History: declaring excluded content

Providers may elect not to make all content available from the history service (e.g. real-time samples). We discussed, at plug-a-thon 18, whether anyone (e.g., consumers, responsible organizations) needs to know what is available and how to disclose this?

One method to disclose available history information is a per-metric extension, for example:

<Metric xsi:type="RealTimeSampleArrayMetricDescriptor" Resolution="1" SamplePeriod="PT0.001S" MetricCategory="Msrmt" MetricAvailability="Cont" Handle="rtsa_metric_0.channel_1.vmd_0.mds_0" DescriptorVersion="1">
  <ext:Extension>
    <hm:History Available="true" Period="PT30S" xmlns:hm="urn:oid:1.3.6.1.4.1.3592.2.6933.5.1"/>
  </ext:Extension>
  <Type Code="67108881"/>
    :
    :

• we weren't able to identify specific use cases where someone needed to know this information but didn't necessarily believe there aren't any; may need discussion with other stakeholders to identify specific use cases.
• tagging the unavailable items with an extension (similar to retrievability) seemed problematic for several reasons
  • historic availability could be considered part of transport information and doesn't belong in the mdib;
  • an alternative view that availability in the history service might be considered part of the mdib description and not transport information
  • providers may modify the subscription so there may be no description present for states that won't be included in the history service in the current mdib at the time a consumer is making a request.
• a separate function in the history service may be needed to indicate what's available. This needs to account for
  • multiple version sequences,
  • parts of the description that come and go,
  • clock-versioning anticipated for non-slewing time-adjustments.