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AvailabilityGuard.java
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AvailabilityGuard.java
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/*
* Copyright (c) 2002-2016 "Neo Technology,"
* Network Engine for Objects in Lund AB [http://neotechnology.com]
*
* This file is part of Neo4j.
*
* Neo4j is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package org.neo4j.kernel;
import java.time.Clock;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArraySet;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Function;
import org.neo4j.helpers.Format;
import org.neo4j.helpers.Listeners;
import org.neo4j.helpers.collection.Iterables;
import org.neo4j.logging.Log;
/**
* The availability guard ensures that the database will only take calls when it is in an ok state.
* It tracks a set of requirements (added via {@link #require(AvailabilityRequirement)}) that must all be marked
* as fulfilled (using {@link #fulfill(AvailabilityRequirement)}) before the database is considered available again.
* Consumers determine if it is ok to call the database using {@link #isAvailable()},
* or await availability using {@link #isAvailable(long)}.
*/
public class AvailabilityGuard
{
public static final String DATABASE_AVAILABLE_MSG = "Fulfilling of requirement makes database available: ";
public static final String DATABASE_UNAVAILABLE_MSG = "Requirement makes database unavailable: ";
public class UnavailableException extends Exception
{
public UnavailableException( String message )
{
super( message );
}
}
public interface AvailabilityListener
{
void available();
void unavailable();
}
/**
* Represents a description of why someone is denying access to the database, to help debugging. Components
* granting and revoking access should use the same denial reason for both method calls, as it is used to track
* who is blocking access to the database.
*/
public interface AvailabilityRequirement
{
String description();
}
public static AvailabilityRequirement availabilityRequirement( final String descriptionWhenBlocking )
{
return new AvailabilityRequirement()
{
@Override
public String description()
{
return descriptionWhenBlocking;
}
@Override
public boolean equals( Object o )
{
if ( this == o )
{
return true;
}
if ( o == null || getClass() != o.getClass() )
{
return false;
}
AvailabilityRequirement that = (AvailabilityRequirement) o;
return descriptionWhenBlocking == null ?
that.description() == null :
descriptionWhenBlocking.equals( that.description() );
}
@Override
public int hashCode()
{
return descriptionWhenBlocking != null ? descriptionWhenBlocking.hashCode() : 0;
}
};
}
private final AtomicInteger requirementCount = new AtomicInteger( 0 );
private final Set<AvailabilityRequirement> blockingRequirements = new CopyOnWriteArraySet<>();
private final AtomicBoolean isShutdown = new AtomicBoolean( false );
private final Listeners<AvailabilityListener> listeners = new Listeners<>();
private final Clock clock;
private final Log log;
public AvailabilityGuard( Clock clock, Log log )
{
this.clock = clock;
this.log = log;
}
/**
* Indicate a requirement that must be fulfilled before the database is considered available.
*
* @param requirement the requirement object
*/
public void require( AvailabilityRequirement requirement )
{
if ( !blockingRequirements.add( requirement ) )
{
return;
}
synchronized ( requirementCount )
{
if ( requirementCount.getAndIncrement() == 0 && !isShutdown.get() )
{
log.info( DATABASE_UNAVAILABLE_MSG + requirement.description() );
listeners.notify( AvailabilityListener::unavailable );
}
}
}
/**
* Indicate that a requirement has been fulfilled.
*
* @param requirement the requirement object
*/
public void fulfill( AvailabilityRequirement requirement )
{
if ( !blockingRequirements.remove( requirement ) )
{
return;
}
synchronized ( requirementCount )
{
if ( requirementCount.getAndDecrement() == 1 && !isShutdown.get() )
{
log.info( DATABASE_AVAILABLE_MSG + requirement.description() );
listeners.notify( AvailabilityListener::available );
}
}
}
/**
* Shutdown the guard. After this method is invoked, the database will always be considered unavailable.
*/
public void shutdown()
{
synchronized ( requirementCount )
{
if ( isShutdown.getAndSet( true ) )
{
return;
}
if ( requirementCount.get() == 0 )
{
listeners.notify( AvailabilityListener::unavailable );
}
}
}
private enum Availability
{
AVAILABLE,
UNAVAILABLE,
SHUTDOWN
}
/**
* Check if the database is available for transactions to use.
*
* @return true if there are no requirements waiting to be fulfilled and the guard has not been shutdown
*/
public boolean isAvailable()
{
return availability() == Availability.AVAILABLE;
}
/**
* Check if the database has been shut down.
*/
public boolean isShutdown()
{
return availability() == Availability.SHUTDOWN;
}
/**
* Check if the database is available for transactions to use.
*
* @param millis to wait for availability
* @return true if there are no requirements waiting to be fulfilled and the guard has not been shutdown
*/
public boolean isAvailable( long millis )
{
return availability( millis ) == Availability.AVAILABLE;
}
/**
* Checks if available. If not then an {@link UnavailableException} is thrown describing why.
* This methods doesn't wait like {@link #await(long)} does.
*
* @throws UnavailableException if not available.
*/
public void checkAvailable() throws UnavailableException
{
await( 0 );
}
/**
* Await the database becoming available.
*
* @param millis to wait for availability
* @throws UnavailableException thrown when the timeout has been exceeded or the guard has been shutdown
*/
public void await( long millis ) throws UnavailableException
{
Availability availability = availability( millis );
if ( availability == Availability.AVAILABLE )
{
return;
}
String description = (availability == Availability.UNAVAILABLE)
? "Timeout waiting for database to become available and allow new transactions. Waited " +
Format.duration( millis ) + ". " + describeWhoIsBlocking()
: "Database not available because it's shutting down";
throw new UnavailableException( description );
}
private Availability availability()
{
if ( isShutdown.get() )
{
return Availability.SHUTDOWN;
}
int count = requirementCount.get();
if ( count == 0 )
{
return Availability.AVAILABLE;
}
assert (count > 0);
return Availability.UNAVAILABLE;
}
private Availability availability( long millis )
{
Availability availability = availability();
if ( availability != Availability.UNAVAILABLE )
{
return availability;
}
long timeout = clock.millis() + millis;
do
{
try
{
Thread.sleep( 10 );
}
catch ( InterruptedException e )
{
Thread.interrupted();
break;
}
availability = availability();
} while ( availability == Availability.UNAVAILABLE && clock.millis() < timeout );
return availability;
}
/**
* Add a listener for changes to availability.
*
* @param listener the listener to receive callbacks when availability changes
*/
public void addListener( AvailabilityListener listener )
{
listeners.add( listener );
}
/**
* Remove a listener for changes to availability.
*
* @param listener the listener to remove
*/
public void removeListener( AvailabilityListener listener )
{
listeners.remove( listener );
}
/**
* @return a textual description of what components, if any, are blocking access
*/
public String describeWhoIsBlocking()
{
if ( blockingRequirements.size() > 0 || requirementCount.get() > 0 )
{
String causes = Iterables.join( ", ", Iterables.map( DESCRIPTION, blockingRequirements ) );
return requirementCount.get() + " reasons for blocking: " + causes + ".";
}
return "No blocking components";
}
public static final Function<AvailabilityRequirement, String> DESCRIPTION =
AvailabilityRequirement::description;
}