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From "Brandon Williams (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-4375) FD incorrectly using RPC timeout to ignore gossip heartbeats
Date Tue, 26 Jun 2012 14:21:44 GMT


Brandon Williams commented on CASSANDRA-4375:

What if we set MAX_INTERVAL_IN_MS to the greater of DD.getRpcTimeout() or the gossip interval
* 2?
> FD incorrectly using RPC timeout to ignore gossip heartbeats
> ------------------------------------------------------------
>                 Key: CASSANDRA-4375
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Peter Schuller
>            Priority: Critical
> Short version: You can't run a cluster with short RPC timeouts because nodes just constantly
flap up/down.
> Long version:
> CASSANDRA-3273 tried to fix a problem resulting from the way the failure detector works,
but did so by introducing a much more sever bug: With low RPC timeouts, that are lower than
the typical gossip propagation time, a cluster will just constantly have all nodes flapping
other nodes up and down.
> The cause is this:
> {code}
> +    // in the event of a long partition, never record an interval longer than the rpc
> +    // since if a host is regularly experiencing connectivity problems lasting this
long we'd
> +    // rather mark it down quickly instead of adapting
> +    private final double MAX_INTERVAL_IN_MS = DatabaseDescriptor.getRpcTimeout();
> {code}
> And then:
> {code}
> -        tLast_ = value;            
> -        arrivalIntervals_.add(interArrivalTime);        
> +        if (interArrivalTime <= MAX_INTERVAL_IN_MS)
> +            arrivalIntervals_.add(interArrivalTime);
> +        else
> +            logger_.debug("Ignoring interval time of {}", interArrivalTime);
> {code}
> Using the RPC timeout to ignore unreasonably long intervals is not correct, as the RPC
timeout is completely orthogonal to gossip propagation delay (see CASSANDRA-3927 for a quick
description of how the FD works).
> In practice, the propagation delay ends up being in the 0-3 second range on a cluster
with good local latency. With a low RPC timeout of say 200 ms, very few heartbeat updates
come in fast enough that it doesn't get ignored by the failure detector. This in turn means
that the FD records a completely skewed average heartbeat interval, which in turn means that
nodes almost always get flapped on interpret() unless they happen to *just* have had their
heartbeat updated. Then they flap back up whenever the next heartbeat comes in (since it gets
brought up immediately).
> In our build, we are replacing the FD with an implementation that simply uses a fixed
{{N}} second time to convict, because this is just one of many ways in which the current FD
hurts, while we still haven't found a way it actually helps relative to the trivial fixed-second
conviction policy.
> For upstream, assuming people won't agree on changing it to a fixed timeout, I suggest,
at minimum, never using a value lower than something like 10 seconds or something, when determining
whether to ignore. Slightly better is to make it a config option.
> (I should note that if propagation delays are significantly off from the expected level,
other things than the FD already breaks - such as the whole concept of {{RING_DELAY}}, which
assumes the propagation time is roughly constant with e.g. cluster size.)

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