public static class LatencyAwarePolicy.Builder extends Object
This helper allows to configure the different parameters used by
LatencyAwarePolicy
. The only mandatory parameter is the child
policy that will be wrapped with latency awareness. The other parameters
can be set through the methods of this builder, but all have defaults (that
are documented in the javadoc of each method) if you don't.
If you observe that the resulting policy excludes hosts too aggressively or
not enough so, the main parameters to check are the exclusion threshold
(withExclusionThreshold(double)
) and scale (withScale(long, java.util.concurrent.TimeUnit)
).
Constructor and Description |
---|
LatencyAwarePolicy.Builder(LoadBalancingPolicy childPolicy)
Creates a new latency aware policy builder given the child policy
that the resulting policy wraps.
|
Modifier and Type | Method and Description |
---|---|
LatencyAwarePolicy |
build()
Builds a new latency aware policy using the options set on this
builder.
|
LatencyAwarePolicy.Builder |
withExclusionThreshold(double exclusionThreshold)
Sets the exclusion threshold to use for the resulting latency aware policy.
|
LatencyAwarePolicy.Builder |
withMininumMeasurements(int minMeasure)
Sets the minimimum number of measurements per-host to consider for
the resulting latency aware policy.
|
LatencyAwarePolicy.Builder |
withRetryPeriod(long retryPeriod,
TimeUnit unit)
Sets the retry period for the resulting latency aware policy.
|
LatencyAwarePolicy.Builder |
withScale(long scale,
TimeUnit unit)
Sets the scale to use for the resulting latency aware policy.
|
LatencyAwarePolicy.Builder |
withUpdateRate(long updateRate,
TimeUnit unit)
Sets the update rate for the resulting latency aware policy.
|
public LatencyAwarePolicy.Builder(LoadBalancingPolicy childPolicy)
childPolicy
- the load balancing policy to wrap with latency
awareness.public LatencyAwarePolicy.Builder withExclusionThreshold(double exclusionThreshold)
The exclusion threshold controls how much worse the average latency of a node must be compared to the fastest performing node for it to be penalized by the policy.
The default exclusion threshold (if this method is not called) is 2. In other words, the resulting policy excludes nodes that are more than twice slower than the fastest node.
exclusionThreshold
- the exclusion threshold to use. Must be
greater or equal to 1.IllegalArgumentException
- if exclusionThreshold < 1
.public LatencyAwarePolicy.Builder withScale(long scale, TimeUnit unit)
The scale
provides control on how the weight given to older latencies
decreases over time. For a given host, if a new latency \(l\) is received at
time \(t\), and the previously calculated average is \(prev\) calculated at
time \(t'\), then the newly calculated average \(avg\) for that host is calculated
thusly:
\[
d = \frac{t - t'}{scale} \\
\alpha = 1 - \left(\frac{\ln(d+1)}{d}\right) \\
avg = \alpha * l + (1-\alpha) * prev
\]
Typically, with a scale
of 100 milliseconds (the default), if a new
latency is measured and the previous measure is 10 millisecond old (so \(d=0.1\)),
then \(\alpha\) will be around \(0.05\). In other words, the new latency will
weight 5% of the updated average. A bigger scale will get less weight to new
measurements (compared to previous ones), a smaller one will give them more weight.
The default scale (if this method is not used) is of 100 milliseconds. If unsure, try this default scale first and experiment only if it doesn't provide acceptable results (hosts are excluded too quickly or not fast enough and tuning the exclusion threshold doesn't help).
scale
- the scale to use.unit
- the unit of scale
.IllegalArgumentException
- if scale <e; 0
.public LatencyAwarePolicy.Builder withRetryPeriod(long retryPeriod, TimeUnit unit)
The retry period defines how long a node may be penalized by the
policy before it is given a 2nd change. More precisely, a node is excluded
from query plans if both his calculated average latency is exclusionThreshold
times slower than the fastest node average latency (at the time the query plan is
computed) and his calculated average latency has been updated since
less than retryPeriod
. Since penalized nodes will likely not see their
latency updated, this is basically how long the policy will exclude a node.
retryPeriod
- the retry period to use.unit
- the unit for retryPeriod
.IllegalArgumentException
- if retryPeriod < 0
.public LatencyAwarePolicy.Builder withUpdateRate(long updateRate, TimeUnit unit)
The default update rate if 100 milliseconds, which should be appropriate for most applications. In particular, note that while we want to avoid to recompute the minimum for every query, that computation is not particularly intensive either and there is no reason to use a very slow rate (more than second is probably unnecessarily slow for instance).
updateRate
- the update rate to use.unit
- the unit for updateRate
.IllegalArgumentException
- if updateRate <e; 0
.public LatencyAwarePolicy.Builder withMininumMeasurements(int minMeasure)
Penalizing nodes is based on an average of their recently measured
average latency. This average is only meaningful if a minimum of
measurements have been collected (moreover, a newly started
Cassandra node will tend to perform relatively poorly on the first
queries due to the JVM warmup). This is what this option controls.
If less that minMeasure
data points have been collected for
a given host, the policy will never penalize that host. Also, the
30% first measurement will be entirely ignored (in other words, the
30% * minMeasure
first measurement to a node are entirely
ignored, while the 70%
next ones are accounted in the latency
computed but the node won't get convicted until we've had at least
minMeasure
measurements).
Note that the number of collected measurements for a given host is reseted if the node is restarted.
The default for this option (if this method is not called) is 50. Note that it is probably not a good idea to put this option too low if only to avoid the influence of JVM warm-up on newly restarted nodes.
minMeasure
- the minimum measurements to consider.IllegalArgumentException
- if minMeasure < 0
.public LatencyAwarePolicy build()
LatencyAwarePolicy
.Copyright © 2014. All Rights Reserved.