Introduction to SharePoint 2016 Add-ins (aka apps)

SharePoint Add-ins (aka apps) are used to extend SharePoint Applications through Client APIs.

Types of SharePoint Add-ins
Based on the type of hosting, the Add-ins are categorized into 2 types:
1) SharePoint-Hosted Add-ins : Contains SharePoint components like Lists, Web Parts, Workflows etc on a sub web called the add-in web (aka app-web). Only client side code can be used to access SharePoint.
2) Provider-Hosted Add-ins :  Contains components that are deployed and hosted outside SharePoint farm. Both client side code and server side code can be used for development.

Packaging Add-ins
1) Organization's add-in catalog - It is a dedicated SharePoint site collection in the SharePoint Online subscription or on-premise farm. This method is used when the add-in is custom-made for a particular organization.
2) Office Store - The store handles the marketing process for you, from discovery to purchase to updates. Microsoft has a Seller Dashboard to help you sell add-ins through the Office Store.

Advantages of Add-in Model
1) There is no dependency on Server Side code since it executes on Client side.
2) Add-ins communicate with SharePoint using Client side object model (CSOM) or Rest Services with OAuth being the authentication mode.
3) Remarkable usability in Tablets and Mobile devices.
4) One step ahead for development in future where cloud hosting could be more preferrable.
5) In SharePoint 2016 everything is an Add-in including Lists, Libraries etc.
6) SharePoint Add-ins can be developed with any programming language or technology, provided they are hosted outside SharePoint.
7) SharePoint Add-ins can be created using PHP, Java, or any other technology capable of communicating with SharePoint via the new REST API and the OAuth protocol.

Types of Configuration
1) Full-Page: These Add-ins have an independent UI irrespective of the Sites on which they are hosted. A "Back" button should be provided to navigate back to the parent site which triggered the Add-in.
2) Add-in Parts: Add-in parts renders inside an IFrame from whithin the pages of the parent site. They are also known as Client Parts.
3) UI command extension: These Add-ins are used to extend the UI of the parent site. For example, you can create an Add-in to add a new button to the ribbon.

Security Perspective
From a security point of view one can trust SharePoint 2016 Add-in model to be thouroughly secured.
A definite set of permissions are requested by the SharePoint Add-in when it is installed. The Add-in will be installed only if the current user can grant the requested permissions, otherwise the Add-in will not install. Pretty straight formward. Isn't it!!

Development Tools for creating Add-ins
1) "Napa" office 365 Dev tools: This tool is purely client side and ideal for creating SP hosted Add-ins. It comes with Office 365 subscription.
2) Visual Studio: For developing Add-ins using Microsoft .NET and Microsoft Visual Studio 2015, you will need the Office Developer Tools for Visual Studio 2015.

Note: You also will need to install the following tools and libraries if you working on a machine outside the SharePoint 2016 environment.
i) SharePoint client components
ii) Windows Identity Foundation SDK
iii) Workflow Tools SDK and Workflow Client SDK
iv) Windows Identity Foundation SDK and Windows Identity Foundation extensions

Programming model for SharePoint
There are three ways that an Add-in can communicate with  SharePoint:
1) .NET / Silverlight API
2) JavaScript API
3) REST API