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The use of <a href="wal.html">WAL mode</a> largely obviates the +need for this asynchronous I/O module. Hence, this module is no longer +supported. The source code continues to exist in the SQLite source tree, +but it is not a part of any standard build and is no longer maintained. +This documentation is retained for historical reference.</font></p><hr> + +<p>Normally, when SQLite writes to a database file, it waits until the write +operation is finished before returning control to the calling application. +Since writing to the file-system is usually very slow compared with CPU +bound operations, this can be a performance bottleneck. The asynchronous I/O +backend is an extension that causes SQLite to perform all write requests +using a separate thread running in the background. Although this does not +reduce the overall system resources (CPU, disk bandwidth etc.), it does +allow SQLite to return control to the caller quickly even when writing to +the database. + +<h2>1.0 FUNCTIONALITY</h2> + +<p>With asynchronous I/O, write requests are handled by a separate thread +running in the background. This means that the thread that initiates +a database write does not have to wait for (sometimes slow) disk I/O +to occur. The write seems to happen very quickly, though in reality +it is happening at its usual slow pace in the background. + +<p>Asynchronous I/O appears to give better responsiveness, but at a price. +You lose the Durable property. With the default I/O backend of SQLite, +once a write completes, you know that the information you wrote is +safely on disk. With the asynchronous I/O, this is not the case. If +your program crashes or if a power loss occurs after the database +write but before the asynchronous write thread has completed, then the +database change might never make it to disk and the next user of the +database might not see your change. + +<p>You lose Durability with asynchronous I/O, but you still retain the +other parts of ACID: Atomic, Consistent, and Isolated. Many +applications get along fine without the Durability. + +<h3>1.1 How it Works</h3> + +<p>Asynchronous I/O works by creating an SQLite <a href="c3ref/vfs.html">VFS object</a> +and registering it with <a href="c3ref/vfs_find.html">sqlite3_vfs_register()</a>. +When files opened via +this VFS are written to (using the vfs xWrite() method), the data is not +written directly to disk, but is placed in the "write-queue" to be +handled by the background thread. + +<p>When files opened with the asynchronous VFS are read from +(using the vfs xRead() method), the data is read from the file on +disk and the write-queue, so that from the point of view of +the vfs reader the xWrite() appears to have already completed. + +<p>The asynchronous I/O VFS is registered (and unregistered) by calls to the +API functions sqlite3async_initialize() and sqlite3async_shutdown(). +See section "Compilation and Usage" below for details. + +<h3>1.2 Limitations</h3> + +<p>In order to gain experience with the main ideas surrounding asynchronous +IO, this implementation is deliberately kept simple. Additional +capabilities may be added in the future. + +<p>For example, as currently implemented, if writes are happening at a +steady stream that exceeds the I/O capability of the background writer +thread, the queue of pending write operations will grow without bound. +If this goes on for long enough, the host system could run out of memory. +A more sophisticated module could to keep track of the quantity of +pending writes and stop accepting new write requests when the queue of +pending writes grows too large. + +<h3>1.3 Locking and Concurrency</h3> + +<p>Multiple connections from within a single process that use this +implementation of asynchronous IO may access a single database +file concurrently. From the point of view of the user, if all +connections are from within a single process, there is no difference +between the concurrency offered by "normal" SQLite and SQLite +using the asynchronous backend. + +<p>If file-locking is enabled (it is enabled by default), then connections +from multiple processes may also read and write the database file. +However concurrency is reduced as follows: + +<ul> +<li><p> When a connection using asynchronous IO begins a database + transaction, the database is locked immediately. However the + lock is not released until after all relevant operations + in the write-queue have been flushed to disk. This means + (for example) that the database may remain locked for some + time after a "<a href="lang_transaction.html">COMMIT</a>" or "<a href="lang_transaction.html">ROLLBACK</a>" is issued. + +<li><p> If an application using asynchronous IO executes transactions + in quick succession, other database users may be effectively + locked out of the database. This is because when a <a href="lang_transaction.html">BEGIN</a> + is executed, a database lock is established immediately. But + when the corresponding COMMIT or ROLLBACK occurs, the lock + is not released until the relevant part of the write-queue + has been flushed through. As a result, if a COMMIT is followed + by a BEGIN before the write-queue is flushed through, the database + is never unlocked,preventing other processes from accessing + the database. +</ul> + +<p>File-locking may be disabled at runtime using the sqlite3async_control() +API (see below). This may improve performance when an NFS or other +network file-system, as the synchronous round-trips to the server be +required to establish file locks are avoided. However, if multiple +connections attempt to access the same database file when file-locking +is disabled, application crashes and database corruption is a likely +outcome. + + +<h2>2.0 COMPILATION AND USAGE</h2> + +<p> +The asynchronous IO extension consists of a single file of C code +(sqlite3async.c), and a header file (sqlite3async.h), located in the +<a href="https://www.sqlite.org/src/dir?name=ext/async"> +<tt>ext/async/</tt> subfolder</a> of the SQLite source tree, that defines the +C API used by applications to activate and control the modules +functionality. + +<p> +To use the asynchronous IO extension, compile sqlite3async.c as +part of the application that uses SQLite. Then use the APIs defined +in sqlite3async.h to initialize and configure the module. + +<p> +The asynchronous IO VFS API is described in detail in comments in +sqlite3async.h. Using the API usually consists of the following steps: + +<ol> +<li><p>Register the asynchronous IO VFS with SQLite by calling the + sqlite3async_initialize() function. + +<li><p>Create a background thread to perform write operations and call + sqlite3async_run(). + +<li><p>Use the normal SQLite API to read and write to databases via + the asynchronous IO VFS. +</ol> + +<p>Refer to comments in the +<a href="https://www.sqlite.org/src/finfo?name=ext/async/sqlite3async.h"> +sqlite3async.h header file</a> for details. + + +<h2>3.0 PORTING</h2> + +<p>Currently the asynchronous IO extension is compatible with win32 systems +and systems that support the pthreads interface, including Mac OS X, Linux, +and other varieties of Unix. + +<p>To port the asynchronous IO extension to another platform, the user must +implement mutex and condition variable primitives for the new platform. +Currently there is no externally available interface to allow this, but +modifying the code within sqlite3async.c to include the new platforms +concurrency primitives is relatively easy. Search within sqlite3async.c +for the comment string "PORTING FUNCTIONS" for details. Then implement +new versions of each of the following: + +<blockquote><pre> +static void async_mutex_enter(int eMutex); +static void async_mutex_leave(int eMutex); +static void async_cond_wait(int eCond, int eMutex); +static void async_cond_signal(int eCond); +static void async_sched_yield(void); +</pre></blockquote> + +<p>The functionality required of each of the above functions is described +in comments in sqlite3async.c. + + |