org.apache.cassandra.service
Class PBSPredictor

java.lang.Object
  org.apache.cassandra.service.PBSPredictor

All Implemented Interfaces:

PBSPredictorMBean

public class PBSPredictor
extends java.lang.Object
implements PBSPredictorMBean

Performs latency and consistency predictions as described in "Probabilistically Bounded Staleness for Practical Partial Quorums" by Bailis et al. in VLDB 2012. The predictions are of the form:

With ReplicationFactor N, read consistency level of R, and write consistency level W, after t seconds, p% of reads will return a version within k versions of the last written; this should result in a latency of L ms.

These predictions should be used as a rough guideline for system operators. This interface is exposed through nodetool.

The class accomplishes this by measuring latencies for reads and writes, then using Monte Carlo simulation to predict behavior under a given N,R, and W based on those latencies.

We capture four distributions:

• W: time from when the coordinator sends a mutation to the time that a replica begins to serve the new value(s)
• A: time from when a replica accepting a mutation sends an acknowledgment to the time the coordinator hears of it
• R: time from when the coordinator sends a read request to the time that the replica performs the read
• S: time from when the replica sends a read response to the time when the coordinator receives it

A and S are mostly network-bound, while W and R depend on both the network and local processing time.

Caveats: Prediction is only as good as the latencies collected. Accurate prediction requires synchronizing clocks between replicas. We collect a running sample of latencies, but, if latencies change dramatically, predictions will be off.

The predictions are conservative, or worst-case, meaning we may predict more staleness than in practice in the following ways:

• We do not account for read repair.
• We do not account for Merkle tree exchange.
• Multi-version staleness is particularly conservative.
• We simulate non-local reads and writes. We assume that the coordinating Cassandra node is not itself a replica for a given key.

The predictions are optimistic in the following ways:

• We do not predict the impact of node failure.
• We do not model hinted handoff.

See Also:

ConsistencyLevel, AbstractReplicationStrategy

Field Summary

static java.lang.String

MBEAN_NAME

Method Summary

void	disableConsistencyPredictionLogging()
PBSPredictionResult	doPrediction(int n, int r, int w, float timeSinceWrite, int numberVersionsStale, float percentileLatency)
void	enableConsistencyPredictionLogging()
void	init()
static PBSPredictor	instance()
boolean	isLoggingEnabled()
void	logReadResponse(java.lang.String id, long constructionTime)
void	logReadResponse(java.lang.String id, long responseCreationTime, long receivedTime)
void	logWriteResponse(java.lang.String id, long constructionTime)
void	logWriteResponse(java.lang.String id, long responseCreationTime, long receivedTime)
void	setMaxLoggedLatenciesForConsistencyPrediction(int maxLogged)
void	setNumberTrialsForConsistencyPrediction(int numTrials)
void	startReadOperation(java.lang.String id)
void	startReadOperation(java.lang.String id, long startTime)
void	startWriteOperation(java.lang.String id)
void	startWriteOperation(java.lang.String id, long startTime)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

MBEAN_NAME

public static final java.lang.String MBEAN_NAME

See Also:

Constant Field Values

Method Detail

instance

public static PBSPredictor instance()

enableConsistencyPredictionLogging

public void enableConsistencyPredictionLogging()

Specified by:

enableConsistencyPredictionLogging in interface PBSPredictorMBean

disableConsistencyPredictionLogging

public void disableConsistencyPredictionLogging()

Specified by:

disableConsistencyPredictionLogging in interface PBSPredictorMBean

isLoggingEnabled

public boolean isLoggingEnabled()

setMaxLoggedLatenciesForConsistencyPrediction

public void setMaxLoggedLatenciesForConsistencyPrediction(int maxLogged)

Specified by:

setMaxLoggedLatenciesForConsistencyPrediction in interface PBSPredictorMBean

setNumberTrialsForConsistencyPrediction

public void setNumberTrialsForConsistencyPrediction(int numTrials)

Specified by:

setNumberTrialsForConsistencyPrediction in interface PBSPredictorMBean

init

public void init()

doPrediction

public PBSPredictionResult doPrediction(int n,
                                        int r,
                                        int w,
                                        float timeSinceWrite,
                                        int numberVersionsStale,
                                        float percentileLatency)

Specified by:

doPrediction in interface PBSPredictorMBean

startWriteOperation

public void startWriteOperation(java.lang.String id)

startWriteOperation

public void startWriteOperation(java.lang.String id,
                                long startTime)

startReadOperation

public void startReadOperation(java.lang.String id)

startReadOperation

public void startReadOperation(java.lang.String id,
                               long startTime)

logWriteResponse

public void logWriteResponse(java.lang.String id,
                             long constructionTime)

logWriteResponse

public void logWriteResponse(java.lang.String id,
                             long responseCreationTime,
                             long receivedTime)

logReadResponse

public void logReadResponse(java.lang.String id,
                            long constructionTime)

logReadResponse

public void logReadResponse(java.lang.String id,
                            long responseCreationTime,
                            long receivedTime)