Multichannel

Concept

The Multichannel QoS Policy is used to partition the data published by a DataWriter across multiple channels. A channel is defined by a filter expression and a sequence of multicast locators.

By using this QosPolicy, a DataWriter can be configured to send data to different multicast groups based on the content of the data. Using syntax similar to those used in Content-Based Filters, you can associate different multicast addresses with filter expressions that operate on the values of the fields within the data. When your application's code calls write(), data is sent to any multicast address for which the data passes the filter.

Example Description

This example shows how to use multichannel DataWriters. You will learn how to:

• Use the STRINGMATCH filter
• Exercise the content-filtered topic (CFT) API
• Configure the MULTICHANNEL QosPolicy
• Observe the performance benefits of using a multichannel DataWriter

The Publisher application publishes round-robin instances with Symbols "A" through "Z" on 8 different channels of a multichannel DataWriter. By default, the XML profile is used to specify the QoS. On the create_datawriter() call, you can toggle between writer_qos and DATAWRITER_QOS_DEFAULT to switch between using code and using XML.

The Subscriber application uses a ContentFilteredTopic (CFT) with the STRINGMATCH filter, then dynamically modifies the CFT to match "A,D", and later just "D". You will notice that the DataReader does not have to specify multiple multicast receive addresses, because it learns about the DataWriter's receive_addresses upon endpoint discovery and then receives on them.

The example includes also two pcap files captured with Wireshark that show the traffic using multichannel and single channel.

• multichannel.pcap shows the wire traffic when multichannel is used.
  Notice that data samples are only being sent on the channels that are necessary for delivering samples that pass the DataReader's filter.
• singlechannel.pcap shows the wire traffic when multichannel is not used and when the DataReader specifies a multicast.receive_address of 239.255.0.1. Observe that all data samples are being sent on the wire (i.e., reader-side filtering), since there is only one channel.

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