Understanding Event Streams

Events are the lifeline of WSO2 CEP/DAS. They not only process data as events, but also interact with external systems using events. Event is a unit of data, and an event stream is a sequence of events of a particular type. The type of events can be defined as an event stream definition. The following sections explain how to work with events in WSO2 CEP.

Event streams

You can manage event streams through event stream definitions.

Event stream definition
Adding an event stream
Using the source view
Deleting an event stream
Editing an event stream
Creating sample events

Event stream definition

Definitions of the event streams are stored in the filesystem as deployable artifacts in the <PRODUCT _HOME>/repository/deployment/server/eventstreams/ directory as .json files. These are hot deployable files and can be added/removed when the server is up and running. A sample event stream definition is as follows.

{
  "streamId": "org.wso2.test:1.0.0",
  "name": "org.wso2.test",
  "version": "1.0.0",
  "nickName": "TestStream",
  "description": "Test Stream",
  "metaData": [
    {
      "name": "ip",
      "type": "STRING"
    }
  ],
  "correlationData": [
    {
      "name": "id",
      "type": "LONG"
    }
  ],
  "payloadData": [
    {
      "name": "testMessage",
      "type": "STRING"
    }
  ]
}

The properties of the above event stream definition are described below.

Property Description

Event Stream Name Name of the event stream.

Event Stream Version Version of the event stream. (Default value is 1.0.0.)

Event Stream Description Description of the events stream. (This is optional.)

Event Stream Nick-Name Nick-names of an event streams separated by commas. (This is optional.)

Stream Attributes

Stream attributes contains the data of the event. Data is divided into the following 3 logical categories for maintenance and usability. It is not required to to have attributes for all 3 categories, but there should be at least one category with at least one attribute defined. The attribute names should be unique within each category.

Meta Data: This refers to meta information of the events (e.g., timestamp, host, IP, etc.). They are received with the meta tag (i.e., in the meta_<attribute name> format).
Correlation Data: This refers to information that is used to correlate multiple events (e.g., correlation_id,temporal_id etc.). They are received with the correlation tag (i.e., in the correlation_<attribute name> format).
Payload Data: Contains the actual data that the event intends to have. (Referred to as <attribute name>.)

e.g., The following attributes exist in a single event.

event_timestamp
request_IP_address
correlation_Id
price
symbol

These attributes can be logically categorized as follows.

Category	Attributes	Notes
Meta Data	`event_timestamp` `request_IP_address`	These attributes provide information about the events themselves.
Correlation Data	`correlation_Id`	This attribute correlates the event with other events from other streams, and it is useful when performing join operations on a stream.
Payload Data	`price` `symbol`	These are information provided via the event.

Note that the internal system does not differentiate the attributes based on this categorization. However, it is recommended to categorize the attributes for the purpose of organizing them in a logical manner within the stream definition.
If you edit an attribute in an event stream or add a new attribute to it, other artifacts that are associated with the stream will be inactive. Therefore, it is recommended to redefine the stream with a new version including the additional/changed attribute.

Adding an event stream

You can create an event stream by creating a new event stream definition using the design view or the source view.

Using the design view

Follow the steps below to add an event stream using the design view.

Log in to the management console, and click Main.
Click Event Streams in the Event Processor menu, and then click Add Event Stream.
Enter details of the stream definition that you want to create as shown in the below example.
Click Add Event Stream, to create the Event Stream in the system. When you click OK in the pop-up message on successful addition of the stream definition, you view it in the Available Event Streams list as shown below.

Using the source view

Follow the steps below to add an event stream using the source view.

Log in to the management console, and click Main .
Click Event Streams in the Event Processor menu, and then click Add Event Stream.
Click switch to source view.

Click switch to design view to add the event stream using the design view.
Enter details of the stream definition that you want to create as shown in the below example.
Click Add Event Stream , to create the Event Stream in the system. Y ou view the new event stream in the Available Event Streams list as shown below.

Deleting an event stream

Follow the steps below to delete an event stream by deleting the corresponding event stream definition.

Log in to the management console, and click Main .
Click Event Streams in the Event Processor menu. You view the Available Event Streams list.
Click the Delete button of the corresponding event stream to delete it.

Editing an event stream

Follow the steps below to edit an event stream by editing the corresponding event stream definition.

Log in to the management console, and click Main.
Click Event Streams in the Event Processor menu. You view the Available Event Streams list.
Click the Edit button of the corresponding event stream to edit it.

Click the switch to source view link to edit an event stream using the source view.

Creating sample events

Follow the steps below to create sample events for a defined event stream.

Log in to the management console, and click Main.
Click Event Streams in the Event Processor menu. You view the Available Event Streams list.
Click the Event Stream Id of the corresponding event stream for which you want to create the sample event.
Select the event format type (i.e. xml, json, or text) from the drop down list, which you want to create the sample event in.
You view details of the event stream as shown below.
Click Generate Event to create the sample event.

Event formats

WSO2 CEP/DAS facilitates the following default and custom event formats.

Default event formats
Custom event formats

Default event formats

By default, WSO2 CEP/DAS represents an event as a WSO2Event object. Furthermore, WSO2 CEP/DAS supports events in XML, JSON, Text and Map formats. The default event formats of the XML, JSON, Text and Map representations for the following sample event stream definition are as follows.

Sample event stream definition

{
  "streamId": "org.wso2.test:1.0.0",
  "name": "org.wso2.test",
  "version": "1.0.0",
  "nickName": "TestStream",
  "description": "Test Stream",
  "metaData": [
    {
      "name": "ip",
      "type": "STRING"
    }
  ],
  "correlationData": [
    {
      "name": "id",
      "type": "LONG"
    }
  ],
  "payloadData": [
    {
      "name": "testMessage",
      "type": "STRING"
    }
  ]
}

Custom event formats

If you receive and publish events with a different format than the default format, you need to provide appropriate mappings for the system to interpret the events.

Custom formats for receiving events

For information on the custom event receiver mappings, see Input Mapping Types.

Custom formats for publishing events

For information on the custom event publisher mappings, see Output Mapping Types.

Event Flow

Event flow visualizes the stream flow in WSO2 CEP/DAS to easily navigate to different WSO2 CEP/DAS components.

Follow the steps below to view the event flow.

Log in to the management console.
Click Main, and then click Event Flow in the Event Processor menu.

This demonstrates how all the active event receivers, event streams, event publishers, and execution plans are connected.