Messaging Services

Messaging Services In Azure

A simple queue service useful to store a large number of small size messages
REST-based HTTP verbs (i.e. GET and PUT) to read and insert messages into the queue
Uses queue concept: application can also look at the message on the head of the queue using the PEEK command
Usage case:
- workloads or tasks to be processed asynchronously
- message order guarantee is not important

Event Grid is an eventing backplane that enables event-driven, reactive programming. It uses the publish-subscribe model
Fully-managed event routing service
Routes descrete events to event handlers
- messages are pushed to event handlers
- handlers should be capable of handling the load or implement a throttling mechanism
Use cases
- an application needs to react to the changes that happened in another service (i.e. process a file when a new file is uploaded in blob container)
- filter and fan out an event to multiple targets (i.e. Azure Function, Azure Logic Apps etc.)

Azure Event Hubs is a big data streaming platform and event ingestion service
It also supports Advanced Message Queuing Protocol (AMQP)
Azure Event hub supports Kafka API (allowing applications to migrate without code change which are using Kafka as message broker)
Usae cases
- high-speed data ingestion (telemetry, distributed data streaming)
- aggregate stream of events
- to publish stream of ‘state change’ events and “any specific action from consumers” is not expected (i.e. time series click-stream from a website to track user)
- stream from millions of IoT devices

Service Bus is a fully managed enterprise message broker with message queues and publish-subscribe topics
Enterprise grade integration message broker: asynchronous transfer of data between applications
It supports Advanced Message Queuing Protocol (AMQP) which is a TCP based efficient and reliable open protocol and makes a solution vendor-neutral
Service Bus includes advanced features that enable you to solve more complex messaging problems: https://docs.microsoft.com/en-us/azure/service-bus-messaging/advanced-features-overview

Event vs Message

Message:
- A message is raw data produced by a service to be consumed or stored elsewhere
- Use “Azure Service Bus”
Event:
- 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
- Discrete event: use “Azure Event Grid”
- Stream of events: use “Azure Event Hub”
See: Event vs. message services

Event Hub vs Event Grid

Event Hubs accept only endpoints for the ingestion of data and they don’t provide a mechanism for sending data back to publishers. On the other hand, Event Grid sends HTTP requests to notify events that happen in publishers
Event Grid can trigger an Azure Function. In the case of Event Hubs, the Azure Function needs to pull and process an event
Drability:
- Event Grid is a distribution system, not a queueing mechanism. If an event is pushed in, it gets pushed out immediately and if it doesn’t get handled, it’s gone forever unless we send the undelivered events to a storage account (This process is known as dead-lettering, by default this option is disabled, so if we need to enable it, we have to specify a storage account at the creation of the Event Grid)
- In Event Hubs, publishers and subscribers read and write from durable storage. The data can be kept in the Event Hubs for up to seven days and then replayed. This gives us the ability to resume from a certain point or to restart from an older point in time and reprocess events when we need it
Event Grid doesn’t guarantee the order of the events. Contrarily, Event Hubs use partitions and these partitions are ordered sequences, so it can maintain the order of the events in the same partition
Courtesy: https://www.cognizantsoftvision.com/blog/azure-event-grid-vs-event-hubs/