Network partitions

The Partitioning element in the configuration allows configuring NetworkPartition elements and mapping topic/partition names to these “network partitions” using PartitionMappings elements.

Network partitions introduce alternative multicast addresses for data and/or restrict the set of unicast addresses (that is, interfaces). In the DDSI discovery protocol, a reader can override the addresses at which it is reachable, which is a feature of the discovery protocol that is used to advertise alternative multicast addresses and/or a subset of the unicast addresses. The writers in the network use the addresses advertised by the reader rather than the default addresses advertised by the reader’s participant.

Unicast and multicast addresses in a network partition play different roles:

  • The multicast addresses specify an alternative set of addresses to be used instead of the participant’s default. This is particularly useful to limit high-bandwidth flows to the parts of a network where the data is needed (for IP/Ethernet, this assumes switches that are configured to do IGMP snooping).

  • The unicast addresses not only influence the set of interfaces are used for unicast, but thereby also the set of interfaces that are considered for use by multicast. For example: specifying a unicast address that matches network interface A, ensures all traffic to that reader uses interface A, whether unicast or multicast.

The typical use of unicast addresses is to force traffic onto certain interfaces, the configuration also allows specifying interface names (using the interface attribute).

The mapping of a data reader or writer to a network partition is indirect:

  1. The partition and topic are matched against a table of partition mappings, partition/topic combinations to obtain the name of a network partition

  2. The network partition name is used to find the addressing information.

This makes it easier to map many different partition/topic combinations to the same multicast address without having to specify the actual multicast address many times over. If no match is found, the default addresses are used.

The matching sequence is in the order in which the partition mappings are specified in the configuration. The first matching mapping is the one that is used. The * and ? wildcards are available for the DCPS partition/topic combination in the partition mapping.

A single reader or writer is associated with a set of partitions, and each partition/topic combination can potentially map to a different network partition. In this case, the first matching network partition is used. This does not affect the data the reader receives, it only affects the addressing on the network.