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The events system provides a mechanism for distributing events between IIS applications and Optimizely Content Management System (CMS) websites in a load-balanced environment. The event system in CMS is provider-based, and you can either use the built-in event provider or develop your own customized one.

## Event providers

### Built-in event provider

The built-in event provider is based on Microsoft Windows Communication Foundation (WCF) and requires some manual WCF configuration steps to enable. It uses either UDP broadcasting or TCP, which requires detailed knowledge about ports, firewalls and endpoints. To use elastic scaling (adding and removing servers on demand) with the WCF provider, you need to use UPD broadcasting because TCP requires you to manually configure all endpoints.

Elastic scaling in cloud environments does not work well using the built-in WCF provider since you may not have detailed control over networks and servers. An event provider, optimized for [Azure Web Apps](🔗), is available on NuGet. The event provider for Azure uses Service Bus for distributing events and requires no configuration of servers or network details. An event provider for [Amazon Web Servers](🔗) also is available on NuGet.

### Customized event provider

Another alternative is to develop your own custom event provider, adapted for your hosting environment.

## Configure an event provider

For the built-in WCF-based event provider, it is enough to define WCF end-points (as described in [Configuring events over WCF](🔗), because it is the default provider.

To use another custom event provider, add the provider configuration to the event element in the **episerver.framework** section of the _web.config_. For example:

See [Use the Event API](🔗).

## Event system process flow

The following procedure describes the logical setup and execution of event system actions.

  1. When an Episerver CMS website starts, it does the following: a. Creates a secret if one does not already exist in the site database. b. Initializes the Event Broker responsible for sending and receiving event messages remotely. c. Initializes each Event Provider that has been configured. d. Enables the Event system when the initialization process has completed (for example when CMS or Optimizely Customized Commerce has completed initialization).

  2. Code in a site obtains an `Event` object instance and subscribes to the Raised and optionally to the Missed event.

  3. Code in a site obtains an `Event` object instance and calls the `Raise` method (the Cache Manager could raise a cache changed event for example).

  4. The `Event` object instance fires the `Raised` event to notify the local subscribers.

  5. The `Event` object instance informs the Event Broker about the event.

  6. The Event Broker sends the event to all the other sites on the same server and other servers in the network using the configured Event Provider. In addition to the event information, a cryptographic HMAC block also is sent in the message to let the receiver test the authenticity of the event (this functionality can be disabled by the provider if a secure channel is already present).

  7. The Event Broker on another instance (either an IIS site on the local server or remote server) receives the sent message and does the following: a. Makes sure the message was sent from another instance sharing the same database. If no match is found, the action will be logged, the message dropped and no further action taken. b. Verifies the HMAC using the event data and the secret stored in the database (unless the provider disabled validation). c. If the verification fails, the action is logged, the message dropped, and no further action taken. d. Obtains the `Event` object instance for the event and calls the Raise method to notify the local subscribers.

## Test the event system

Examples of CMS functionality that use the events system are Cache updates. To test cache events, edit a page on one CMS site and ensure the change is reflected on the 2nd load-balanced CMS website:

  1. On CMS #1, navigate to a page in the example website.

  2. On CMS #2, enter edit view and navigate to the same page as in step 1.

  3. On CMS #2, add/edit/delete some text on the page and click **Publish**.

  4. On CMS #1, refresh the page and ensure it reflects the new information saved in step 3.