228 lines
8.2 KiB
HTML
228 lines
8.2 KiB
HTML
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<link href="sqlite.css" rel="stylesheet">
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<title>Using SQLite In Multi-Threaded Applications</title>
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<body>
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Small. Fast. Reliable.<br>Choose any three.
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<div class=fancy>
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<div class=nosearch>
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<div class="fancy_title">
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Using SQLite In Multi-Threaded Applications
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</div>
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</div>
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<h1 id="overview"><span>1. </span>Overview</h1>
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<p>SQLite supports three different threading modes:</p>
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<ol>
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<li><p><b>Single-thread</b>.
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In this mode, all mutexes are disabled and SQLite is unsafe to use in
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more than a single thread at once.</p></li>
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<li><p><b>Multi-thread</b>.
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In this mode, SQLite can be safely used by multiple threads provided that
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no single <a href="c3ref/sqlite3.html">database connection</a> nor any object derived from database connection,
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such as a <a href="c3ref/stmt.html">prepared statement</a>,
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is used in two or more threads at the same time.
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</p></li>
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<li><p><b>Serialized</b>.
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In serialized mode, API calls to control SQLite any <a href="c3ref/sqlite3.html">database connection</a> or
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any object derived from a database connection
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can be made safely from multiple threads.
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The effect on an individual object is the same as if the API calls had all
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been made in the same order from a single thread. The name "serialized"
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arises from the fact that SQLite uses mutexes to serialize access to each
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object.</p></li>
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</ol>
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<p>
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The threading mode can be selected at compile-time (when the SQLite
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library is being compiled from source code) or at start-time (when the
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application that intends to use SQLite is initializing) or at
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run-time (when a new SQLite database connection is being created).
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Generally speaking, run-time overrides start-time and start-time
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overrides compile-time. Except, single-thread mode cannot be
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overridden once selected.
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</p>
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<p>
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The default mode is serialized.
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</p>
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<h1 id="compile_time_selection_of_threading_mode"><span>2. </span>Compile-time selection of threading mode</h1>
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<p>
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Use the <a href="compile.html#threadsafe">SQLITE_THREADSAFE</a> compile-time parameter to select the
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threading mode. If no <a href="compile.html#threadsafe">SQLITE_THREADSAFE</a> compile-time parameter is
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present, then serialized mode is used.
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This can be made explicit with
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<a href="compile.html#threadsafe">-DSQLITE_THREADSAFE=1</a>.
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With
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<a href="compile.html#threadsafe">-DSQLITE_THREADSAFE=0</a> the threading mode is
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single-thread. With
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<a href="compile.html#threadsafe">-DSQLITE_THREADSAFE=2</a> the threading mode is
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multi-thread.
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</p>
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<p>
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The return value of the <a href="c3ref/threadsafe.html">sqlite3_threadsafe()</a> interface is the value
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of SQLITE_THREADSAFE set at compile-time. It does not reflect changes
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to the threading mode made at runtime via the <a href="c3ref/config.html">sqlite3_config()</a>
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interface or by flags given as the third argument to <a href="c3ref/open.html">sqlite3_open_v2()</a>.
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</p>
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<p>
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If single-thread mode is selected at compile-time, then critical
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mutexing logic is omitted from the build and it is impossible to
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enable either multi-thread or serialized modes at start-time or
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run-time.
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</p>
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<h1 id="start_time_selection_of_threading_mode"><span>3. </span>Start-time selection of threading mode</h1>
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<p>
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Assuming that the compile-time threading mode is not single-thread, then
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the threading mode can be changed during initialization using the
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<a href="c3ref/config.html">sqlite3_config()</a> interface. The <a href="c3ref/c_config_covering_index_scan.html#sqliteconfigsinglethread">SQLITE_CONFIG_SINGLETHREAD</a> verb
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puts SQLite into single-thread mode, the <a href="c3ref/c_config_covering_index_scan.html#sqliteconfigmultithread">SQLITE_CONFIG_MULTITHREAD</a>
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verb sets multi-thread mode, and the <a href="c3ref/c_config_covering_index_scan.html#sqliteconfigserialized">SQLITE_CONFIG_SERIALIZED</a> verb
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sets serialized mode.
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</p>
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<h1 id="run_time_selection_of_threading_mode"><span>4. </span>Run-time selection of threading mode</h1>
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<p>If single-thread mode has not been selected at compile-time or start-time,
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then individual database connections can be created as either multi-thread
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or serialized. It is not possible to downgrade an individual database
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connection to single-thread mode. Nor is it possible to escalate an
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individual database connection if the compile-time or start-time mode
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is single-thread.</p>
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<p>The threading mode for an individual database connection is determined
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by flags given as the third argument to <a href="c3ref/open.html">sqlite3_open_v2()</a>. The
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<a href="c3ref/c_open_autoproxy.html">SQLITE_OPEN_NOMUTEX</a> flag causes the database connection to be in the
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multi-thread mode and the <a href="c3ref/c_open_autoproxy.html">SQLITE_OPEN_FULLMUTEX</a> flag causes the connection
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to be in serialized mode. If neither flag is specified or if
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<a href="c3ref/open.html">sqlite3_open()</a> or <a href="c3ref/open.html">sqlite3_open16()</a> are used instead of
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<a href="c3ref/open.html">sqlite3_open_v2()</a>, then the default
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mode determined by the compile-time and start-time settings is used.
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</p>
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