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Multi-channel DataWriters

In Connext DDS, producers publish data to a Topic, identified by a topic name; consumers subscribe to a Topic and optionally to specific content by means of a content-filter expression.

A Market Data Example:

A producer can publish data on the Topic "MarketData" which can be defined as a structured record containing fields that identify the exchange (e.g., "NYSE" or "NASDAQ"), the stock symbol (e.g., "APPL" or "JPM"), volume, bid and ask prices, etc.

Similarly, a consumer may want to subscribe to data on the "MarketData" Topic, but only if the exchange is "NYSE" or the symbol starts with the letter "M." Or the consumer may want all the data from the "NYSE" whose volume exceeds a certain threshold, or may want MarketData for a specific stock symbol, regardless of the exchange, and so on.

The middleware’s efficient implementation of content-filtering is critical for scenarios such as the above "Market Data" example, where there are large numbers of consumers, large volumes of data, or Topics that transmit information about many data-objects or subjects (e.g., individual stocks).

Traditionally, middleware products use four approaches to implement content filtering: Producer-based, Consumer-based, Server-based, and Network Switch-based.

Producer-based approaches push the burden of filtering to the producer side. The producer knows what each consumer wants and delivers to the consumer only the data that matches the consumer's filter. This approach is suitable when using point-to-point protocols such as TCP—it saves bandwidth and lowers the load on the consumer—but it does not work if data is distributed via multicast. Also, this approach does not scale to large numbers of consumers, because the producer would be overburdened by the need to filter for each individual consumer.
Consumer-based approaches push the burden of filtering to the consumer side. The producer sends all the data to every consumer and the middleware on the consumer side decides whether the application wants it or not, automatically filtering the unwanted data. This approach is simple and fits well in systems that use multicast protocols as a transport. But the approach is not efficient for consumers that want small subsets of the data, since the consumers have to spend a lot of time filtering unwanted data. This approach is also unsuitable for systems with large volumes of data, such as the above Market Data system.
Server-based approaches push the burden of filtering to a third component: a server or broker. This approach has some scalability advantages—the server can be run on a more powerful computer and can be federated to handle a large number of consumers. Some providers also provide hardware-assisted filtering in the server. However, the server-based approach significantly increases latency and jitter. It is also far more expensive to deploy and manage.
Network Switch-based approaches leverage the network hardware, specifically advanced (IGMP snooping) network switches, to offload most of the burden of filtering from the producers and consumers without introducing additional hardware, servers or proxies. This approach preserves the low latency and ease of deployment of the brokerless approaches while still providing most of the off-loading and scalability benefits of the broker.

RTI supports the producer-based, consumer-based and network-switch approaches to content filtering:

RTI automatically uses the producer-based and consumer-based approaches as soon as it detects a consumer that specifies a content filter. The producer-based approach is used if the consumer is receiving data over a point-to-point protocol (i.e., not multicast) and the number of consumers that specify filters is reasonably low (below 32). Otherwise, RTI uses a subscriber-based approach.
To use the more scalable network-switched based approach, an application must configure the DataWriter as a Multi-channel DataWriter. This concept is described in the following section.