- From: Carsten Bormann <cabo@tzi.org>
- Date: Mon, 11 Jul 2016 08:54:16 +0200
- To: Christian Groves <Christian.Groves@nteczone.com>
- CC: Public Web of Things IG <public-wot-ig@w3.org>
Christian Groves wrote: > [CNG] I'm familiar with metrology, however the Cache-Control Max-Age is > defined to be a non-negative decimal integer. If the primary purpose of > stability option is to map to the cache control and http max-age then it > doesn't seem to follow that you make it a sub-multiple of a second. I don't agree with this premise. Indeed max-age has limited resolution, mainly for simplicity and because the benefit of caching in the network decreases with very low max-age values. But the stability attribute is not about mapping max-age values, it is about the data source itself. If the sensor is designed to support a 4 Hz control loop, a stability of 250 ms is quite appropriate. (A numeric implementation problem is that binary floating point values cannot exactly represent decimal fractions. It can therefore make a non-trivial difference whether the base unit is seconds or, say, milliseconds. If we assume that real-world sensor systems are using milliseconds as their time base and not "mibiseconds", i.e. 1024ths of a second. And if we are stuck with binary floating point, which of course neither CBOR nor EXI are.) > It > means that for a given number of bits the time represented won't be the > same. 32bits of ms doesn't equal the same length of time as 32bit s. > Also it means the mapping is more complicated, e.g. is 1750ms mapped to > a Max-Age value of 1s or 2s? It can't be mapped to 1.75s. What is the use case for mapping between the TD stability values and a max-age value? In any case, the fact that max-age doesn't have a resolution that is appropriate for high-rate, low-latency communication doesn't mean that we are stuck with the limitations of this option for all applications. On the other hand, stability would be a more abstract TD attribute where it would be nice if it could span a wide array of applications. I should note that I don't have a strong opinion on the data type or base unit of the stability attribute; I just want to be sure that we consider the implications of any choice we make here. (And that we don't make asymptotic errors such as choosing 0 for static.) Grüße, Carsten
Received on Monday, 11 July 2016 06:54:45 UTC