Locking Maps
Hazelcast Distributed Map (IMap) is thread-safe to meet your thread safety requirements. When these requirements increase or you want to have more control on the concurrency, consider the following Hazelcast features and solutions.
Let's work on a sample case as shown below.
public class RacyUpdateMember {
public static void main( String[] args ) throws Exception {
HazelcastInstance hz = Hazelcast.newHazelcastInstance();
IMap<String, Value> map = hz.getMap( "map" );
String key = "1";
map.put( key, new Value() );
System.out.println( "Starting" );
for ( int k = 0; k < 1000; k++ ) {
if ( k % 100 == 0 ) System.out.println( "At: " + k );
Value value = map.get( key );
Thread.sleep( 10 );
value.amount++;
map.put( key, value );
}
System.out.println( "Finished! Result = " + map.get(key).amount );
}
static class Value implements Serializable {
public int amount;
}
}
If the above code is run by more than one cluster member simultaneously, there will be likely a race condition. You can solve this with Hazelcast.
Pessimistic Locking
One way to solve the race issue is with pessimistic locking: lock the map entry until you are finished with it.
To perform pessimistic locking, use the lock mechanism provided by Hazelcast distributed map, i.e. the map.lock and map.unlock methods. See the below example code.
public class PessimisticUpdateMember {
public static void main( String[] args ) throws Exception {
HazelcastInstance hz = Hazelcast.newHazelcastInstance();
IMap<String, Value> map = hz.getMap( "map" );
String key = "1";
map.put( key, new Value() );
System.out.println( "Starting" );
for ( int k = 0; k < 1000; k++ ) {
map.lock( key );
try {
Value value = map.get( key );
Thread.sleep( 10 );
value.amount++;
map.put( key, value );
} finally {
map.unlock( key );
}
}
System.out.println( "Finished! Result = " + map.get( key ).amount );
}
static class Value implements Serializable {
public int amount;
}
}
The IMap lock will automatically be collected by the garbage collector when the lock is released and no other waiting conditions exist on the lock.
The IMap lock is reentrant, but it does not support fairness.
Another way to solve the race issue can be acquiring a predictable Lock object from Hazelcast. This way, every value in the map can be given a lock or you can create a stripe of locks.
Optimistic Locking
In Hazelcast, you can apply the optimistic locking strategy with the map replace method. This method compares values in object or data forms depending on the in-memory format configuration. If the values are equal, it replaces the old value with the new one. If you want to use your defined equals method, in-memory-format should be OBJECT. Otherwise, Hazelcast serializes objects to BINARY forms and compares them.
See the below example code.
public class OptimisticMember {
public static void main( String[] args ) throws Exception {
HazelcastInstance hz = Hazelcast.newHazelcastInstance();
IMap<String, Value> map = hz.getMap( "map" );
String key = "1";
map.put( key, new Value() );
System.out.println( "Starting" );
for ( int k = 0; k < 1000; k++ ) {
if ( k % 10 == 0 ) System.out.println( "At: " + k );
for (; ; ) {
Value oldValue = map.get( key );
Value newValue = new Value( oldValue );
Thread.sleep( 10 );
newValue.amount++;
if ( map.replace( key, oldValue, newValue ) )
break;
}
}
System.out.println( "Finished! Result = " + map.get( key ).amount );
}
static class Value implements Serializable {
public int amount;
public Value() {
}
public Value( Value that ) {
this.amount = that.amount;
}
public boolean equals( Object o ) {
if ( o == this ) return true;
if ( !( o instanceof Value ) ) return false;
Value that = ( Value ) o;
return that.amount == this.amount;
}
}
}
NOTE: The above example code is intentionally broken.
Pessimistic vs. Optimistic Locking
Depending on your locking requirements, you can pick one locking strategy.
Optimistic locking is better for mostly read-only systems. It has a performance boost over pessimistic locking.
Pessimistic locking is good if there are lots of updates on the same key. It is more robust than optimistic locking from the perspective of data consistency.
In Hazelcast, use IExecutorService to submit a task to a key owner, or to a member or members. This is the recommended way to perform task executions, rather than using pessimistic or optimistic locking techniques. IExecutorService will have less network hops and less data over wire, and tasks will be executed very near to the data. Please refer to the Data Affinity section.
Solving the ABA Problem
The ABA problem occurs in environments when a shared resource is open to change by multiple threads. Even if one thread sees the same value for a particular key in consecutive reads, it does not mean that nothing has changed between the reads. Another thread may change the value, do work, and change the value back, while the first thread thinks that nothing has changed.
To prevent these kind of problems, one solution is to use a version number and to check it before any write to be sure that nothing has changed between consecutive reads. Although all the other fields will be equal, the version field will prevent objects from being seen as equal. This is the optimistic locking strategy, and it is used in environments which do not expect intensive concurrent changes on a specific key.
In Hazelcast, you can apply the optimistic locking strategy with the map replace method.