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Editing: isolation.html

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Isolation</title>
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    <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
    <link rel="start" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
    <link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
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    <div xmlns="" class="navheader">
      <div class="libver">
        <p>Library Version 11.2.5.3</p>
      </div>
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">Isolation</th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="lockingsubsystem.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 4. Concurrency</th>
          <td width="20%" align="right"> <a accesskey="n" href="txn_ccursor.html">Next</a></td>
        </tr>
      </table>
      <hr />
    </div>
    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="isolation"></a>Isolation</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="isolation.html#degreesofisolation">Supported Degrees of Isolation</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="isolation.html#dirtyreads">Reading Uncommitted Data</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="isolation.html#readcommitted">Committed Reads</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="isolation.html#snapshot_isolation">Using Snapshot Isolation</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
            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.
        </p>
      <p>
            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 <span class="emphasis"><em>potentially</em></span> improve your application's throughput.
            Whether you actually see any improvement depends, of course, on
            the nature of your application's data and transactions.
        </p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="degreesofisolation"></a>Supported Degrees of Isolation</h3>
            </div>
          </div>
        </div>
        <p>
                DB supports the following levels of isolation:
            </p>
        <div class="informaltable">
          <table border="1" width="80%">
            <colgroup>
              <col />
              <col />
              <col />
            </colgroup>
            <thead>
              <tr>
                <th>Degree</th>
                <th>ANSI Term</th>
                <th>Definition</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td>1</td>
                <td>READ UNCOMMITTED</td>
                <td>
                    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 database.
                </td>
              </tr>
              <tr>
                <td>2</td>
                <td>READ COMMITTED</td>
                <td>
                    <p>
                    Committed read isolation means that degree 1 is observed, except that dirty data is never read. 
                    </p>
                    <p>
                    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.
                    </p>
                </td>
              </tr>
              <tr>
                <td>3</td>
                <td>SERIALIZABLE</td>
                <td>
                    <p>
                    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.
                    </p>
                    <p>
                        <span>
                        In addition, 
                        </span>

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.
                    </p>
                    <p>
                        This is DB's default isolation guarantee.
                    </p>
                </td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>

By default, DB transactions and transactional cursors offer 
    <span>
        serializable isolation. 
    </span>
    
    
    You can optionally reduce your isolation level by configuring DB to use
    uncommitted read isolation. See 
        <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a> 
     for more information.

You can also configure DB to use committed read isolation. See
            <a class="xref" href="isolation.html#readcommitted" title="Committed Reads">Committed Reads</a>
        for more information.
        
  </p>
        <p>
          Finally, in addition to DB's normal degrees of isolation, you
          can also use <span class="emphasis"><em>snapshot isolation</em></span>. This allows
          you to avoid the read locks that serializable isolation requires. See
          <a class="xref" href="isolation.html#snapshot_isolation" title="Using Snapshot Isolation">Using Snapshot Isolation</a>
          for details.
  </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="dirtyreads"></a>Reading Uncommitted Data</h3>
            </div>
          </div>
        </div>
        <p>
                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.
            </p>
        <p>
                When used with transactions, uncommitted reads means that one transaction can see data
                modified but not yet committed by another transaction. When
                used with transactional cursors, uncommitted reads means
                that any database reader can see data modified by the
                cursor before the cursor's transaction has committed.
            </p>
        <p>
                Because of this, uncommitted reads allow a transaction to read data
                that may subsequently be aborted by another transaction. In
                this case, the reading transaction will have read data that
                never really existed in the database.
            </p>
        <p>
                To configure your application to read uncommitted data:
            </p>
        <div class="orderedlist">
          <ol type="1">
            <li>
              <p>
                        Open your database such that it will allow uncommitted reads. You do this by
                            <span>
                                specifying <code class="literal">DB_READ_UNCOMMITTED</code> when you open your database.
                            </span>
                            
                    </p>
            </li>
            <li>
              <p>
                            <span>
                                Specify <code class="literal">DB_READ_UNCOMMITTED</code>
                                when you create the transaction, 
                                open the cursor, or read a record from the database.
                            </span>

</p>
            </li>
          </ol>
        </div>
        <p>
                For example, the following opens the database such that it supports uncommitted reads, and then creates a
                transaction that causes all reads performed within it to use uncommitted reads. Remember that simply opening
                the database to support uncommitted reads is not enough; you must also declare your read operations to be
                performed using uncommitted reads. 
            </p>
        <pre class="programlisting">#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;

#include "db.h"

int
main(void)
{
    int ret, ret_c;
    u_int32_t db_flags, env_flags;
    DB *dbp;
    DB_ENV *envp;
    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;

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_THREAD     |  /* Free-thread the env handle. */
                DB_INIT_TXN;     /* Initialize transactions */

ret = envp-&gt;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(&amp;dbp, envp, 0);
    if (ret != 0) {
        envp-&gt;err(envp, ret, "Database creation failed");
        goto err;
    }

db_flags = DB_CREATE |             /* Create the db if it does not 
                                        * exist */
               DB_AUTO_COMMIT |        /* Enable auto commit */
               DB_READ_UNCOMMITTED;    /* Enable uncommitted reads */

ret = dbp-&gt;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-&gt;err(envp, ret, "Database '%s' open failed",
            file_name);
        goto err;
    }

/* Get the txn handle */
    txn = NULL;
    ret = envp-&gt;txn_begin(envp, NULL, &amp;txn, DB_READ_UNCOMMITTED);
    if (ret != 0) {
        envp-&gt;err(envp, ret, "Transaction begin failed.");
        goto err;
    }

...  </pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="readcommitted"></a>Committed Reads</h3>
            </div>
          </div>
        </div>
        <p>
                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 
                    
                    <span>
                        record (that is, reading records from the page that it currently has locked),
                    </span>
                    
                the cursor releases its lock on that
                    
                    <span>
                        record or page.
                    </span>
                    
                 This means that the data the cursor has read and released
                 may change before the cursor's transaction has completed.
              </p>
        <p>
                For example,
                suppose you have two transactions, <code class="literal">Ta</code> and <code class="literal">Tb</code>. Suppose further that
                <code class="literal">Ta</code> has a cursor that reads <code class="literal">record R</code>, but does not modify it. Normally,
                <code class="literal">Tb</code> would then be unable to write <code class="literal">record R</code> because
                <code class="literal">Ta</code> would be holding a read lock on it. But when you configure your transaction for
                committed reads, <code class="literal">Tb</code> <span class="emphasis"><em>can</em></span> modify <code class="literal">record
                R</code> before <code class="literal">Ta</code> completes, so long as the reading cursor is no longer
                addressing the 
                    
                    <span>
                        record or page.
                    </span>
                    
             </p>
        <p>
                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 DB to release read locks as it goes, rather than hold them for the life of the
                transaction.
             </p>
        <p>
                To configure your application to use committed reads, do one of the following:
            </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                        Create your transaction such that it allows committed reads. You do this by
                            <span>
                                specifying <code class="literal">DB_READ_COMMITTED</code> when you open the transaction.
                            </span>
                            
                    </p>
            </li>
            <li>
              <p>
                            <span>
                                Specify <code class="literal">DB_READ_COMMITTED</code>
                                when you open the cursor. 
                            </span>
                            
                    </p>
            </li>
          </ul>
        </div>
        <p>
                For example, the following creates a transaction that allows committed reads:
            </p>
        <pre class="programlisting">#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;

#include "db.h"

ret = envp-&gt;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(&amp;dbp, envp, 0);
    if (ret != 0) {
        envp-&gt;err(envp, ret, "Database creation failed");
        goto err;
    }

/*
     * Notice that we do not have to specify any flags to the database to
     * allow committed reads (this is as opposed to uncommitted reads
     * where we DO have to specify a flag on the database open.
     */
    db_flags = DB_CREATE | DB_AUTO_COMMIT;
    ret = dbp-&gt;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-&gt;err(envp, ret, "Database '%s' open failed",
            file_name);
        goto err;
    }

/* Get the txn handle */
    txn = NULL;
    /*
     * Open the transaction and enable committed reads. All cursors open
     * with this transaction handle will use read committed isolation.
     */
    ret = envp-&gt;txn_begin(envp, NULL, &amp;txn, DB_READ_COMMITTED);
    if (ret != 0) {
        envp-&gt;err(envp, ret, "Transaction begin failed.");
        goto err;
    }

/*
     * From here, you perform your database reads and writes as normal,
     * committing and aborting the transactions as is necessary, and 
     * testing for deadlock exceptions as normal (omitted for brevity). 
     *
     * Using transactional cursors with concurrent applications is 
     * described in more detail in the following section.
     */

...  </pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="snapshot_isolation"></a>Using Snapshot Isolation</h3>
            </div>
          </div>
        </div>
        <p>
                    By default DB 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. 
                    
                    <span>
                    When you are using transactional cursors, this 
                    means that read locks are held until the transaction commits or
                    aborts. In that case, over time a transactional cursor
                    can gradually block all other transactions from writing
                    to the database.
                    </span>
            </p>
        <p>
                    You can avoid this by using snapshot isolation.
                    Snapshot isolation uses <span class="emphasis"><em>multiversion
                    concurrency control</em></span> 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.
            </p>
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
          <h3 class="title">Note</h3>
          <p>
                        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 DB reads.
                    </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="sisolation_cost"></a>Snapshot Isolation Cost</h4>
              </div>
            </div>
          </div>
          <p>
                    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.
            </p>
          <p>
                    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.
            </p>
          <p>
                    You can estimate how large your cache should be by
                    taking a checkpoint, followed by a call to the 
                    <code class="methodname">DB_ENV-&gt;log_archive()</code>
                    
                    
                    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).
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="sisolation_maxtxn"></a>Snapshot Isolation Transactional Requirements</h4>
              </div>
            </div>
          </div>
          <p>
                    In addition to an increased cache size, you may also
                    need to increase the number of transactions
                    that your application supports. (See 
                    <a class="xref" href="maxtxns.html" title="Configuring the Transaction Subsystem">Configuring the Transaction Subsystem</a>
                    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.
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="sisolation_whenuse"></a>When to Use Snapshot Isolation</h4>
              </div>
            </div>
          </div>
          <p>
                           Snapshot isolation is best used when all or most
                           of the following conditions are true:
                   </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                        You can have a large cache relative to your working data set size. 
                                   </p>
              </li>
              <li>
                <p>
                                        You require repeatable reads. 
                                   </p>
              </li>
              <li>
                <p>
                                        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.
                                   </p>
              </li>
              <li>
                <p>
                                           Read/write contention is
                                           limiting your application's
                                           throughput, or the application
                                           is all or mostly read-only and
                                           contention for the lock manager
                                           mutex is limiting throughput.
                                   </p>
              </li>
            </ul>
          </div>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="sisolation_howuse"></a>How to use Snapshot Isolation</h4>
              </div>
            </div>
          </div>
          <p>
                           You use snapshot isolation by:
                   </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                           Opening the database  with
                                           multiversion support. You can
                                           configure this either when you
                                           open your environment or when
                                           you open your 
                                           
                                           <span>
                                           database.
                                           </span>

<span>
                                            Use the
                                           <code class="literal">DB_MULTIVERSION</code>
                                           flag to configure this support.
                                            </span>

</p>
              </li>
              <li>
                <p>
                                           Configure your <span>cursor or</span>
                                           transaction to use snapshot isolation.
                                   </p>
                <p>
                                           To do this, 
                                           
                                           <span>
                                                pass the <code class="literal">DB_TXN_SNAPSHOT</code> flag 
                                                when you

<span>
                                                  open the cursor or
                                                </span>

create the transaction. 
                                            
                                                <span>
                                                    If configured for the transaction,
                                                     then this flag is not required
                                                     when the cursor is opened.
                                                </span>

</span>

</p>
              </li>
            </ul>
          </div>
          <p>
                           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.
                   </p>
          <p>
                           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 
                           <a class="xref" href="lockingsubsystem.html#deadlockresolve" title="Resolving Deadlocks">Resolving Deadlocks</a>
                           for details.
                   </p>
          <p>
                           The following code fragment turns
                           on snapshot isolation for a transaction:
                   </p>
          <pre class="programlisting">#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;

#include "db.h"

int
main(void)
{
    int ret, ret_c;
    u_int32_t db_flags, env_flags;
    DB *dbp;
    DB_ENV *envp;
    const char *db_home_dir = "/tmp/myEnvironment";
    const char *file_name = "mydb.db";
    
    dbp = NULL;
    envp = NULL;

/* Open the environment */
    ret = db_env_create(&amp;envp, 0);
    if (ret != 0) {
        fprintf(stderr, "Error creating environment handle: %s\n",
            db_strerror(ret));
        return (EXIT_FAILURE);
    }
    <strong class="userinput"><code>/* Support snapshot isolation */
    envp-&gt;set_flags(envp, DB_MULTIVERSION, 1);</code></strong>
    env_flags = DB_CREATE |    /* Create the environment if it does 
                                * not already exist. */
                
                DB_INIT_LOCK | /* Initialize locking. */
                DB_INIT_LOG  | /* Initialize logging */
                DB_INIT_MPOOL| /* Initialize the in-memory cache. */
                DB_INIT_TXN;   /* Initialize transactions */

ret = envp-&gt;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(&amp;dbp, envp, 0);
    if (ret != 0) {
        envp-&gt;err(envp, ret, "Database creation failed");
        goto err;
    }

/* 
     * Nothing needs to be supplied here to support snapshot isolation. 
     * The environment does, so its databases will too.
     */
    db_flags = DB_CREATE | DB_AUTO_COMMIT;
    ret = dbp-&gt;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-&gt;err(envp, ret, "Database '%s' open failed",
            file_name);
        goto err;
    }

....

ret = envp-&gt;txn_begin(envp, NULL, &amp;txn, <strong class="userinput"><code>DB_TXN_SNAPSHOT</code></strong>);

/* remainder of the program omitted for brevity */

</pre>
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