Sindbad~EG File Manager
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<p>Library Version 18.1.40</p>
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<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="rep_ryw"></a>Read your writes consistency</h2>
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<dl>
<dt>
<span class="sect2">
<a href="rep_ryw.html#gettoken">Getting a token</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_ryw.html#tokenhandling">Token handling</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_ryw.html#usingtoken">Using a token to check or wait for a transaction</a>
</span>
</dt>
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<p>
Some applications require the ability to read replicated
data at a client site, and determine whether it is consistent
with data that has been written previously at the master site.
</p>
<p>
For example, a web application may be backed by multiple
database environments, linked to form a replication group, in
order to share the workload. Web requests that update data
must be served by the replication master, but any site in the
group may serve a read-only request. Consider a work flow of a
series of web requests from one specific user at a web
browser: the first request generates a database update, but
the second request merely reads data. If the read-only request
is served by a replication client database environment, it may
be important to make sure that the updated data has been
replicated to the client before performing the read (or to
wait until it has been replicated) in order to show this user
a consistent view of the data.
</p>
<p>
Berkeley DB supports this requirement through the use of
transaction "tokens". A token is a form of identification for
a transaction within the scope of the replication group. The
application may request a copy of the transaction's token at
the master site during the execution of the transaction.
Later, the application running on a client site can use a copy
of the token to determine whether the transaction has been
applied at that site.
</p>
<p>
It is the application's responsibility to keep track of the
token during the interim. In the web example, the token might
be sent to the browser as a "cookie", or stored on the
application server in the user's session context.
</p>
<p>
The operations described here are supported both for
Replication Manager applications and for applications that use
the replication Base API.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="gettoken"></a>Getting a token</h3>
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<p>
In order to get a token, the application must supply a
small memory buffer, using the <a href="../api_reference/C/txnset_commit_token.html" class="olink">DB_TXN->set_commit_token()</a> method. </p>
<p>
Note that a token is generated only upon a successful
commit operation, and therefore the token buffer content
is valid only after a successful commit. Also, if a
transaction does not perform any update operations it does
not generate a useful token.
</p>
<p>
In the Berkeley DB Java and C# API, getting a token is
simpler. The application need only invoke the
<a class="ulink" href="../java/com/sleepycat/db/Transaction.html#getCommitToken()" target="_top">Transaction.getCommitToken()</a> method, after the transaction has
committed.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="tokenhandling"></a>Token handling</h3>
</div>
</div>
</div>
<p>
The application should not try to interpret the content
of the token buffer, but may store and/or transmit it
freely between systems. However, since the buffer contains
binary data it may be necessary to apply some encoding for
transmission (e.g., base 64).
</p>
<p>
The data is resilient to differences in byte order
between different systems. It does not expire: it may be
retained indefinitely for later use, even across Berkeley
DB version upgrades.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="usingtoken"></a>Using a token to check or wait for a transaction</h3>
</div>
</div>
</div>
<p>
The <a href="../api_reference/C/envtxn_applied.html" class="olink">DB_ENV->txn_applied()</a> method takes a copy of a token, and
determines whether the corresponding transaction is
currently applied at the local site. The timeout argument
allows the application to block for a bounded amount of
time for cases where the transaction has not yet been
applied.
</p>
<p>
Depending on the transaction durability levels
implemented or configured by the application, it is
sometimes possible for a transaction to disappear from a
replication group if an original master site fails and a
different site becomes the new master without having
received the transaction. When the <a href="../api_reference/C/envtxn_applied.html" class="olink">DB_ENV->txn_applied()</a> method
discovers this, it produces the
<code class="literal">DB_NOTFOUND</code> return code.
</p>
<p>
This means that the results of <a href="../api_reference/C/envtxn_applied.html" class="olink">DB_ENV->txn_applied()</a> are not
guaranteed forever. Even after a successful call to
<a href="../api_reference/C/envtxn_applied.html" class="olink">DB_ENV->txn_applied()</a>, it is possible that by the time the
application tries to read the data, the transaction and
its data could have disappeared.
</p>
<p>
To avoid this problem the application should do the
read operations in the context of a transaction, and hold
the transaction handle open during the <a href="../api_reference/C/envtxn_applied.html" class="olink">DB_ENV->txn_applied()</a> call.
The <a href="../api_reference/C/envtxn_applied.html" class="olink">DB_ENV->txn_applied()</a> method itself does not actually execute
in the context of the transaction; but no rollbacks due to
new master synchronization ever occur while a transaction
is active, even a read-only transaction at a client site.
</p>
<p>
Note that the <a href="../api_reference/C/envtxn_applied.html" class="olink">DB_ENV->txn_applied()</a> method can return
<code class="literal">DB_LOCK_DEADLOCK</code>. The application
should respond to this situation just as it does for any
other normal operation: abort any existing transaction,
and then pause briefly before retrying.
</p>
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Sindbad File Manager Version 1.0, Coded By Sindbad EG ~ The Terrorists