An add-on contains components that extend the functionality of an Optimizely Content Management System (CMS) website, such as initializable modules, gadgets, visitor group criteria, virtual path providers, page and search providers, and so on. See also [Apps and integrations](🔗) and [Add-ons platform compatibility](🔗).
## Configure shell modules
An add-on is packaged and deployed as a shell module. Configure modules in _web.config_ to associate a virtual path to an assembly that contains gadgets and other plug-ins. The following example defines a module of the assembly `QuickChat.dll` and associates it with the virtual path _/modules/QuickChat_.
The following example configures a `PublicTemplates`  module. The module’s disk location is the root of the application (~/), which means that relative references to client resources and menu items are based on this URL. Furthermore, Optimizely maps the module to an assembly, `EPiServer.Templates.Public`, which means Optimizely routes controllers in this assembly through the path format \_\*/modules/PublicTemplates/{controller}/{action}\_ The shell initialization module set up this route.
## protectedModules and publicModules
These elements contain collections of shell modules. Optimizely maps `protectedModules` to a path protected by ASP.NET authorized rules, and maps `publicModules` to a path that is open to everyone.
Attributes:
`
rootPath` – The root path where modules are located and routed. Required.`
autoDiscovery` – Option for probing the module folder and loading module assemblies automatically on start-up. The default is Minimal. If you change this to Modules, folders are automatically added as modules below the root path.
### Module (add element)
The `publicModules` and `protectedModules` elements in _web.config_ contain a collection of registered modules. These modules define assemblies and virtual paths. By default, a module is expected to be found in a folder whose name matches the module. This folder should be located in the configured `rootPath` (as configured per public and protected modules).
Attributes:
`
name` – Required. The name of the module used to find the module directory.`
resourcePath` – Optional. The path of the module to override the default location of the module.`
clientResourcePath` – Optional. An alternative location for client resources. By default, this path is the same as `resourcePath`.
Elements:
`
assemblies` – Assemblies to load and associate with the module. You can combine this value with assemblies defined by the module depending on the auto discovery option.
Example (_web.config_):
## Module manifest
Each module has a manifest file (_module.config_) in the root directory, where you can specify additional module-specific settings.