Two-Phase Commit Transactions

ScalarDB also supports two-phase commit style transactions called Two-phase Commit Transactions. With Two-phase Commit Transactions, you can execute a transaction that spans multiple processes/applications (e.g., Microservices).

This document briefly explains how to execute Two-phase Commit Transactions in ScalarDB.

Configuration

The configuration for Two-phase Commit Transactions is the same as the one for the transaction API.

For example, you can set the following configuration when you use Cassandra:

# Comma separated contact points
scalar.db.contact_points=cassandra

# Port number for all the contact points. Default port number for each database is used if empty.
scalar.db.contact_port=9042

# Credential information to access the database
scalar.db.username=cassandra
scalar.db.password=cassandra

# Storage implementation. Either cassandra or cosmos or dynamo or jdbc can be set. Default storage is cassandra.
scalar.db.storage=cassandra

Please see Getting Started for configurations of other databases/storages.

ScalarDB Server

You can also execute Two-phase Commit Transactions through the ScalarDB Server. You don't need a special configuration for Two-phase Commit Transactions, so you can follow the ScalarDB Server document to use it.

How to execute Two-phase Commit Transactions

This section explains how to execute Two-phase Commit Transactions.

Like a well-known two-phase commit protocol, there are two roles, a coordinator and a participant, that collaboratively execute a single transaction. The coordinator process first starts a transaction, and the participant processes join the transaction after that.

Get a TwoPhaseCommitTransactionManager instance

First, you need to get a TwoPhaseCommitTransactionManager instance to execute Two-phase Commit Transactions. You can use TransactionFactory to get a TwoPhaseCommitTransactionManager instance as follows:

TransactionFactory factory = new TransactionFactory(new DatabaseConfig(new File("<configuration file path>")));
TwoPhaseCommitTransactionManager manager = factory.getTwoPhaseCommitTransactionManager();

Start a transaction (coordinator only)

You can start a transaction as follows:

TwoPhaseCommitTransaction tx = manager.start();

The process/application that starts the transaction acts as a coordinator, as mentioned.

You can also start a transaction by specifying a transaction ID as follows:

TwoPhaseCommitTransaction tx = manager.start("<transaction ID>");

And, you can get the transaction ID with getId() as follows:

tx.getId();

Join the transaction (participant only)

If you are a participant, you can join the transaction that has been started by the coordinator as follows:

TwoPhaseCommitTransaction tx = manager.join("<transaction ID>")

You need to specify the transaction ID associated with the transaction that the coordinator has started.

Resume the transaction (participant only)

You can get the transaction object (the TwoPhaseCommitTransaction instance) that you have already joined with TwoPhaseCommitTransactionManager.resume():

TwoPhaseCommitTransaction tx = manager.resume("<transaction ID>")

TwoPhaseCommitTransactionManager manages the transaction objects that you have joined, and you can get it with the transaction ID.

CRUD operations for the transaction

The way to execute CRUD operations in Two-phase Commit Transactions is the same as the transaction API. TwoPhaseCommitTransacton has get()/put()/delete()/mutate() to execute CRUD operations.

This is an example code for CRUD operations in Two-phase Commit Transactions:

TwoPhaseCommitTransaction tx = ...

// Retrieve the current balances for ids
Get fromGet = new Get(new Key(ID, fromId));
Get toGet = new Get(new Key(ID, toId));
Optional<Result> fromResult = tx.get(fromGet);
Optional<Result> toResult = tx.get(toGet);

// Calculate the balances (it assumes that both accounts exist)
int newFromBalance = fromResult.get().getValue(BALANCE).get().getAsInt() - amount;
int newToBalance = toResult.get().getValue(BALANCE).get().getAsInt() + amount;

// Update the balances
Put fromPut = new Put(new Key(ID, fromId)).withValue(BALANCE, newFromBalance);
Put toPut = new Put(new Key(ID, toId)).withValue(BALANCE, newToBalance);
tx.put(fromPut);
tx.put(toPut);

Prepare/Commit/Rollback the transaction

After finishing CRUD operations, you need to commit the transaction. Like a well-known two-phase commit protocol, there are two phases: prepare and commit phases. You first need to prepare the transaction in all the coordinator/participant processes, then you need to call in the order of coordinator's commit() and the participants' commit() as follows:

TwoPhaseCommitTransaction tx = ...

try {
  // Execute CRUD operations in the coordinator/participant processes
  ...

  // Prepare phase: Prepare the transaction in all the coordinator/participant processes
  tx.prepare();
  ...

  // Commit phase: Commit the transaction in all the coordinator/participant processes
  tx.commit()
  ...
} catch (TransactionException e) {
  // When an error happans, you need to rollback the transaction in all the coordinator/participant processes
  tx.rollback();
  ...
}

If an error happens, you need to call rollback() in all the coordinator/participant processes. Note that you need to call it in the coordinator process first, and then call it in the participant processes in parallel.

You can call prepare() in the coordinator/participant processes in parallel. Similarly, you can also call commit() in the participant processes in parallel.

Validate the transaction

Depending on the concurrency control protocol, you need to call validate() in all the coordinator/participant processes after prepare() and before commit():

// Prepare phase 1: Prepare the transaction in all the coordinator/participant processes
tx.prepare();
...

// Prepare phase 2: Validate the transaction in all the coordinator/participant processes
tx.validate()
...

// Commit phase: Commit the transaction in all the coordinator/participant processes
tx.commit()
...

Similar to prepare(), you can call validate() in the coordinator/participant processes in parallel.

Currently, you need to call validate() when you use the Consensus Commit transaction manager with EXTRA_READ serializable strategy in SERIALIZABLE isolation level. In other cases, validate() does nothing.

Further documentation

One of the use cases for Two-phase Commit Transactions is Microservice Transaction. Please see the following sample to learn Two-phase Commit Transactions further:

• Microservice Transaction Sample