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A driver for Apache Cassandra 1.2+ that works exclusively with the Cassandra Query Language version 3
(CQL3) and Cassandra's binary protocol.
/*
* Copyright (C) 2012-2015 DataStax Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.datastax.driver.core;
import com.datastax.driver.core.exceptions.TraceRetrievalException;
import com.google.common.util.concurrent.Uninterruptibles;
import java.net.InetAddress;
import java.util.*;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* The Cassandra trace for a query.
*
* A trace is generated by Cassandra when query tracing is enabled for the
* query. The trace itself is stored in Cassandra in the {@code sessions} and
* {@code events} table in the {@code system_traces} keyspace and can be
* retrieve manually using the trace identifier (the one returned by
* {@link #getTraceId}).
*
* This class provides facilities to fetch the traces from Cassandra. Please
* note that the writing of the trace is done asynchronously in Cassandra. So
* accessing the trace too soon after the query may result in the trace being
* incomplete.
*/
public class QueryTrace {
private static final String SELECT_SESSIONS_FORMAT = "SELECT * FROM system_traces.sessions WHERE session_id = %s";
private static final String SELECT_EVENTS_FORMAT = "SELECT * FROM system_traces.events WHERE session_id = %s";
private static final int MAX_TRIES = 5;
private static final long BASE_SLEEP_BETWEEN_TRIES_IN_MS = 3;
private final UUID traceId;
private volatile String requestType;
// We use the duration to figure out if the trace is complete, because
// that's the last event that is written (and it is written asynchronously
// so it's possible that a fetch gets all the trace except the duration).
private volatile int duration = Integer.MIN_VALUE;
private volatile InetAddress coordinator;
private volatile Map parameters;
private volatile long startedAt;
private volatile List events;
private final SessionManager session;
private final Lock fetchLock = new ReentrantLock();
QueryTrace(UUID traceId, SessionManager session) {
this.traceId = traceId;
this.session = session;
}
/**
* Returns the identifier of this trace.
*
* Note that contrary to the other methods in this class, this
* does not entail fetching query trace details from Cassandra.
*
* @return the identifier of this trace.
*/
public UUID getTraceId() {
return traceId;
}
/**
* Returns the type of request.
*
* @return the type of request or {@code null} if the request
* type is not yet available.
* @throws TraceRetrievalException if the trace details cannot be retrieve
* from Cassandra successfully.
*/
public String getRequestType() {
maybeFetchTrace();
return requestType;
}
/**
* Returns the server-side duration of the query in microseconds.
*
* @return the (server side) duration of the query in microseconds. This
* method will return {@code Integer.MIN_VALUE} if the duration is not yet
* available.
* @throws TraceRetrievalException if the trace details cannot be retrieve
* from Cassandra successfully.
*/
public int getDurationMicros() {
maybeFetchTrace();
return duration;
}
/**
* Returns the coordinator host of the query.
*
* @return the coordinator host of the query or {@code null}
* if the coordinator is not yet available.
* @throws TraceRetrievalException if the trace details cannot be retrieve
* from Cassandra successfully.
*/
public InetAddress getCoordinator() {
maybeFetchTrace();
return coordinator;
}
/**
* Returns the parameters attached to this trace.
*
* @return the parameters attached to this trace. or
* {@code null} if the coordinator is not yet available.
* @throws TraceRetrievalException if the trace details cannot be retrieve
* from Cassandra successfully.
*/
public Map getParameters() {
maybeFetchTrace();
return parameters;
}
/**
* Returns the server-side timestamp of the start of this query.
*
* @return the server side timestamp of the start of this query or
* 0 if the start timestamp is not available.
* @throws TraceRetrievalException if the trace details cannot be retrieve
* from Cassandra successfully.
*/
public long getStartedAt() {
maybeFetchTrace();
return startedAt;
}
/**
* Returns the events contained in this trace.
*
* Query tracing is asynchronous in Cassandra. Hence, it
* is possible for the list returned to be missing some events for some of
* the replica involved in the query if the query trace is requested just
* after the return of the query it is a trace of (the only guarantee being
* that the list will contain the events pertaining to the coordinator of
* the query).
*
* @return the events contained in this trace.
* @throws TraceRetrievalException if the trace details cannot be retrieve
* from Cassandra successfully.
*/
public List getEvents() {
maybeFetchTrace();
return events;
}
@Override
public String toString() {
maybeFetchTrace();
return String.format("%s [%s] - %dµs", requestType, traceId, duration);
}
private void maybeFetchTrace() {
if (duration != Integer.MIN_VALUE)
return;
fetchLock.lock();
try {
doFetchTrace();
} finally {
fetchLock.unlock();
}
}
private void doFetchTrace() {
int tries = 0;
try {
// We cannot guarantee the trace is complete. But we can't at least wait until we have all the information
// the coordinator log in the trace. Since the duration is the last thing the coordinator log, that's
// what we check to know if the trace is "complete" (again, it may not contain the log of replicas).
while (duration == Integer.MIN_VALUE && tries <= MAX_TRIES) {
++tries;
ResultSetFuture sessionsFuture = session.executeQuery(new Requests.Query(String.format(SELECT_SESSIONS_FORMAT, traceId)), Statement.DEFAULT);
ResultSetFuture eventsFuture = session.executeQuery(new Requests.Query(String.format(SELECT_EVENTS_FORMAT, traceId)), Statement.DEFAULT);
Row sessRow = sessionsFuture.get().one();
if (sessRow != null && !sessRow.isNull("duration")) {
requestType = sessRow.getString("request");
coordinator = sessRow.getInet("coordinator");
if (!sessRow.isNull("parameters"))
parameters = Collections.unmodifiableMap(sessRow.getMap("parameters", String.class, String.class));
startedAt = sessRow.getTimestamp("started_at").getTime();
events = new ArrayList();
for (Row evRow : eventsFuture.get()) {
events.add(new Event(evRow.getString("activity"),
evRow.getUUID("event_id").timestamp(),
evRow.getInet("source"),
evRow.getInt("source_elapsed"),
evRow.getString("thread")));
}
events = Collections.unmodifiableList(events);
// Set the duration last as it's our test to know if the trace is complete
duration = sessRow.getInt("duration");
} else {
// The trace is not ready. Give it a few milliseconds before trying again.
// Notes: granted, sleeping uninterruptibly is bad, but having all method propagate
// InterruptedException bothers me.
Uninterruptibles.sleepUninterruptibly(tries * BASE_SLEEP_BETWEEN_TRIES_IN_MS, TimeUnit.MILLISECONDS);
}
}
} catch (Exception e) {
throw new TraceRetrievalException("Unexpected exception while fetching query trace", e);
}
if (tries > MAX_TRIES)
throw new TraceRetrievalException(String.format("Unable to retrieve complete query trace for id %s after %d tries", traceId, MAX_TRIES));
}
/**
* A trace event.
*
* A query trace is composed of a list of trace events.
*/
public static class Event {
private final String name;
private final long timestamp;
private final InetAddress source;
private final int sourceElapsed;
private final String threadName;
private Event(String name, long timestamp, InetAddress source, int sourceElapsed, String threadName) {
this.name = name;
// Convert the UUID timestamp to an epoch timestamp; I stole this seemingly random value from cqlsh, hopefully it's correct.
this.timestamp = (timestamp - 0x01b21dd213814000L) / 10000;
this.source = source;
this.sourceElapsed = sourceElapsed;
this.threadName = threadName;
}
/**
* The event description, that is which activity this event correspond to.
*
* @return the event description.
*/
public String getDescription() {
return name;
}
/**
* Returns the server side timestamp of the event.
*
* @return the server side timestamp of the event.
*/
public long getTimestamp() {
return timestamp;
}
/**
* Returns the address of the host having generated this event.
*
* @return the address of the host having generated this event.
*/
public InetAddress getSource() {
return source;
}
/**
* Returns the number of microseconds elapsed on the source when this event
* occurred since when the source started handling the query.
*
* @return the elapsed time on the source host when that event happened
* in microseconds.
*/
public int getSourceElapsedMicros() {
return sourceElapsed;
}
/**
* Returns the name of the thread on which this event occurred.
*
* @return the name of the thread on which this event occurred.
*/
public String getThreadName() {
return threadName;
}
@Override
public String toString() {
return String.format("%s on %s[%s] at %s", name, source, threadName, new Date(timestamp));
}
}
}