Messages? Events? or Both?

Most of what I discuss here is culled from https://docs.microsoft.com/en-us/learn/modules/choose-a-messaging-model-in-azure-to-connect-your-services/2-choose-whether-to-use-message-queues-or-events.

If you are building a distributed application you will want to ensure the application is both reliable and scalable. You will often leverage communication components to accomplish these objectives. This will necessitate a choice between messages, events or some combination of both? So how do you choose?

Let’s first define what we mean by messages and events.

Messages

Messages have the following characteristics:

  • Contains the raw data, all of it, just not a reference to the data, that the receiving component will consume.
  • The sending component expects the receiving component to process the message content in a specific way.
  • Integrity of the system may depend on the both the sending component and receiving component doing their job.

Events

Events have the following characteristics:

  • Lightweight notifications that indicate something occurred within the system.
  • May be sent to multiple receiving components or none at all.
  • Intended to “fan out”, multiple receiving components (subscribers) for each sending component (publisher).
  • No processing expectations from the sending component.
  • Some events are discrete and unrelated to other events, while some may be part of a related or ordered series of events.

How do I choose?

It is very common for an application to use BOTH events and messages, the best choice should be based on a specific use case.

Messages should be used if the processing of communications need a guarantee that they occurred, while events should be used for broadcasts.

For each communication, ask yourself, does the sending component expect the communication to be processed in a particular way by the destination component?

  • If the answer is YES, then choose to use messages.
  • If the answer is NO, then choose to use events.

See https://docs.microsoft.com/en-us/learn/modules/choose-a-messaging-model-in-azure-to-connect-your-services/2-choose-whether-to-use-message-queues-or-events for more information.

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