History QoS

This QosPolicy controls which samples will be stored when the value of an instance changes (one or more times) before it is finally communicated.

Attributes

Value	Meaning	Concerns	RxO	Changeable
A HistoryQosPolicyKind: kind	Specifies specifies the type of history, which may be KEEP_LAST or KEEP_ALL.	Topic, DataReader, DataWriter	No	No
KEEP_LAST and optional long "depth"	On the publishing side, the Service will only attempt to keep the most recent “depth” samples of each instance of data (identified by its key) managed by the DataWriter. On the subscribing side, the DataReader will only attempt to keep the most recent “depth” samples received for each instance (identified by its key) until the application “takes” them via the DataReader’s take operation. KEEP_LAST is the default kind. The default value of depth is 1. If a value other than 1 is specified, it should be consistent with the settings of the RESOURCE_LIMITS QoS policy.
KEEP_ALL	On the publishing side, the Service will attempt to keep all samples (representing each value written) of each instance of data (identified by its key) managed by the DataWriter until they can be delivered to all subscribers. On the subscribing side, the Service will attempt to keep all samples of each instance of data (identified by its key) managed by the DataReader. These samples are kept until the application “takes” them from the Service via the take operation. The setting of depth has no effect. Its implied value is LENGTH_UNLIMITED.

1. This policy controls the behaviour of the Service when the value of an instance changes before it is finally communicated to some of its existing DataReader entities.
2. If the kind is set to KEEP_LAST, then the Service will only attempt to keep the latest values of the instance and discard the older ones. In this case, the value of depth regulates the maximum number of values (up to and including the most current one) the Service will maintain and deliver. The default (and most common setting) for depth is one, indicating that only the most recent value should be delivered.
3. If the kind is set to KEEP_ALL, then the Service will attempt to maintain and deliver all the values of the instance to existing subscribers. The resources that the Service can use to keep this history are limited by the settings of the RESOURCE_LIMITS QoS. If the limit is reached, then the behaviour of the Service will depend on the RELIABILITY QoS. If the reliability kind is BEST_EFFORT, then the old values will be discarded. If reliability is RELIABLE, then the Service will block the DataWriter until it can deliver the necessary old values to all subscribers.

The setting of HISTORY depth must be consistent with the RESOURCE_LIMITS max_samples_per_instance. For these two QoS to be consistent, they must verify that depth <= max_samples_per_instance.