====== F.17 Destination Order  ======
[[dido:public:ra:xapend:xapend.f_qos:start| Return to DDS Quality of Service ]]

The DESTINATION_ORDER [[dido:public:ra:xapend:xapend.a_glossary:p:policy|policy]] controls how each [[dido:public:ra:xapend:xapend.a_glossary:s:subscriber|subscriber]] resolves the final value of a data [[dido:public:ra:xapend:xapend.a_glossary:i:instance|instance]] that is written by multiple ''DataWriter'' objects (which may be associated with different ''[[dido:public:ra:xapend:xapend.a_glossary:p:publisher|Publisher]]'' objects) running on different nodes. 

The setting BY_RECEPTION_TIMESTAMP indicates that, assuming the [[dido:public:ra:xapend:xapend.a_glossary:o:ownership_qos|OWNERSHIP]] policy allows it, the latest received value for the instance should be the one whose value is kept. This is the default value.

The setting BY_SOURCE_TIMESTAMP indicates that, assuming the OWNERSHIP policy allows it, a timestamp placed at the source should be used. This is the only setting that, in the case of concurrent same-[[dido:public:ra:xapend:xapend.f_qos:ownership_strength|strength]] ''DataWriter'' objects updating the same instance, ensures all subscribers will end up with the same final value for the instance. The mechanism to set the source timestamp is [[dido:public:ra:xapend:xapend.a_glossary:m:midware|middleware]] dependent.

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 DESTINATION_ORDER ''**kind**'' are considered ordered such that ''BY_RECEPTION_TIMESTAMP < BY_SOURCE_TIMESTAMP''. 

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


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