====== F.18 History  ======
[[dido:public:ra:xapend:xapend.f_qos:start| Return to DDS Quality of Service ]]

  - The HISTORY [[dido:public:ra:xapend:xapend.a_glossary:p:policy|policy]] controls the behavior of the Service when the value of an [[dido:public:ra:xapend:xapend.a_glossary:i:instance|instance]] changes before it is finally communicated to some of its existing ''DataReader'' entities.
  - 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.
  - 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 [[dido:public:ra:xapend:xapend.a_glossary:q:qos|QoS]]. If the limit is reached, then the behavior 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''.

Source: [[https://www.omg.org/spec/DDS/1.4/PDF | DDS 1.4 Spec]]


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