Clock Selection
[RTI Data Distribution Service API Reference]

APIs related to clock selection. More... RTI Data Distribution Service uses clocks to measure time and generate timestamps.

The middleware uses two clocks, an internal clock and an external clock. The internal clock is used to measure time and handles all timing in the middleware. The external clock is used solely to generate timestamps, such as the source timestamp and the reception timestamp, in addition to providing the time given by DDSDomainParticipant::get_current_time.

Available Clocks

Two clock implementations are generally available, the monotonic clock and the realtime clock.

The monotonic clock provides times that are monotonic from a clock that is not adjustable. This clock is useful to use in order to not be subject to changes in the system or realtime clock, which may be adjusted by the user or via time synchronization protocols. However, this time generally starts from an arbitrary point in time, such as system startup. Note that this clock is not available for all architectures. Please see the Platform Notes for the architectures on which it is supported. For the purposes of clock selection, this clock can be referenced by the name "monotonic".

The realtime clock provides the realtime of the system. This clock may generally be monotonic but may not be guaranteed to be so. It is adjustable and may be subject to small and large changes in time. The time obtained from this clock is generally a meaningful time in that it is the amount of time from a known epoch. For the purposes of clock selection, this clock can be referenced by the names "realtime" or "system".

Clock Selection Strategy

By default, both the internal and external clocks use the realtime clock. If you want your application to be robust to changes in the system time, you may use the monotonic clock as the internal clock, and leave the system clock as the external clock. Note, however, that this may slightly diminish performance in that both the send and receive paths may need to obtain times from both clocks. Since the monotonic clock is not available on all architectures, you may want to specify "monotonic,realtime" for the internal_clock (see the table below). By doing so, the middleware will attempt to use the monotonic clock if available, and will fall back to the realtime clock if the monotonic clock is not available.

If you want your application to be robust to changes in the system time, you are not relying on source timestamps, and you want to avoid obtaining times from both clocks, you may use the monotonic clock for both the internal and external clocks.

Configuring Clock Selection

To configure the clock selection, use the PROPERTY QoS policy associated with the DDSDomainParticipant.

See also:

DDS_PropertyQosPolicy

The following table lists the supported clock selection properties.

Clock Selection Properties

Property	Description
dds.clock.external_clock	Comma-delimited list of clocks to use for the external clock, in the order of preference. Valid clock names are "realtime", "system", and "monotonic". Default: "realtime"
dds.clock.internal_clock	Comma-delimited list of clocks to use for the internal clock, in the order of preference. Valid clock names are "realtime", "system", and "monotonic". Default: "realtime"