====== F.14 Reliability ======
[[dido:public:ra:xapend:xapend.f_qos:start| Return to DDS Quality of Service ]]

The RELIABILITY [[dido:public:ra:xapend:xapend.a_glossary:p:policy|policy]] indicates the level of reliability requested by a ''DataReader'' or offered by a ''DataWriter''. These levels are ordered, BEST_EFFORT being lower than RELIABLE. A DataWriter offering a level is implicitly offering all levels below.

The setting of this policy has a dependency on the setting of the RESOURCE_LIMITS policy. In case the RELIABILITY ''kind'' is set to RELIABLE the ''write'' operation on the DataWriter may block if the modification would cause data to be lost or else cause one of the limits in specified in the RESOURCE_LIMITS to be exceeded. Under these circumstances, the RELIABILITY max_blocking_time configures the maximum duration the ''write'' operation may block.

If the RELIABILITY ''kind'' is set to RELIABLE, data-samples originating from a single ''DataWriter'' cannot be made available to the ''DataReader'' if there are previous data-samples that have not been received yet due to a communication error. In other words, the service will repair the error and re-transmit data-samples as needed in order to re-construct a correct [[dido:public:ra:xapend:xapend.a_glossary:s:snapshot|snapshot]] of the ''DataWriter'' history before it is accessible by the ''DataReader''.

If the RELIABILITY ''kind'' is set to BEST_EFFORT, the service will not re-transmit missing data-samples. However for datasamples originating from any one ''DataWriter'' the service will ensure they are stored in the ''DataReader'' history in the same order they originated in the ''DataWriter''. In other words, the ''DataReader'' may miss some data-samples but it will never see the value of a data-object change from a newer value to an order value.

The value offered is considered compatible with the value requested if and only if the inequality ''offered kind >= requested kind'' evaluates to ''TRUE''. For the purposes of this inequality, the values of RELIABILITY ''kind'' are considered ordered such that ''BEST_EFFORT < RELIABLE''.

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


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