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<div>
<h2 class="title"><a id="usingtxns"></a>Chapter 3. Transaction Basics</h2>
</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>
<dt>
<span class="sect1">
<a href="nodurabletxn.html">Non-Durable Transactions</a>
</span>
</dt>
<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><code class="methodname">DB_ENV->txn_begin()</code> 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><code class="methodname">DB_TXN->commit()</code> method.</span>
</p>
<p>
If, for any reason, you want to abandon the transaction, you abort
it using
<span><code class="methodname">DB_TXN->abort()</code>.</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 class="xref" href="autocommit.html" title="Auto Commit">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 <stdio.h>
#include <stdlib.h>
#include "db.h"
int
main(void)
{
int ret, ret_c;
u_int32_t db_flags, env_flags;
DB *dbp;
DB_ENV *envp;
DBT key, data;
DB_TXN *txn;
const char *db_home_dir = "/tmp/myEnvironment";
const char *file_name = "mydb.db";
const char *keystr ="thekey";
const char *datastr = "thedata";
dbp = NULL;
envp = NULL;
/* Open the environment */
ret = db_env_create(&envp, 0);
if (ret != 0) {
fprintf(stderr, "Error creating environment handle: %s\n",
db_strerror(ret));
return (EXIT_FAILURE);
}
env_flags = DB_CREATE | /* Create the environment if it does
* not already exist. */
DB_INIT_TXN | /* Initialize transactions */
DB_INIT_LOCK | /* Initialize locking. */
DB_INIT_LOG | /* Initialize logging */
DB_INIT_MPOOL; /* Initialize the in-memory cache. */
ret = envp->open(envp, db_home_dir, env_flags, 0);
if (ret != 0) {
fprintf(stderr, "Error opening environment: %s\n",
db_strerror(ret));
goto err;
}
/* Initialize the DB handle */
ret = db_create(&dbp, envp, 0);
if (ret != 0) {
envp->err(envp, ret, "Database creation failed");
goto err;
}
db_flags = DB_CREATE | DB_AUTO_COMMIT;
/*
* Open the database. Note that we are using auto commit for the open,
* so the database is able to support transactions.
*/
ret = dbp->open(dbp, /* Pointer to the database */
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 */
if (ret != 0) {
envp->err(envp, ret, "Database '%s' open failed",
file_name);
goto err;
}
/* Prepare the DBTs */
memset(&key, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
key.data = &keystr;
key.size = strlen(keystr) + 1;
data.data = &datastr;
data.size = strlen(datastr) + 1;
/* Get the txn handle */
txn = NULL;
ret = envp->txn_begin(envp, NULL, &txn, 0);
if (ret != 0) {
envp->err(envp, ret, "Transaction begin failed.");
goto err;
}
/*
* Perform the database write. If this fails, abort the transaction.
*/
ret = dbp->put(dbp, txn, &key, &data, 0);
if (ret != 0) {
envp->err(envp, ret, "Database put failed.");
txn->abort(txn);
goto err;
}
/*
* Commit the transaction. Note that the transaction handle
* can no longer be used.
*/
ret = txn->commit(txn, 0);
if (ret != 0) {
envp->err(envp, ret, "Transaction commit failed.");
goto err;
}
err:
/* Close the database */
if (dbp != NULL) {
ret_c = dbp->close(dbp, 0);
if (ret_c != 0) {
envp->err(envp, ret_c, "Database close failed.");
ret = ret_c
}
}
/* Close the environment */
if (envp != NULL) {
ret_c = envp->close(envp, 0);
if (ret_c != 0) {
fprintf(stderr, "environment close failed: %s\n",
db_strerror(ret_c));
ret = ret_c;
}
}
return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
} </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>
<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 class="xref" href="nodurabletxn.html" title="Non-Durable Transactions">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 class="xref" href="logconfig.html#inmemorylogging" title="Configuring In-Memory Logging">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 class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a> for more information).
</p>
</li>
</ul>
</div>
<p>
To commit a transaction, you simply call
<span><code class="methodname">DB_TXN->commit()</code>.</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 class="xref" href="filemanagement.html#checkpoints" title="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>
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<span>Database</span>
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