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Activities signify an event or action occurring within your application. The Optimizely Community API allows for activities to be published, within an application, resulting in the generation of corresponding feed items for subscribing users. It also provides the opportunity for your application to react to these activities in real-time.

An activity is represented in the system using the Activity class.

## Identify an actor and target

An `Activity` identifies both an actor and a target:

  • `Actor` – User who triggered activity.

  • `Target` – Resource or user upon which the activity occurred.

The actor is identified with an instance of the Reference class.

The target is also identified with an instance of the Reference class, which may represent a resource, such as a page or product or a user within your application.

The scheme applied to define these target and references is designed within your application.

For more information on how to construct References, including best practices, see _References and IDs_ in [Discover the platform](🔗).

## Design an activity

The Activity Streams feature leverages the platform's concept of Composites, providing developers with complete control over the design of their application's activities.

Activities published within the platform are accompanied by data of your design. This data, intended to encapsulate information defining the particular activity that occurred, is available upon retrieval of feeds for subscribing users.

For example, an application wants to broadcast an announcement to interested users whenever a product is rated. The application adds an activity representing this action through the Activity Streams system, resulting in the generation of feed items for subscribing users and the notification of handlers registered to react to the event in real-time.

An example of a class definition, representing the activity of rating a page, appears below.

For information and best practices on designing such types, see _Defining extension data_ in [Discover the platform](🔗).

## Manage activities

In the Optimizely Community API, activities are managed through a service implementing the interface `IActivityService`. This service provides the ability to add and respond to activities in your application.

### Access an IActivityService

When the Activity Streams feature is installed to an Optimizely Content Management System (CMS) site, with the feature's site integration package, you can get an instance of this service from the inversion of control (IoC) container.


When the feature is installed to a non-Optimizely CMS site, you can get an instance of a service from the default factory class provided within the package.


## Add an activity

You can publish an activity to the Optimizely Community Activity Streams system by using the `Add<TExtension>(Activity,TExtension)` method of the `IActivityService`. This method accepts an instance of the Activity class and an instance of `TExtension``TExtension` is a type, defined within your application, used to describe the activity that occurred.

When the system receives an activity, it scans existing subscriptions to identify subscribers that are interested in the received activity. If any matching subscriptions are found, the system generates a feed item for all eligible subscribers.

A feed item is generated for:

  • Any subscription following the target of the activity.

  • Any subscription following the actor who performed the activity.

Adding an activity also results in the notification of any handlers registered to watch for activities of the type represented by `TExtension`.

Adding an activity is a non-blocking operation. The activity is queued, and control is immediately returned to your application. The generation of feed items is not instantaneous; the items are generated as the system is able to process them.

The example below demonstrates the announcement of the contribution of a rating to a page, using the PageRated definition described in Designing an activity.

In the next example, a similar approach is used to add an activity targeting a user. Consider an activity using the Vote definition, described below:

To add an activity, whereby the user "Chris" votes for the user "Carlos", consider the sample below:

## Watch activities

In addition to generating feed items, the Activity Streams feature handles activity notifications. A developer may register a handler, which the platform invokes upon receipt of activities that the handler is capable of processing.

You register a handler to receive activity notifications for a particular type of activity by using the `Watch<TExtension>(IActivityHandler)` method. The method accepts an instance of `IActivityHandler<TExtension>`, defining the operation to be executed when an activity is received. The type parameter, `TExtension`, identifies the type of activity payload that the handler is capable of handling.

The execution of handlers is not an instantaneous operation. Their execution does not block a request, and they are executed as the system can process them. The order of handler execution is not guaranteed. Implement handlers so that their behavior is not dependent on the execution of another handler.

The example below demonstrates an implementation of `IActivityHandler` for the PageRated activity, defined in previous examples.

An instance of this handler can be registered to receive notification upon receipt of PageRated activities as shown below.

A handler only needs to be registered once in the lifecycle of your application, typically during application startup or initialization. (Registering the same handler multiple times results in multiple invocations of that handler for a single activity.)

If you are registering a handler in an Optimizely site, you could use an initialization module to perform this setup.

## Best practices for watching activities

### Stateless operations

It is important to consider that handlers are executed within the context of your application. It is advantageous to implement handlers as stateless operations to account for the possibility that instances of your application may be distributed.

### Short-lived operations

The execution of a handler occurs in the background and does not block the execution of the thread in which an activity is being added into the system. However, the execution of one handler may block the execution of subsequent handlers. It is recommended that handlers registered to watch activities execute only short-lived operations. Longer running operations should be offloaded to services or systems more suitable to accommodating them.