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From "Ariel Weisberg (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-8732) Make inter-node timeouts tolerate time skew
Date Tue, 03 Feb 2015 20:14:35 GMT
Ariel Weisberg created CASSANDRA-8732:

             Summary: Make inter-node timeouts tolerate time skew
                 Key: CASSANDRA-8732
             Project: Cassandra
          Issue Type: Improvement
            Reporter: Ariel Weisberg

Right now internode timeouts rely on currentTimeMillis() (and NTP) to make sure that tasks
don't expire before they arrive.

Every receiver needs to deduce the offset between its nanoTime and the remote nanoTime. I
don't think currentTimeMillis is a good choice because it is designed to be manipulated by
operators and NTP. I would probably be comfortable assuming that nanoTime isn't going to move
in significant ways without something that could be classified as operator error happening.

I suspect the one timing method you can rely on being accurate is nanoTime within a node (on
average) and that a node can report on its own scheduling jitter (on average).

Finding the offset requires knowing what the network latency is in one direction.

One way to do this would be to periodically send a ping request which generates a series of
ping responses at fixed intervals (maybe by UDP?). The responses should corrected for scheduling
jitter since the fixed intervals may not be exactly achieved by the sender. By measuring the
time deviation between ping responses and their expected arrival time (based on the interval)
and correcting for the remotely reported scheduling jitter, you should be able to measure
latency in one direction.

A weighted moving average (only correct for drift, not readjustment) of these measurements
would eventually converge on a close answer and would not be impacted by outlier measurements.
It may also make sense to drop the largest N samples to improve accuracy.

One you know network latency you can add that to the timestamp of each ping and compare to
the local clock and know what the offset is.

These measurements won't calculate the offset to be too small (timeouts fire early), but could
calculate the offset to be too large (timeouts fire late). The conditions where you the offset
won't be accurate are the conditions where you also want them firing reliably. This and bootstrapping
in bad conditions is what I am most uncertain of.

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