Basic Examples

This section guides the developer of Omid-based transactional applications over the different steps required to execute multi-row transactions on HBase and the different APIs involved in the process.

Click here to go directly to a complete example

Obtaining the Transaction Manager

In order to use transactions, a client application needs to create an instance of the TransactionManager interface with is part of the Transactional Client described in the architecture (See section About Omid). The current Omid version offers an implementation of a transaction manager for HBase. Please, make sure core-site.xml and hbase-site.xml HBase configuration files are present in the CLASSPATH of your client application.

To create a transaction manager just add the following code to your application:

    ...
    TransactionManager tm = HBaseTransactionManager.newInstance();
    ...

If nothing is specified, the HBase transaction manager instance is created with default configuration settings loaded from the default-hbase-omid-client-config.yml file included in the HBase Omid client jar. To change the client default settings, there are two possibilities:

1. Put the specific configuration settings in a file named hbase-omid-client-config.yml and include it in the CLASSPATH. The file has the following format:

#HBase related
commitTableName: MY_OWN_OMID_COMMIT_TABLE_NAME

#TSO/ZK connection
omidClientConfiguration: !!com.yahoo.omid.tsoclient.OmidClientConfiguration
    connectionType: !!com.yahoo.omid.tsoclient.OmidClientConfiguration$ConnType ZK
    connectionString: "my_zk_cluster_conn_string"

#Instrumentation
metrics: !!com.yahoo.omid.metrics.NullMetricsProvider [ ]

2. Create an instance of the HBaseOmidClientConfiguration class in the application code and pass it in the creation of the transaction manager instance:

    ...    
    HBaseOmidClientConfiguration omidClientConfiguration = new HBaseOmidClientConfiguration();
    omidClientConfiguration.setConnectionType(DIRECT);
    omidClientConfiguration.setConnectionString("my_tso_server_host:54758");
    omidClientConfiguration.setRetryDelayMs(3000);
    
    TransactionManager tm = HBaseTransactionManager.newInstance(omidClientConfiguration);
    ...

Please, refer to the ConfigurationExample in the source code to experiment with the configuration options.

Creating Transactions

Once the TransactionManager is created, client applications can use its interface to demarcate transactional boundaries.

In order to create a transaction the TransactionManager.begin() method is used:

    ...

    Transaction tx = tm.begin();

    ...

The transaction manager will return an instance of the Transaction interface representing the recently created transaction. This instance is necessary to instruct the operations on the data source, in which transactional context they should operate (See next section).

Executing Transactional Operations

In order to perform transaction operations on data, the client application requires to use a wrapper on the HBase's HTableInterface abstraction. The wrapper is called TTable and is also part of what is described as Transactional Client in the Omid architecture (See section About Omid). TTable basically offers the same interface as HTableInterface enhanced with a parameter representing the transactional context. As was previously described, a Transaction instance is obtained on return of TransactionManager.begin() method calls.

    ...
    TTable txTable = new TTable(conf, "EXAMPLE_TABLE"); 
    ...

Once the access point to the data has been created, applications can use it to trigger transactional operations:

    private final byte[] family = Bytes.toBytes("EXAMPLE_CF");    
    private final byte[] qualifier = Bytes.toBytes("foo");

    ...

	// Retrieve transactionally a specific cell
    Get get = new Get(Bytes.toBytes("EXAMPLE_ROW");
    get.add(family, qualifier);
    Result txGetResult = txTable.get(tx, get);

    ...

    // Add a new cell value inside a transactional context
    Put updatedRow = new Put(Bytes.toBytes("EXAMPLE_ROW");
    updatedRow.add(family, qualifier, Bytes.toBytes("Another_value"));
    txTable.put(tx, updatedRow);

    ...

Committing and Aborting Transactions

Once the client application has finished reading/writting from/into the datasource, it must decide whether to make the changes visible or to discard them. In order to do this, it must instruct the Omid TransactionManager either to commit() or to rollback() the transactional context where the changes were produced. In case of commit, the TSO server will be notified to perform the SI validation phase. If the validation succeeds the changes will be visible to new transactions started from that point on. Otherwise, it will roll back the changes.

In order to commit the data, the client application should do something like this:

    ...

    try {

        ...

        tm.commit(tx);

    } catch (RollbackException e) {

        // Here the transaction was rolled-back when
        // trying to commit due to conflicts with other
        // some other concurrent transaction
        // 
        // The client application should do whatever is
        // required according to its business logic

        ...

    }

A transaction can also be specifically aborted, for example if something goes wrong when executing the business logic:

    ...

    try {

        ...

	 	if( ! some_business_logic_condition ) {
            tm.rollback(tx);
            throw AnyApplicationException("Changes aborted due to...");
        } 

        tm.commit(tx);

    } catch (RollbackException e) {

        ...

    }

Complete Example

This example summarizes the steps described above.

import org.apache.hadoop.hbase.client.Put;
import org.apache.hadoop.hbase.util.Bytes;
import com.yahoo.omid.transaction.HBaseTransactionManager;
import com.yahoo.omid.transaction.TTable;
import com.yahoo.omid.transaction.Transaction;
import com.yahoo.omid.transaction.TransactionManager;

public class Example {

    public static final byte[] exampleRow = Bytes.toBytes("EXAMPLE_ROW");
    public static final byte[] family = Bytes.toBytes("EXAMPLE_CF");
    public static final byte[] qualifier = Bytes.toBytes("foo");
    public static final byte[] dataValueToAvoid = Bytes.toBytes("valToAvoid");
    public static final byte[] dataValue = Bytes.toBytes("val");

    public static void main(String[] args) throws Exception {

        try (TransactionManager tm = HBaseTransactionManager.newInstance();
             TTable txTable = new TTable("EXAMPLE_TABLE")) {

            Transaction tx = tm.begin();
            System.out.printl("Transaction started");

            // Retrieve data transactionally
            Get get = new Get(exampleRow);
            get.add(family, qualifier);
            Result txGetResult = txTable.get(tx, get);
            byte[] retrievedValue = txGetResult.getValue(family, qualifier);

            // Add a condition in the application logic to show explicit transaction
            // aborts just for illustrative purposes
            if (Bytes.equals(retrievedValue, dataValueToAvoid)) {
                tm.rollback(tx);
                throw new RuntimeException("Illegal value found in database!");
            }

            // Otherwise, add a value in other column and try to commit the transaction
            try {
                Put putOnRow = new Put(exampleRow);
                putOnRow.add(family, qualifier, dataValue);
        	    txTable.put(tx, putOnRow);
                tm.commit(tx);
                System.out.println("Transaction committed. New value written to example row");
            } catch(RollbackException e) {
                System.out.println("Transaction aborted due to conflicts. Changes to row aborted");
            }

        }

    }

}