much data, which requires the receiving component to do unnecessary work. In this sense, the sender and receiver of a message are often coupled by a strict data contract.
In a pizza company, when a pizza order is entered, the company would likely use messages. The web front end or mobile app would send a message to the back-end processing components. In the back end, steps like routing to the store near the customer and charging the credit card would take place.
Events
An event triggers a notification that something has occurred. Events are "lighter" than messages and are most often used for broadcast communications.
Events have the following characteristics:
- The event may be sent to multiple receivers, or to none at all
- Events are often intended to "fan out," or have a large number of subscribers for each publisher
- The publisher of the event has no expectation about the action a receiving component takes
The pizza chain would likely use events for notifications to users about status changes. Status change events could be sent to Azure Event Grid, then on to Azure Functions, and to Azure Notification Hubs for a completely serverless solution.
This difference between events and messages is fundamental because communications platforms are generally designed to handle one or the other. Service Bus is designed to handle messages. If you want to send events, you would likely choose Event Grid.
Azure also has Azure Event Hubs, but it is most often used for a specific type of high-flow stream of communications used for analytics. For example, if we had networked sensors on our pizza ovens, we could use Event Hubs coupled with Azure Stream Analytics to watch for patterns in the temperature changes that might indicate an unwanted fire or component wear.
How to choose a communications technology
Consider the following questions:
- Is the communication an event? If so, consider using Event Grid or Event Hubs.
- Should a single message be delivered to more than one destination? If so, use a Service Bus topic. Otherwise, use a queue.
If you decide that you need a queue:
Choose Service Bus queues if:
- You need an at-most-once delivery guarantee
- You need a FIFO guarantee
- 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 would like to be able to publish and consume batches of messages
Choose queue storage if:
- You need a simple queue with no particular additional requirements
- 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
Although the components of a distributed application can communicate directly, you can often increase the reliability of that communication by using an intermediate communication platform such as Azure Service Bus or Azure Event Grid.
Event Grid is designed for events, which notify recipients only of an event and do not contain the raw data associated with that event. Azure Event Hubs is designed for high-flow analytics types of events. Azure Service Bus and storage queues are for messages, which can be used for binding the core pieces of any application workflow.
If your requirements are simple, if you want to send each message to only one destination, or if you want to write code as quickly as possible, a storage queue may be the best option. Otherwise, Service Bus queues provide many more options and flexibility.
If you want to send messages to multiple subscribers, use a Service Bus topic.
