Understanding Event Streams and Event Tables

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 DAS.

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.

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.

1. Log in to the management console, and click Main.
2. Click Event Streams in the Event Processor menu, and then click Add Event Stream.
3. Enter details of the stream definition that you want to create as shown in the below example.
   
4. 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.

1. Log in to the management console, and click Main.
2. Click Event Streams in the Event Processor menu, and then click Add Event Stream.

3. Click switch to source view.

   Click switch to design view to add the event stream using the design view.

4. Enter details of the stream definition that you want to create as shown in the below example.
   
5. Click Add Event Stream, to create the event stream in the system. You can 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.

1. Log in to the management console, and click  Main .
2. Click Event Streams in the Event Processor menu. You view the Available Event Streams list.
3. 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.

1. Log in to the management console, and click Main.
2. Click Event Streams in the Event Processor menu. You view the Available Event Streams list.

3. 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.

1. Log in to the management console, and click Main.
2. Click Event Streams in the Event Processor menu. You view the Available Event Streams list.
3. Click the Event Stream Id of the corresponding event stream for which you want to create the sample event. 
4. 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. 
5. You view details of the event stream as shown below. 
   
6. Click Generate Event to create the sample event.

Event formats

{
  "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"
    }
  ]
}

<events>
    <event>
        <metaData>
            <ip>data4</ip>
        </metaData>
        <correlationData>
            <id>56783</id>
        </correlationData>
        <payloadData>
            <testMessage>data1</testMessage>
        </payloadData>
    </event>
</events>

{
    "event": {
        "metaData": {
            "ip": "data4"
        },
        "correlationData": {
            "id": "545455"
        },
        "payloadData": {
            "testMessage": "data1"
        }
    }
}

meta_ip:data1,
correlation_id:323232,
testMessage:data2

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.

1. Log in to the management console.
2. 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.

Analytics event table

In batch analytics, an event table is used to persist events from a stream, and to later lookup/update/delete from it. The Data Analytics Server contains an event table implementation based on its Data Access Layer, where users can create an event table based on the underlying configured data source of the server.

The analytics event table will follow the best approach in carrying out its tasks, e.g. using the primary keys if there are no non-equal conditional expressions in the queries etc.. or else, for conditions that require less than, greater than, less than or equal, greater than or equal, contains, then the respective fields must be marked as indexed.

The syntax in using the analytics event table is as follows:-

@from(eventtable = 'analytics.table' , table.name = <analytics_table_name>, primary.keys = <primary_keys>, indices = <indices>, wait.for.indexing = <wait_for_indexing_flag>, merge.schema = <merge_schema_flag>)
define table <EventTableName> (<schema>);

@from(eventtable = 'analytics.table' , table.name = 'stocks', primary.keys = 'symbol', indices = 'price, volume -sp', wait.for.indexing = 'true', merge.schema = 'false')
define table StockTable (symbol string, price float, volume long);