Event Driven Development in Office 365
On webhooks
The beginning: Event Receivers
Since the early days of SharePoint on-premises and we can easily register event receivers on the web, list and item levels to trigger custom actions when a particular event occurs. These abilities were moved along to SharePoint online where the custom action was hosted in a remote endpoint (Remote Event Receivers).
You can register event receivers based on the list template or item content type. Similar technique exists in Project Server and Project Online with a slight difference in naming (Event Handlers). So basically we've been doing event driven development in SharePoint Online since the beginning of Office 365.
What's not good about Event Receivers
1. Unlike SharePoint on-Premises remote event receivers is loosely coupled from SharePoint online and if the event receiver endpoint is down for some reason SharePoint online won't retry executing the event again with the exception of all App related remote event receiver.
2. Remote event receivers and Project online event handlers are built as WCF endpoints so SharePoint online and Project online send SOAP message to these endpoint (not very portable huh!)
A Whole new world!
Microsoft is gradually replacing traditional remote event receivers by one of the following options:
Microsoft Flow
Easy to use tool with a nice user-friendly interface which allows super users and IT pros to build event driven scenarios with more than 446 templates and 85+ triggers including generic HTTP endpoint which opens the door for unlimited possibilities. Microsoft flows offer an admin interface via https://flow.microsoft.com where the IT pros can build self-service flows using the browser interface.
Azure Logic Apps
Microsoft flows is built on top of azure logic apps. They both have the same designer and same list of connectors. Azure logic apps is the preferable option in B2B mission critical scenarios. Azure logic apps is managed like any other azure service via Azure portal. Flows can be designed using browser or Visual Studio using azure logic apps extension for visual studio
which requires Azure Resource manager SDK to be installed first
here is the designer within Visual studio 2015.
Webhooks
Webhooks is another option which also more suitable for developers and B2B applications. Webhooks gives you a mean to build an event driven application on a massive scale without the need to have an azure subscription. Webhooks introduced first within Microsoft graph outlook resources like mail, contact, and calendar. In addition to Microsoft Graph resources, it has been introduced to SharePoint as well.
You can create a subscription via a simple POST request
Webhooks will enable you to build more complex solutions where the subscription or the flow-trigger can be created on the fly based on another trigger.