31–40 of 52 posts
First, CockroachDB's time api is based on the Hybrid Logical Clocks paper: http://www.cse.buffalo.edu/tech-reports/2014-04.pdf This paper is one of the most interesting published in the last couple of years IMO. I remain surprised at how many people have overlooked it. I'm not sure why this blog post makes no mention of the work, in the past the Cockroach folks have been quite explicit about crediting the research do…
Earlier quoted context omitted.
Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?
Absolute velocity does not affect time, simply because there is no such thing as absolute velocity. Relative velocity, though, does. For instance, if you look at a clock on a GPS satellite, moving fast relative to you, you can see it run slower than the one on your wrist. Similarly, someone stationary on the surface of the sun would see us here on earth moving in slow motion. However, someone moving on the satellite…
Actually, the general relativity effects of weaker gravitational field dominate the special relativity effects of velocity[1]. So the GPS satellite clock actually runs faster, not slower.
[1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....
I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…
Nice writeup. I disagree that we need chip-scale, atomic clocks. My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. You can plug them into each machine in the cluster periodically to sync them. Or you can plug it into a master node that connects with low-latency management inter…
> My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. Google have a variant on this, where they use a GPS receiver in each data centre to provide an accurate time source for local machines.
Earlier quoted context omitted.
Absolute velocity does not affect time, simply because there is no such thing as absolute velocity. Relative velocity, though, does. For instance, if you look at a clock on a GPS satellite, moving fast relative to you, you can see it run slower than the one on your wrist. Similarly, someone stationary on the surface of the sun would see us here on earth moving in slow motion. However, someone moving on the satellite…
> For instance, if you look at a clock on a GPS satellite, moving fast relative to you, you can see it run slower than the one on your wrist. Actually, the general relativity effects of weaker gravitational field dominate the special relativity effects of velocity[1]. So the GPS satellite clock actually runs faster, not slower. [1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....
Earlier quoted context omitted.
> For instance, if you look at a clock on a GPS satellite, moving fast relative to you, you can see it run slower than the one on your wrist. Actually, the general relativity effects of weaker gravitational field dominate the special relativity effects of velocity[1]. So the GPS satellite clock actually runs faster, not slower. [1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....
There's a really neat thing about this, the relationship balances out at a certain orbital height, before it flips over so there's an orbit where your chronologically in synch with the ground.
The spanner paper is unclear how they deal with this, but my guess is that since they have accurately synchronized clocks, you'll never have to block long for that commitment to hold. Spanner also uses pessimistic locking, so for a R/W transaction, you can rely on locking reads to prevent the anomaly.
With cockroachdb, wouldn't this commitment imply that poorly synchronized clocks would lead to poor performance?
First, CockroachDB's time api is based on the Hybrid Logical Clocks paper: http://www.cse.buffalo.edu/tech-reports/2014-04.pdf This paper is one of the most interesting published in the last couple of years IMO. I remain surprised at how many people have overlooked it. I'm not sure why this blog post makes no mention of the work, in the past the Cockroach folks have been quite explicit about crediting the research do…
Kudu uses a similar algorithm which we call HybridTime. You can read the tech report here: http://pdsl.ece.utexas.edu/david/hybrid-time-tech-report-01.... and the source here: https://github.com/apache/incubator-kudu/blob/master/src/kud... Both are probably more readable than patent-ese :)
Nice writeup. I disagree that we need chip-scale, atomic clocks. My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. You can plug them into each machine in the cluster periodically to sync them. Or you can plug it into a master node that connects with low-latency management inter…
Nice writeup. I disagree that we need chip-scale, atomic clocks. My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. You can plug them into each machine in the cluster periodically to sync them. Or you can plug it into a master node that connects with low-latency management inter…
That's already a thing, Google GPS based NTP servers with a TCXO or Rubidium reference. You can buy one for ~2000$ that will use GPS, or Cellular as a time reference, and can add options like a Temperature controlled crystal oscillator (TCXO), or a Rubidium reference (basically, small atomic reference).