# 13.4. Topics with many subscribers¶

By default, every time a DataWriter publishes a data change on a Topic, it sends a unicast message for every DataReader that is subscribed to the Topic. If there are several DataReaders subscribed, it is recommendable to use multicast instead of unicast. By doing so, only one network package will be sent for each sample. This will improve both CPU and network usage.

This solution can be implemented with UDP Transport or Shared Memory Transport (SHM). SHM transport is multicast by default, but is only available between DataWriters and DataReaders on the same machine. UDP transport needs some extra configuration. The example below shows how to set a DataWriterQos to configure a DataWriter to use a multicast transport on UDP. More information about configuring local and remote locators on endpoints can be found in RTPSEndpointQos.

Note

Multicast over UDP can be problematic on some scenarios, mainly WiFi and complex networks with multiple network links.

 C++ DataWriterQos qos; // Add new multicast locator with IP 239.255.0.4 and port 7900 eprosima::fastrtps::rtps::Locator_t new_multicast_locator; eprosima::fastrtps::rtps::IPLocator::setIPv4(new_multicast_locator, "239.255.0.4"); new_multicast_locator.port = 7900; qos.endpoint().multicast_locator_list.push_back(new_multicast_locator);  XML 
239.255.0.4
7900