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Transactions

You can use Hazelcast in transactional context.

Transaction Interface

You create a TransactionContext to begin, commit, and rollback a transaction. You can obtain transaction-aware instances of queues, maps, sets, lists, multimaps via TransactionContext, work with them, and commit/rollback in one shot.

Hazelcast supports two types of transactions: LOCAL (One Phase) and TWO_PHASE. With the type, you have influence over how much guarantee you get when a member crashes while a transaction is committing. The default behavior is TWO_PHASE.

  • LOCAL: Unlike the name suggests, LOCAL is a two phase commit. First, all cohorts are asked to prepare; if everyone agrees, then all cohorts are asked to commit. A problem can happen during the commit phase: if one or more members crash, then the system could be left in an inconsistent state.

  • TWO_PHASE: The TWO_PHASE commit is more than the classic two phase commit (if you want a regular two phase commit, use local). Before TWO_PHASE commits, it copies the commit-log to other members, so in case of member failure, another member can complete the commit.

import java.util.Queue;
import java.util.Map;
import java.util.Set;
import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.Transaction; 

HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();

TransactionOptions options = new TransactionOptions()
    .setTransactionType( TransactionType.LOCAL );

TransactionContext context = hazelcastInstance.newTransactionContext( options )
context.beginTransaction();

TransactionalQueue queue = context.getQueue( "myqueue" );
TransactionalMap map = context.getMap( "mymap" );
TransactionalSet set = context.getSet( "myset" );

try {
  Object obj = queue.poll();
  //process obj
  map.put( "1", "value1" );
  set.add( "value" );
  //do other things..
  context.commitTransaction();
} catch ( Throwable t ) {
  context.rollbackTransaction();
}

In a transaction, operations will not be executed immediately. Their changes will be local to the TransactionContext until committed. However, they will ensure the changes via locks.

For the above example, when map.put is executed, no data will be put to the map but the key will get locked for changes. While committing, operations will be executed, the value will be put to the map, and the key will be unlocked.

Isolation level in Hazelcast Transactions is READ_COMMITTED. If you are in a transaction, you can read the data in your transaction and the data that is already committed. If you are not in a transaction, you can only read the committed data.

image NOTE: The REPEATABLE_READ isolation level can also be exercised using the method getForUpdate() of TransactionalMap.

Implementation is different for queue/set/list and map/multimap. For queue operations (offer, poll), offered and/or polled objects are copied to the owner member in order to safely commit/rollback. For map/multimap, Hazelcast first acquires the locks for the write operations (put, remove) and holds the differences (what is added/removed/updated) locally for each transaction. When the transaction is set to commit, Hazelcast will release the locks and apply the differences. When rolling back, Hazelcast will release the locks and discard the differences.

MapStore and QueueStore does not participate in transactions. Hazelcast will suppress exceptions thrown by store in a transaction. Please refer to the XA Transactions section for further information.