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<article data-swiftype-index='true'>
<a class='edit-link' href="https://github.com/apache/ignite/tree/IGNITE-7595/docs/_docs/transactions/mvcc.adoc" target="_blank">Edit</a>
<h1>Multiversion Concurrency Control</h1>
<div id="preamble">
<div class="sectionbody">
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<div class="title">Important</div>
</td>
<td class="content">
MVCC is currently in beta.
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="overview">Overview</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Caches with the <code>TRANSACTIONAL_SNAPSHOT</code> atomicity mode support SQL transactions as well as <a href="/docs/2.9.0/key-value-api/transactions">key-value transactions</a> and enable multiversion concurrency control (MVCC) for both types of transactions.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="multiversion-concurrency-control">Multiversion Concurrency Control</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Multiversion Concurrency Control (MVCC) is a method of controlling the consistency of data accessed by multiple users concurrently. MVCC implements the <a href="https://en.wikipedia.org/wiki/Snapshot_isolation">snapshot isolation</a> guarantee which ensures that each transaction always sees a consistent snapshot of data.</p>
</div>
<div class="paragraph">
<p>Each transaction obtains a consistent snapshot of data when it starts and can only view and modify data in this snapshot.
When the transaction updates an entry, Ignite verifies that the entry has not been updated by other transactions and creates a new version of the entry.
The new version becomes visible to other transactions only when and if this transaction commits successfully.
If the entry has been updated, the current transaction fails with an exception (see the <a href="#concurrent-updates">Concurrent Updates</a> section for the information on how to handle update conflicts).</p>
</div>
<div class="paragraph">
<p>The snapshots are not physical snapshots but logical snapshots that are generated by the MVCC-coordinator: a cluster node that coordinates transactional activity in the cluster. The coordinator keeps track of all active transactions and is notified when each transaction finishes. All operations with an MVCC-enabled cache request a snapshot of data from the coordinator.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="enabling-mvcc">Enabling MVCC</h2>
<div class="sectionbody">
<div class="paragraph">
<p>To enable MVCC for a cache, use the <code>TRANSACTIONAL_SNAPSHOT</code> atomicity mode in the cache configuration. If you create a table with the <code>CREATE TABLE</code> command, specify the atomicity mode as a parameter in the <code>WITH</code> part of the command:</p>
</div>
<code-tabs><code-tab data-tab='XML'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="xml"><span class="nt">&lt;bean</span> <span class="na">class=</span><span class="s">"org.apache.ignite.configuration.IgniteConfiguration"</span><span class="nt">&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"cacheConfiguration"</span><span class="nt">&gt;</span>
<span class="nt">&lt;bean</span> <span class="na">class=</span><span class="s">"org.apache.ignite.configuration.CacheConfiguration"</span><span class="nt">&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"name"</span> <span class="na">value=</span><span class="s">"myCache"</span><span class="nt">/&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"atomicityMode"</span> <span class="na">value=</span><span class="s">"TRANSACTIONAL_SNAPSHOT"</span><span class="nt">/&gt;</span>
<span class="nt">&lt;/bean&gt;</span>
<span class="nt">&lt;/property&gt;</span>
<span class="nt">&lt;/bean&gt;</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='SQL'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sql"><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">Person</span> <span class="k">WITH</span> <span class="nv">"ATOMICITY=TRANSACTIONAL_SNAPSHOT"</span></code></pre>
</div>
</div></code-tab></code-tabs>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
The <code>TRANSACTIONAL_SNAPSHOT</code> mode only supports the default concurrency mode (<code>PESSIMISTIC</code>) and default isolation level (<code>REPEATABLE_READ</code>). See <a href="/docs/2.9.0/key-value-api/transactions#concurrency-modes-and-isolation-levels">Concurrency modes and isolation levels</a> for details.
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="concurrent-updates">Concurrent Updates</h2>
<div class="sectionbody">
<div class="paragraph">
<p>If an entry is read and then updated within a single transaction, it is possible that another transaction could be processed in between the two operations and update the entry first. In this case, an exception is thrown when the first transaction attempts to update the entry and the transaction is marked as "rollback only". You have to retry the transaction.</p>
</div>
<div class="paragraph">
<p>This is how to tell that an update conflict has occurred:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>When Java transaction API is used, a <code>CacheException</code> is thrown with the message <code>Cannot serialize transaction due to write conflict (transaction is marked for rollback)</code> and the <code>Transaction.rollbackOnly</code> flag is set to <code>true</code>.</p>
</li>
<li>
<p>When SQL transactions are executed through the JDBC or ODBC driver, the <code>SQLSTATE:40001</code> error code is returned.</p>
</li>
</ul>
</div>
<code-tabs><code-tab data-tab='Ignite Java'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="java"><span class="k">for</span><span class="o">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">1</span><span class="o">;</span> <span class="n">i</span> <span class="o">&lt;=</span><span class="mi">5</span> <span class="o">;</span> <span class="n">i</span><span class="o">++)</span> <span class="o">{</span>
<span class="k">try</span> <span class="o">(</span><span class="nc">Transaction</span> <span class="n">tx</span> <span class="o">=</span> <span class="nc">Ignition</span><span class="o">.</span><span class="na">ignite</span><span class="o">().</span><span class="na">transactions</span><span class="o">().</span><span class="na">txStart</span><span class="o">())</span> <span class="o">{</span>
<span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"attempt #"</span> <span class="o">+</span> <span class="n">i</span> <span class="o">+</span> <span class="s">", value: "</span> <span class="o">+</span> <span class="n">cache</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="mi">1</span><span class="o">));</span>
<span class="k">try</span> <span class="o">{</span>
<span class="n">cache</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="s">"new value"</span><span class="o">);</span>
<span class="n">tx</span><span class="o">.</span><span class="na">commit</span><span class="o">();</span>
<span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"attempt #"</span> <span class="o">+</span> <span class="n">i</span> <span class="o">+</span> <span class="s">" succeeded"</span><span class="o">);</span>
<span class="k">break</span><span class="o">;</span>
<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="nc">CacheException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(!</span><span class="n">tx</span><span class="o">.</span><span class="na">isRollbackOnly</span><span class="o">())</span> <span class="o">{</span>
<span class="c1">// Transaction was not marked as "rollback only",</span>
<span class="c1">// so it's not a concurrent update issue.</span>
<span class="c1">// Process the exception here.</span>
<span class="k">break</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='JDBC'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="java"><span class="nc">Class</span><span class="o">.</span><span class="na">forName</span><span class="o">(</span><span class="s">"org.apache.ignite.IgniteJdbcThinDriver"</span><span class="o">);</span>
<span class="c1">// Open JDBC connection.</span>
<span class="nc">Connection</span> <span class="n">conn</span> <span class="o">=</span> <span class="nc">DriverManager</span><span class="o">.</span><span class="na">getConnection</span><span class="o">(</span><span class="s">"jdbc:ignite:thin://127.0.0.1"</span><span class="o">);</span>
<span class="nc">PreparedStatement</span> <span class="n">updateStmt</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
<span class="nc">PreparedStatement</span> <span class="n">selectStmt</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
<span class="k">try</span> <span class="o">{</span>
<span class="c1">// starting a transaction</span>
<span class="n">conn</span><span class="o">.</span><span class="na">setAutoCommit</span><span class="o">(</span><span class="kc">false</span><span class="o">);</span>
<span class="n">selectStmt</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="na">prepareStatement</span><span class="o">(</span><span class="s">"select name from Person where id = 1"</span><span class="o">);</span>
<span class="n">selectStmt</span><span class="o">.</span><span class="na">setInt</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mi">1</span><span class="o">);</span>
<span class="nc">ResultSet</span> <span class="n">rs</span> <span class="o">=</span> <span class="n">selectStmt</span><span class="o">.</span><span class="na">executeQuery</span><span class="o">();</span>
<span class="k">if</span> <span class="o">(</span><span class="n">rs</span><span class="o">.</span><span class="na">next</span><span class="o">())</span>
<span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"name = "</span> <span class="o">+</span> <span class="n">rs</span><span class="o">.</span><span class="na">getString</span><span class="o">(</span><span class="s">"name"</span><span class="o">));</span>
<span class="n">updateStmt</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="na">prepareStatement</span><span class="o">(</span><span class="s">"update Person set name = ? where id = ? "</span><span class="o">);</span>
<span class="n">updateStmt</span><span class="o">.</span><span class="na">setString</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="s">"New Name"</span><span class="o">);</span>
<span class="n">updateStmt</span><span class="o">.</span><span class="na">setInt</span><span class="o">(</span><span class="mi">2</span><span class="o">,</span> <span class="mi">1</span><span class="o">);</span>
<span class="n">updateStmt</span><span class="o">.</span><span class="na">executeUpdate</span><span class="o">();</span>
<span class="c1">// committing the transaction</span>
<span class="n">conn</span><span class="o">.</span><span class="na">commit</span><span class="o">();</span>
<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="nc">SQLException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="s">"40001"</span><span class="o">.</span><span class="na">equals</span><span class="o">(</span><span class="n">e</span><span class="o">.</span><span class="na">getSQLState</span><span class="o">()))</span> <span class="o">{</span>
<span class="c1">// retry the transaction</span>
<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
<span class="c1">// process the exception</span>
<span class="o">}</span>
<span class="o">}</span> <span class="k">finally</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">updateStmt</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="n">updateStmt</span><span class="o">.</span><span class="na">close</span><span class="o">();</span>
<span class="k">if</span> <span class="o">(</span><span class="n">selectStmt</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="n">selectStmt</span><span class="o">.</span><span class="na">close</span><span class="o">();</span>
<span class="o">}</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='C#/.NET'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="csharp"><span class="k">for</span> <span class="p">(</span><span class="kt">var</span> <span class="n">i</span> <span class="p">=</span> <span class="m">1</span><span class="p">;</span> <span class="n">i</span> <span class="p">&lt;=</span> <span class="m">5</span><span class="p">;</span> <span class="n">i</span><span class="p">++)</span>
<span class="p">{</span>
<span class="k">using</span> <span class="p">(</span><span class="kt">var</span> <span class="n">tx</span> <span class="p">=</span> <span class="n">ignite</span><span class="p">.</span><span class="nf">GetTransactions</span><span class="p">().</span><span class="nf">TxStart</span><span class="p">())</span>
<span class="p">{</span>
<span class="n">Console</span><span class="p">.</span><span class="nf">WriteLine</span><span class="p">(</span><span class="s">$"attempt #</span><span class="p">{</span><span class="n">i</span><span class="p">}</span><span class="s">, value: </span><span class="p">{</span><span class="n">cache</span><span class="p">.</span><span class="nf">Get</span><span class="p">(</span><span class="m">1</span><span class="p">)}</span><span class="s">"</span><span class="p">);</span>
<span class="k">try</span>
<span class="p">{</span>
<span class="n">cache</span><span class="p">.</span><span class="nf">Put</span><span class="p">(</span><span class="m">1</span><span class="p">,</span> <span class="s">"new value"</span><span class="p">);</span>
<span class="n">tx</span><span class="p">.</span><span class="nf">Commit</span><span class="p">();</span>
<span class="n">Console</span><span class="p">.</span><span class="nf">WriteLine</span><span class="p">(</span><span class="s">$"attempt #</span><span class="p">{</span><span class="n">i</span><span class="p">}</span><span class="s"> succeeded"</span><span class="p">);</span>
<span class="k">break</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">catch</span> <span class="p">(</span><span class="n">CacheException</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">if</span> <span class="p">(!</span><span class="n">tx</span><span class="p">.</span><span class="n">IsRollbackOnly</span><span class="p">)</span>
<span class="p">{</span>
<span class="c1">// Transaction was not marked as "rollback only",</span>
<span class="c1">// so it's not a concurrent update issue.</span>
<span class="c1">// Process the exception here.</span>
<span class="k">break</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span></code></pre>
</div>