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<div class="chapter" lang="en" xml:lang="en">
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<div>
<div>
<h2 class="title"><a id="usingtxns"></a>Chapter 3. Transaction Basics</h2>
</div>
</div>
<div></div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="usingtxns.html#commitresults">Committing a Transaction</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="usingtxns.html#nodurabletxn">Non-Durable Transactions</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="abortresults.html">Aborting a Transaction</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="autocommit.html">Auto Commit</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="nestedtxn.html">Nested Transactions</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="txncursor.html">Transactional Cursors</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="txnindices.html">Secondary Indices with Transaction Applications</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="maxtxns.html">Configuring the Transaction Subsystem</a>
</span>
</dt>
</dl>
</div>
<p>
Once you have enabled transactions for your environment and your databases,
you can use them to protect your database operations. You do this by
acquiring a transaction handle and then using that handle for any
database operation that you want to participate in that transaction.
</p>
<p>
You obtain a transaction handle using the
<span><tt class="methodname">DbEnv::txn_begin()</tt> method.</span>
</p>
<p>
Once you have completed all of the operations that you want to include
in the transaction, you must commit the transaction using the
<span><tt class="methodname">DbTxn::commit()</tt> method.</span>
</p>
<p>
If, for any reason, you want to abandon the transaction, you abort
it using
<span><tt class="methodname">DbTxn::abort()</tt>.</span>
</p>
<p>
Any transaction handle that has been committed or aborted can no longer
be used by your application.
</p>
<p>
Finally, you must make sure that all transaction handles are either
committed or aborted before closing your databases and environment.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If you only want to transaction protect a single database write operation, you can use auto commit to
perform the transaction administration. When you use auto commit, you do not need an explicit transaction
handle. See <a href="autocommit.html">Auto Commit</a> for more information.
</p>
</div>
<p>
For example, the following example opens a transactional-enabled environment and
database, obtains a transaction handle, and then performs a write
operation under its protection. In the event of any failure in the
write operation, the transaction is aborted and the database is left in a
state as if no operations had ever been attempted in the first place.
</p>
<pre class="programlisting">#include "db_cxx.h"
...
int main(void)
{
u_int32_t env_flags = DB_CREATE | // If the environment does not
// exist, create it.
DB_INIT_LOCK | // Initialize locking
DB_INIT_LOG | // Initialize logging
DB_INIT_MPOOL | // Initialize the cache
DB_INIT_TXN; // Initialize transactions
u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
Db *dbp = NULL;
const char *file_name = "mydb.db";
const char *keystr ="thekey";
const char *datastr = "thedata";
std::string envHome("/export1/testEnv");
DbEnv myEnv(0);
try {
myEnv.open(envHome.c_str(), env_flags, 0);
dbp = new Db(&myEnv, 0);
// Open the database. Note that we are using auto commit for
// the open, so the database is able to support transactions.
dbp->open(NULL, // Txn pointer
file_name, // File name
NULL, // Logical db name
DB_BTREE, // Database type (using btree)
db_flags, // Open flags
0); // File mode. Using defaults
Dbt key, data;
key.set_data(keystr);
key.set_size((strlen(keystr) + 1) * sizeof(char));
key.set_data(datastr);
key.set_size((strlen(datastr) + 1) * sizeof(char));
DbTxn *txn = NULL;
myEnv.txn_begin(NULL, &txn, 0);
try {
db->put(txn, &key, &data, 0);
txn->commit(0);
} catch (DbException &e) {
std::cerr << "Error in transaction: "
<< e.what() << std::endl;
txn->abort();
}
} catch(DbException &e) {
std::cerr << "Error opening database and environment: "
<< file_name << ", "
<< envHome << std::endl;
std::cerr << e.what() << std::endl;
}
try {
if (dbp != NULL)
dbp->close(0);
myEnv.close(0);
} catch(DbException &e) {
std::cerr << "Error closing database and environment: "
<< file_name << ", "
<< envHome << std::endl;
std::cerr << e.what() << std::endl;
return (EXIT_FAILURE);
}
return (EXIT_SUCCESS);
} </pre>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="commitresults"></a>Committing a Transaction</h2>
</div>
</div>
<div></div>
</div>
<p>
In order to fully understand what is happening when you commit
a transaction, you must first understand a little about what
DB is doing with
<span>
the logging subsystem.
</span>
Logging causes all database write operations to be identified in
<span>logs, and by default these
logs are backed by files on disk. These logs are used to restore your databases
</span>
in the event of a system or application failure, so by performing
logging, DB ensures the integrity of your data.
</p>
<p>
Moreover, DB performs <span class="emphasis"><em>write-ahead</em></span>
logging. This means that information is written to the logs
<span class="emphasis"><em>before</em></span> the actual database
is changed.
This means that all write activity performed under the
protection of the transaction is noted in the log before
the transaction is committed. Be aware, however, that database
maintains logs in-memory. If you are backing your logs on
disk, the log information will eventually be written to the log
files, but while the transaction is on-going the log data may be
held only in memory.
</p>
<p>
When you commit a transaction, the following occurs:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
A commit record is written to the log. This
indicates that the modifications made by the
transaction are now permanent. By default, this write is performed synchronously to disk so the
commit record arrives in the log files before any other actions are taken.
</p>
</li>
<li>
<p>
Any log information held in memory is (by default)
synchronously written to disk. Note that this requirement can be
relaxed, depending on the type of commit you perform.
See <a href="usingtxns.html#nodurabletxn">Non-Durable Transactions</a> for
more information.
<span>Also, if you are
maintaining your logs entirely in-memory, then this
step will of course not be taken. To configure your
logging system for in-memory usage, see
<a href="logconfig.html#inmemorylogging">Configuring In-Memory Logging</a>.
</span>
</p>
</li>
<li>
<p>
All locks held by the transaction are released. This means
that read operations performed by other transactions or
threads of control can now see the modifications without
resorting to uncommitted reads (see <a href="isolation.html#dirtyreads">Reading Uncommitted Data</a> for more information).
</p>
</li>
</ul>
</div>
<p>
To commit a transaction, you simply call
<span><tt class="methodname">DbTxn::commit()</tt>.</span>
</p>
<p>
Notice that committing a transaction does not necessarily cause data
modified in your memory cache to be written to the files
backing your databases on disk. Dirtied database pages are written
for a number of reasons, but a transactional
commit is not one of them. The following are the things that can cause a dirtied
database page to be written to the backing database file:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Checkpoints.
</p>
<p>
Checkpoints cause all dirtied pages currently existing
in the cache to be written to disk, and a checkpoint
record is then written to the logs. You can run checkpoints
explicitly. For more information on checkpoints,
see <a href="filemanagement.html#checkpoints">Checkpoints</a>.
</p>
</li>
<li>
<p>
Cache is full.
</p>
<p>
If the in-memory cache fills up, then dirtied pages
might be written to disk in order to free up space for other
pages that your application needs to use. Note that if
dirtied pages are written to the database files, then
any log records that describe how those pages were
dirtied are written to disk before the database
pages are written.
</p>
</li>
</ul>
</div>
<p>
Be aware that because your transaction commit caused database
modifications recorded in your logs to be forced to disk, your modifications
are by default "persistent" in that they can be recovered in the event of
an application or system failure. However, recovery time is
gated by how much data has been modified since the last
checkpoint, so for applications that perform a lot of writes,
you may want to run a checkpoint with some frequency.
</p>
<p>
Note that once you have committed a transaction, the transaction
handle that you used for the transaction is no longer valid. To
perform database activities under the control of a new
transaction, you must obtain a fresh transaction handle.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="nodurabletxn"></a>Non-Durable Transactions</h3>
</div>
</div>
<div></div>
</div>
<p>
As previously noted, by default transaction commits are
durable because they cause the modifications performed
under the transaction to be synchronously recorded in
your on-disk log files. However, it is possible to use
non-durable transactions.
</p>
<p>
You may want non-durable transactions for performance
reasons. For example, you might be using transactions
simply for the isolation guarantee.
<span>
In this case, you might
not want a durability guarantee and so you may want to
prevent the disk I/O that normally accompanies a
transaction commit.
</span>
</p>
<p>
There are several ways to remove the durability guarantee
for your transactions:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Specify
<span>
<tt class="literal">DB_TXN_NOSYNC</tt> using the
<tt class="methodname">DbEnv::set_flags()</tt>
method.
</span>
This causes DB to not synchronously force any
<span>
log
</span>
data to disk upon transaction commit.
<span>
That is, the modifications are held entirely
in the in-memory cache and the logging
information is not forced to the filesystem for
long-term storage.
</span>
Note, however, that the
<span>
logging
</span>
data will eventually make it to the filesystem (assuming no
application or OS crashes) as a part of DB's
management of its logging buffers and/or cache.
</p>
<p>
This form of a commit provides a weak durability
guarantee because data loss can occur due to
an application
or OS crash.
</p>
<p>
This behavior is specified on a per-environment
handle basis. In order for your application to exhibit consistent
behavior, you need to specify this
<span>flag</span>
for all of the environment handles used in your application.
</p>
<p>
You can achieve this behavior on a transaction by transaction basis by
<span>
specifying <tt class="literal">DB_TXN_NOSYNC</tt> to the
<tt class="methodname">DbTxn::commit()</tt>
method.
</span>
</p>
</li>
<li>
<p>
Specify
<span>
<tt class="literal">DB_TXN_WRITE_NOSYNC</tt> using the
<tt class="methodname">DbEnv::set_flags()</tt>
method.
</span>
This causes
<span>
logging
</span>
data to be synchronously
written to the OS's file system buffers upon
transaction commit. The data will eventually be
written to disk, but this occurs when the
operating system chooses to schedule the
activity; the transaction commit can complete
successfully before this disk I/O is performed
by the OS.
</p>
<p>
This form of commit protects you against application
crashes, but not against OS
crashes. This method offers less room for the possibility of data loss than does
<span><tt class="literal">DB_TXN_NOSYNC</tt>.</span>
</p>
<p>
This behavior is specified on a per-environment
handle basis. In order for your application to exhibit consistent
behavior, you need to specify this
<span>flag</span>
for all of the environment handles used in your application.
</p>
</li>
<li>
<p>
Maintain your logs entirely in-memory. In this
case, your logs are never written to disk. The
result is that you lose all durability guarantees.
See
<a href="logconfig.html#inmemorylogging">Configuring In-Memory Logging</a>
for more information.
</p>
</li>
</ul>
</div>
</div>
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