Indicate to Global Caches how long a resource should be cached for

[6a6d74]

As discussed at ET-W2AT meeting, 16-Oct-2023.

WIS2 Manual (Tech Regs) states:

"""
4.5.4 Based on received notifications, a Global Cache shall download core data from WIS nodes or other Global Caches and store for a duration compatible with the real-time or near real-time schedule of the data and not less than 24-hours.
"""

The default period for caching resources is 24-hours. But some (near) real time might need to be kept for longer based on the "real-time or near real-time schedule". Its a WMO Programme decision about how long data should be cached for, agreed with the WIS2 operators - in a process that's yet to be defined.

A mechanism is required to inform Global Caches to cache content for a duration other than 24-hours - usually longer, but perhaps shorter too.

It seems likely that this would be a per-object policy, i.e., each object that is cached should indicate the length of time it should be cached. The simplest way to do this would be to add a cache-duration attribute to the notification message.

A Global Cache may ignore the cache-duration statement, for example, if the duration was set to an excessively long period that compromised the ability of the Global Cache to perform. In "tech regs speak": the Global Cache should respect the cache-duration value.

[amilan17]

note: current guide differs from text above content. https://wmo-im.github.io/wis2-guide/guide/wis2-guide-DRAFT.html#_2_4_3_global_cache