Isolation guarantees are an important aspect of transactional protection. Transactions ensure the data your transaction is working with will not be changed by some other transaction. Moreover, the modifications made by a transaction will never be viewable outside of that transaction until the changes have been committed.
That said, there are different degrees of isolation, and you can choose to relax your isolation guarantees to one degree or another depending on your application's requirements. The primary reason why you might want to do this is because of performance; the more isolation you ask your transactions to provide, the more locking that your application must do. With more locking comes a greater chance of blocking, which in turn causes your threads to pause while waiting for a lock. Therefore, by relaxing your isolation guarantees, you can potentially improve your application's throughput. Whether you actually see any improvement depends, of course, on the nature of your application's data and transactions.
BDB XML supports the following levels of isolation:
| Degree | ANSI Term | Definition |
|---|---|---|
| 1 | READ UNCOMMITTED | Uncommitted reads means that one transaction will never overwrite another transaction's dirty data. Dirty data is data that a transaction has modified but not yet committed to the underlying data store. However, uncommitted reads allows a transaction to see data dirtied by another transaction. In addition, a transaction may read data dirtied by another transaction, but which subsequently is aborted by that other transaction. In this latter case, the reading transaction may be reading data that never really existed in the container. |
| 2 | READ COMMITTED |
Committed read isolation means that degree 1 is observed, except that dirty data is never read. In addition, this isolation level guarantees that data will never change so long as it is addressed by the cursor, but the data may change before the reading cursor is closed. In the case of a transaction, data at the current cursor position will not change, but once the cursor moves, the previous referenced data can change. This means that readers release read locks before the cursor is closed, and therefore, before the transaction completes. Note that this level of isolation causes the cursor to operate in exactly the same way as it does in the absence of a transaction. |
| 3 | SERIALIZABLE |
Committed read is observed, plus the data read by a transaction, T, will never be dirtied by another transaction before T completes. This means that both read and write locks are not released until the transaction completes. In addition, no transactions will see phantoms. Phantoms are records returned as a result of a search, but which were not seen by the same transaction when the identical search criteria was previously used. This is BDB XML's default isolation guarantee. |
By default, BDB XML transactions and transactional cursors offer serializable isolation. You can optionally reduce your isolation level by configuring BDB XML to use uncommitted read isolation. See Reading Uncommitted Data for more information. You can also configure BDB XML to use committed read isolation. See Committed Reads for more information.
Finally, in addition to BDB XML's normal degrees of isolation, you can also use snapshot isolation. This allows you to avoid the read locks that serializable isolation requires. See Using Snapshot Isolation for details.
Berkeley DB allows you to configure your application to read data that has been modified but not yet committed by another transaction; that is, dirty data. When you do this, you may see a performance benefit by allowing your application to not have to block waiting for write locks. On the other hand, the data that your application is reading may change before the transaction has completed.
That said, configuring BDB XML to read uncommitted data can result in internal inconsistencies which lead to random errors. For best results, you should avoid configuring your BDB XML transactions to read uncommitted data.
Committed Reads control the behavior of a Berkeley DB mechanism called a cursor. Cursors are not something you would normally be using directly with your BDB XML application, unless you are using Berkeley DB databases alongside of your BDB XML containers. For that reason, this section is potentially not of great interest to you. Still, we present it here for the sake of completeness.
For a thorough description of cursors, see the Getting Started with Berkeley DB guide.
You can configure your transaction so that the data being read by a transactional cursor is consistent so long as it is being addressed by the cursor. However, once the cursor is done reading the record (that is, reading records from the page that it currently has locked), the cursor releases its lock on that record or page. This means that the data the cursor has read and released may change before the cursor's transaction has completed.
For example,
suppose you have two transactions, Ta and Tb. Suppose further that
Ta has a cursor that reads record R, but does not modify it. Normally,
Tb would then be unable to write record R because
Ta would be holding a read lock on it. But when you configure your transaction for
committed reads, Tb can modify record
R before Ta completes, so long as the reading cursor is no longer
addressing the
record or page.
When you configure your application for this level of isolation, you may see better performance throughput because there are fewer read locks being held by your transactions. Read committed isolation is most useful when you have a cursor that is reading and/or writing records in a single direction, and that does not ever have to go back to re-read those same records. In this case, you can allow BDB XML to release read locks as it goes, rather than hold them for the life of the transaction.
To configure your application to use committed reads, do one of the following:
Create your transaction such that it allows committed reads. You do this by
specifying true to
TransactionConfig.setReadCommitted().
Specify true to
CursorConfig.setReadCommitted().
For example, the following creates a transaction that allows committed reads:
package dbxml.txn;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import com.sleepycat.db.TransactionConfig;
import com.sleepycat.dbxml.XmlContainer;
import com.sleepycat.dbxml.XmlContainerConfig;
import com.sleepycat.dbxml.XmlException;
import com.sleepycat.dbxml.XmlManager;
import com.sleepycat.dbxml.XmlManagerConfig;
import com.sleepycat.dbxml.XmlTransaction;
import java.io.File;
import java.io.FileNotFoundException;
...
Environment myEnv = null;
File envHome = new File("/export1/testEnv");
XmlManager myManager = null;
XmlContainer myContainer = null;
try {
EnvironmentConfig envConf = new EnvironmentConfig();
envConf.setAllowCreate(true); // If the environment does not
// exits, create it.
envConf.setInitializeCache(true); // Turn on the shared memory
// region.
envConf.setInitializeLocking(true); // Turn on the locking subsystem.
envConf.setInitializeLogging(true); // Turn on the logging subsystem.
envConf.setTransactional(true); // Turn on the transactional
// subsystem.
myEnv = new Environment(envHome, envConf);
XmlManagerConfig managerConfig = new XmlManagerConfig();
myManager = new XmlManager(myEnv, managerConfig);
// Notice that we do not have to specify any properties to the
// container to allow committed reads (this is as opposed
// to uncommitted reads where we DO have to specify a
// property on the container open).
XmlContainerConfig containerConf = new XmlContainerConfig();
containerConf.setTransactional(true);
containerConf.setAllowCreate(true);
String containerName = "myContainer.dbxml";
myContainer = myManager.openContainer(containerName, containerConf);
String file = "doc1.xml";
TransactionConfig txnConfig = new TransactionConfig();
// Open the transaction and enable committed reads. All
// queries performed with this transaction handle will
// use read committed isolation.
txnConfig.setReadCommitted(true);
XmlTransaction txn = myManager.createTransaction(null, txnConfig);
// From here, you perform your container reads and writes as normal,
// committing and aborting the transactions as is necessary, as well as
// testing for deadlock exceptions as normal (omitted for brevity).
...
By default BDB XML uses serializable isolation. An important side effect of this isolation level is that read operations obtain read locks on database pages, and then hold those locks until the read operation is completed.
You can avoid this by using snapshot isolation. Snapshot isolation uses multiversion concurrency control to guarantee repeatable reads. What this means is that every time a writer would take a read lock on a page, instead a copy of the page is made and the writer operates on that page copy. This frees other writers from blocking due to a read lock held on the page.
Snapshot isolation is strongly recommended for read-only threads when writer threads are also running, as this will eliminate read-write contention and greatly improve transaction throughput for your writer threads. However, in order for snapshot isolation to work for your reader-only threads, you must of course use transactions for your BDB XML reads.
Snapshot isolation does not come without a cost. Because pages are being duplicated before being operated upon, the cache will fill up faster. This means that you might need a larger cache in order to hold the entire working set in memory.
If the cache becomes full of page copies before old copies can be discarded, additional I/O will occur as pages are written to temporary "freezer" files on disk. This can substantially reduce throughput, and should be avoided if possible by configuring a large cache and keeping snapshot isolation transactions short.
You can estimate how large your cache should be by
taking a checkpoint, followed by a call to the
Environment.getArchiveLogFiles()
method. The amount of cache required is approximately
double the size of the remaining log files (that is,
the log files that cannot be archived).
In addition to an increased cache size, you may also need to increase the maximum number of transactions that your application supports. (See Configuring the Transaction Subsystem for details on how to set this.) In the worst case scenario, you might need to configure your application for one more transaction for every page in the cache. This is because transactions are retained until the last page they created is evicted from the cache.
Snapshot isolation is best used when all or most of the following conditions are true:
You can have a large cache relative to your working data set size.
You require repeatable reads.
You will be using transactions that routinely work on the entire database, or more commonly, there is data in your database that will be very frequently written by more than one transaction.
If your application uses a single write thread and multiple readers, then snapshot isolation can help performance. However, if your application uses multiple write threads, then snapshot isolation can result in additional deadlocks that may harm your application's performance.
You use snapshot isolation by:
Opening the container with
multiversion support. You can
configure this either when you
open your environment or when
you open your
container.
Use the
DB_MULTIVERSION
flag to configure this support.
Use the
XmlContainerConfig::setMultiversion()
option to configure this support when you open your
container. To configure multiversion support when you
open your environment, use the
DB_MULTIVERSION flag on the
environment open.
Use either the
EnvironmentConfig.setMultiversion()
or the
XmlContainerConfig.setMultiversion()
option to configure
this support.
Configure your transaction to use snapshot isolation.
To do this,
specify the
TransactionConfig.setSnapshot()
option when you configure your transaction.
The simplest way to take advantage of snapshot isolation is for queries: keep update transactions using full read/write locking and use snapshot isolation on read-only transactions or cursors. This should minimize blocking of snapshot isolation transactions and will avoid deadlock errors.
If the application has update transactions which read many items and only update a small set (for example, scanning until a desired record is found, then modifying it), throughput may be improved by running some updates at snapshot isolation as well. But doing this means that you must manage deadlock errors. See Resolving Deadlocks for details.
The following code fragment turns on snapshot isolation for a transaction:
package dbxml.txn;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import com.sleepycat.dbxml.XmlContainer;
import com.sleepycat.dbxml.XmlContainerConfig;
import com.sleepycat.dbxml.XmlException;
import com.sleepycat.dbxml.XmlManager;
import com.sleepycat.dbxml.XmlManagerConfig;
import com.sleepycat.dbxml.XmlTransaction;
import java.io.File;
import java.io.FileNotFoundException;
...
Environment myEnv = null;
File envHome = new File("/export1/testEnv");
XmlManager myManager = null;
XmlContainer myContainer = null;
try {
EnvironmentConfig envConf = new EnvironmentConfig();
envConf.setAllowCreate(true); // If the environment does not
// exits, create it.
envConf.setInitializeCache(true); // Turn on the shared memory
// region.
envConf.setInitializeLocking(true); // Turn on the locking subsystem.
envConf.setInitializeLogging(true); // Turn on the logging subsystem.
envConf.setTransactional(true); // Turn on the transactional
// subsystem.
envConf.setMultiversion(true); // Turn on snapshot isolation.
myEnv = new Environment(envHome, envConf);
XmlManagerConfig managerConfig = new XmlManagerConfig();
myManager = new XmlManager(myEnv, managerConfig);
// Nothing new is required here for snapshot isolation.
XmlContainerConfig containerConf = new XmlContainerConfig();
containerConf.setTransactional(true);
containerConf.setAllowCreate(true);
String containerName = "myContainer.dbxml";
myContainer = myManager.openContainer(containerName, containerConf);
TransactionConfig txnConfig = new TransactionConfig();
txnConfig.setSnapshot(true);
XmlTransaction txn = myManager.createTransaction(NULL, txnConfig);
...
// Remainder of program omitted for brevity.