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History QoS Is a QoS policy that specifies the number of samples kept by the middleware engine. Data Distribution Service (DDS) will only attempt to keep the most recent ‘depth’ of samples (KEEP_LAST) or ALL samples (KEEP_ALL) of each instance of data identified by its [dido:public:ra:xapend:xapend.a_glossary:k:key|key]].
Source: OpenSplice Glossary