com.sleepycat.collections
Class TransactionRunner

java.lang.Object
  com.sleepycat.collections.TransactionRunner

public class TransactionRunner
extends Object

Starts a transaction, calls TransactionWorker.doWork(), and handles transaction retry and exceptions. To perform a transaction, the user implements the TransactionWorker interface and passes an instance of that class to the run method.

A single TransactionRunner instance may be used by any number of threads for any number of transactions.

The behavior of the run() method depends on whether the environment is transactional, whether nested transactions are enabled, and whether a transaction is already active.

• When the run() method is called in a transactional environment and no transaction is active for the current thread, a new transaction is started before calling doWork(). If DeadlockException is thrown by doWork(), the transaction will be aborted and the process will be repeated up to the maximum number of retries. If another exception is thrown by doWork() or the maximum number of retries has occurred, the transaction will be aborted and the exception will be rethrown by the run() method. If no exception is thrown by doWork(), the transaction will be committed. The run() method will not attempt to commit or abort a transaction if it has already been committed or aborted by doWork().
• When the run() method is called and a transaction is active for the current thread, and nested transactions are enabled, a nested transaction is started before calling doWork(). The transaction that is active when calling the run() method will become the parent of the nested transaction. The nested transaction will be committed or aborted by the run() method following the same rules described above. Note that nested transactions may not be enabled for the JE product, since JE does not support nested transactions.
• When the run() method is called in a non-transactional environment, the doWork() method is called without starting a transaction. The run() method will return without committing or aborting a transaction, and any exceptions thrown by the doWork() method will be thrown by the run() method.
• When the run() method is called and a transaction is active for the current thread and nested transactions are not enabled (the default) the same rules as above apply. All the operations performed by the doWork() method will be part of the currently active transaction.

In a transactional environment, the rules described above support nested calls to the run() method and guarantee that the outermost call will cause the transaction to be committed or aborted. This is true whether or not nested transactions are supported or enabled. Note that nested transactions are provided as an optimization for improving concurrency but do not change the meaning of the outermost transaction. Nested transactions are not currently supported by the JE product.

Field Summary

static int

DEFAULT_MAX_RETRIES The default maximum number of retries.

Constructor Summary

TransactionRunner(Environment env)
Creates a transaction runner for a given Berkeley DB environment.

TransactionRunner(Environment env, int maxRetries, TransactionConfig config)
Creates a transaction runner for a given Berkeley DB environment and with a given number of maximum retries.

Method Summary

boolean	getAllowNestedTransactions() Returns whether nested transactions will be created if run() is called when a transaction is already active for the current thread.
int	getMaxRetries() Returns the maximum number of retries that will be performed when deadlocks are detected.
TransactionConfig	getTransactionConfig() Returns the transaction configuration used for calling Environment.beginTransaction(com.sleepycat.db.Transaction, com.sleepycat.db.TransactionConfig).
int	handleException(Exception exception, int retries, int maxRetries) Handles exceptions that occur during a transaction, and may implement transaction retry policy.
void	run(TransactionWorker worker) Calls the TransactionWorker.doWork() method and, for transactional environments, may begin and end a transaction.
void	setAllowNestedTransactions(boolean allowNestedTxn) Changes whether nested transactions will be created if run() is called when a transaction is already active for the current thread.
void	setMaxRetries(int maxRetries) Changes the maximum number of retries that will be performed when deadlocks are detected.
void	setTransactionConfig(TransactionConfig config) Changes the transaction configuration used for calling Environment.beginTransaction(com.sleepycat.db.Transaction, com.sleepycat.db.TransactionConfig).

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DEFAULT_MAX_RETRIES

public static final int DEFAULT_MAX_RETRIES

The default maximum number of retries.

See Also:

Constant Field Values

Constructor Detail

TransactionRunner

public TransactionRunner(Environment env)

Creates a transaction runner for a given Berkeley DB environment. The default maximum number of retries (DEFAULT_MAX_RETRIES) and a null (default) TransactionConfig will be used.

Parameters:

env - is the environment for running transactions.

TransactionRunner

public TransactionRunner(Environment env,
                         int maxRetries,
                         TransactionConfig config)

Creates a transaction runner for a given Berkeley DB environment and with a given number of maximum retries.

Parameters:

env - is the environment for running transactions.

maxRetries - is the maximum number of retries that will be performed when deadlocks are detected.

config - the transaction configuration used for calling Environment.beginTransaction(com.sleepycat.db.Transaction, com.sleepycat.db.TransactionConfig), or null to use the default configuration. The configuration object is not cloned, and any modifications to it will impact subsequent transactions.

Method Detail

getMaxRetries

public int getMaxRetries()

Returns the maximum number of retries that will be performed when deadlocks are detected.

setMaxRetries

public void setMaxRetries(int maxRetries)

Changes the maximum number of retries that will be performed when deadlocks are detected. Calling this method does not impact transactions already running.

getAllowNestedTransactions

public boolean getAllowNestedTransactions()

Returns whether nested transactions will be created if run() is called when a transaction is already active for the current thread. By default this property is false.

Note that this method always returns false in the JE product, since nested transactions are not supported by JE.

setAllowNestedTransactions

public void setAllowNestedTransactions(boolean allowNestedTxn)

Changes whether nested transactions will be created if run() is called when a transaction is already active for the current thread. Calling this method does not impact transactions already running.

Note that true may not be passed to this method in the JE product, since nested transactions are not supported by JE.

getTransactionConfig

public TransactionConfig getTransactionConfig()

Returns the transaction configuration used for calling Environment.beginTransaction(com.sleepycat.db.Transaction, com.sleepycat.db.TransactionConfig).

If this property is null, the default configuration is used. The configuration object is not cloned, and any modifications to it will impact subsequent transactions.

Returns:

the transaction configuration.

setTransactionConfig

public void setTransactionConfig(TransactionConfig config)

Changes the transaction configuration used for calling Environment.beginTransaction(com.sleepycat.db.Transaction, com.sleepycat.db.TransactionConfig).

If this property is null, the default configuration is used. The configuration object is not cloned, and any modifications to it will impact subsequent transactions.

Parameters:

config - the transaction configuration.

run

public void run(TransactionWorker worker)
         throws DatabaseException,
                Exception

Calls the TransactionWorker.doWork() method and, for transactional environments, may begin and end a transaction. If the environment given is non-transactional, a transaction will not be used but the doWork() method will still be called. See the class description for more information.

Throws:

DeadlockException - when it is thrown by doWork() and the maximum number of retries has occurred. The transaction will have been aborted by this method.

Exception - when any other exception is thrown by doWork(). The exception will first be unwrapped by calling ExceptionUnwrapper.unwrap(java.lang.Exception). The transaction will have been aborted by this method.

DatabaseException

handleException

public int handleException(Exception exception,
                           int retries,
                           int maxRetries)
                    throws Exception

Handles exceptions that occur during a transaction, and may implement transaction retry policy. The transaction is aborted by the run method before calling this method.

The default implementation of this method throws the exception parameter if it is not an instance of DeadlockException and otherwise returns the maxRetries parameter value. This method can be overridden to throw a different exception or return a different number of retries. For example:

• This method could call Thread.sleep for a short interval to allow other transactions to finish.
• This method could return a different maxRetries value depending on the exception that occurred.
• This method could throw an application-defined exception when the retries value is greater or equal to the maxRetries and a DeadlockException occurs, to override the default behavior which is to throw the DeadlockException.

Parameters:

exception - an exception that was thrown by the TransactionWorker.doWork() method or thrown when beginning or committing the transaction. If the retries value is greater or equal to maxRetries when this method returns normally, this exception will be thrown by the run method.

retries - the current value of a counter that starts out at zero and is incremented when each retry is performed.

maxRetries - the maximum retries to be performed. By default, this value is set to getMaxRetries(). This method may return a different maximum retries value to override that default.

Returns:

the maximum number of retries to perform. The default policy is to return the maxRetries parameter value if the exception parameter value is an instance of DeadlockException.

Throws:

Exception - to cause the exception to be thrown by the run method. The default policy is to throw the exception parameter value if it is not an instance of DeadlockException.

Since:

3.4