Isn’t that just describing reintroducing waiting out the uncertainty window in order gain back external consistency? I’m not sure that ends up being substantially different from TrueTime.
Yeah, that's why I was asking. I've seen some various comments about the details of this but nothing definitive. I'm hoping when I have the mental bandwidth to sit down and go through the TLA proofs of HLC I'll fully grok it.
One immediate difference between TrueTime and HLC springs to mind though: TT knows the clock ambiguity when the timestamp was generated, HLC does not. So in practice I think TT may see much shorter wait intervals, than HLC which would have to conservative wait out the maximum clock drift estimate allowed.
A skim through the cdb source is consistent with this. It looks like when they hit a read/write conflict where the write has a non-zero logical component in it's HLC stamp, then they wait out any remaining portion of the ambiguity window. Since the max offset is around 400ms, that seems like it could hurt pretty bad under contended writes.
I think HLC would get more interesting if the stamps were extended with the ambiguity interval at stamp time.
One immediate difference between TrueTime and HLC springs to mind though: TT knows the clock ambiguity when the timestamp was generated, HLC does not. So in practice I think TT may see much shorter wait intervals, than HLC which would have to conservative wait out the maximum clock drift estimate allowed.
A skim through the cdb source is consistent with this. It looks like when they hit a read/write conflict where the write has a non-zero logical component in it's HLC stamp, then they wait out any remaining portion of the ambiguity window. Since the max offset is around 400ms, that seems like it could hurt pretty bad under contended writes.
I think HLC would get more interesting if the stamps were extended with the ambiguity interval at stamp time.