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Channel Configuration
to be aware of its shape (that is, no DDL is required) and does not need the channel to
act as intermediary.
In general, use a channel between components when:
• Buffering is needed between the emitting component and the receiver.
• Queueing or concurrency is needed for the receiving component.
• If a custom adapter is used and thread control is necessary.
5.2 Channel Configuration
When you add a channel to your Event Processing Network (EPN), it has a default
configuration. The default channel has a name, an ID, is a system time-stamped stream
channel, and has a default heartbeat time out of 100 milliseconds or 100,000,000
nanoseconds.
The default configuration is adequate for most applications.You can modify the
configuration by editing the application assembly file or by editing the component
configuration file.
When a channel is time stamped by the system, Oracle Stream Explorer assigns a new
time from the CPU clock when a new event arrives and when the configurable
heartbeat time out expires.
When a channel is time stamped by an application, the time stamp of an event is
determined by the wlevs:expression element. A common example of an
expression is a reference to a property on the event. If no expression is specified, then
the time stamp can be propagated from a prior event. For example, when a channel
that is time stamped by the system from one Oracle CQL processor feeds events into a
channel that is time stamped by an application of another downstream Oracle CQL
processor. In addition, an application can use the StreamSender.sendHeartbeat
method to send an event of type heart-beat downstream to StreamSink listeners
in the EPN.
Note:
When a channel is both application time stamped and map-based (uses a hash
map event type), Oracle Stream Explorer adds a time stamp. A delete or
update operation without a key does not work on a channel with this
configuration because application time stamped events hold an always
changing timestamp property.
This chapter describes some of the assembly and configuration file channel settings
For a complete reference, see Schema Reference for Oracle Stream Explorer.
5.2.1 Assembly File
The assembly file shows the channel settings for the helloworldInputChannel. The
settings indicate that helloworldProcessor listens to the channel for events, and that
events flow into the channel from helloworldAdapter.
<wlevs:channel id="helloworldInputChannel" event-type="HelloWorldEvent" >
<wlevs:listener ref="helloworldProcessor"/>
<wlevs:source ref="helloworldAdapter"/>
</wlevs:channel>
5-2 Developing Applications for Event Processing with Oracle Stream Explorer