32. Reliability Models for Sending Data

The DCPS reliability model recognizes that the optimal balance between time-determinism and data-delivery reliability varies widely among applications and can vary among different publications within the same application. For example, individual DDS samples of signal data can often be dropped because their value disappears when the next DDS sample is sent. However, each DDS sample of command data must be received and it must be received in the order sent.

The QosPolicies provide a way to customize the determinism/reliability trade-off on a per Topic basis, or even on a per DataWriter/DataReader basis.

There are two delivery models:

• Best-effort delivery model: “I’m not concerned about missed or unordered DDS samples.”
• Reliable delivery model: “Make sure all DDS samples get there, in order.”

Connext uses best-effort delivery by default. The other type of delivery that Connext supports is called reliable. This chapter provides instructions on how to set up and use reliable communication.