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Events / Notification Hub
The Notification Hub uses the Publish/Subscribe approach to message handling.
For a given event, you can opt to subscribe using specific API calls.
The currently available events will be either Create, Delete or Amend against the following entities.
Worker
WorkerPost
AbsenceHeader
Post
Grade
WorkPattern
PositionStatus
ServiceCondition
Location
MyFormInstance
Subscribing to Events
A webhook is a user-defined Hypertext Transfer Protocol (HTTP) call-back that is triggered when a particular event occurs at the source site. When the event occurs, the source site makes a HTTP request to the URL specified in the subscription.
A subscription is a contract between a source and destination site that is triggered whenever changes happen to the subscriptions. A webhook is then used to send out an event notification to your service. A subscription can be created for a particular event so that subscribers get notified only when something very specific happens.
Alternatively, notifications can be controlled by event type. For example, if you are not interested in delete events, you can set a flag to ignore them.
Event Grid
Please note that the ecosystem of Notification Hub includes Microsoft Event Grid resource as shown in the architecture diagram above. It is logically placed between the instance of the Notification Hub application and the client webhook and serves the purpose of providing resilience and an additional layer of security.
Upon receiving a subscription request from an authorised client, Notification Hub registers the webhook on Event Grid as a topic, which is a collection of related events. Customers (subscribers) can decide which topics to subscribe to. The following is an example response when subscribing to a topic:
Dead Letter Queue
As part of the Notification Hub internal audit, we have implemented a Dead Letter Queue which enables a subscribing service to assess undelivered notifications and action them as appropriate. Event Grid will manage to retry attempts and based on parameters will attempt to retry messages a set number of times over a set period. Once those parameters have been exceeded with continued failure, the event will be posted to the queue and will then be available for action should the need arise.
Validation
Upon receiving the registration of a webhook, Event Grid sends out a request to validate the authenticity of a webhook and seek consent for sending future notifications. The following is an example of a sample request sent.
Once you’ve configured the URL, the Notification Hub will post the event as a JavaScript Object Notation (JSON) payload to your webhook URL via Event Grid.
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