用于 SQLite 的异步 I/O 模块（二十四）

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注意：将 PRAGMA 同步设置为 NORMAL 的 WAL 模式可避免对 fsync（）并且仅在检查点操作。WAL 模式的使用在很大程度上避免了需要这个异步 I/O 模块。因此，此模块不再支持。源代码继续存在于 SQLite 源代码树中，但它不是任何标准构建的一部分，也不再维护。保留此文档以供历史参考。

通常，当SQLite写入数据库文件时，它会等到写入在将控制权返回给调用应用程序之前，操作已完成。由于与 CPU 相比，写入文件系统通常非常慢绑定操作，这可能是性能瓶颈。异步 I/O backend 是一个扩展，它使 SQLite 执行所有写入请求使用在后台运行的单独线程。虽然这没有减少整体系统资源（CPU、磁盘带宽等），确实如此允许 SQLite 快速将控制权返回给调用方，即使在写入数据库。

1.0 功能

使用异步 I/O 时，写入请求由单独的线程处理在后台运行。这意味着启动的线程数据库写入不必等待（有时很慢）磁盘 I/O 发生。写入似乎发生得非常快，尽管实际上它在后台以通常的缓慢速度发生。

异步 I/O 似乎提供了更好的响应能力，但要付出代价。您将失去 Durable 属性。使用 SQLite 的默认 I/O 后端，写入完成后，您就知道您写入的信息是安全地在磁盘上。对于异步 I/O，情况并非如此。如果程序崩溃或数据库断电后写入，但在异步写入线程完成之前，则数据库更改可能永远不会进入磁盘，而数据库可能看不到您的更改。

异步 I/O 会失去持久性，但仍会保留 ACID 的其他部分：原子、一致和隔离。多应用程序在没有耐用性的情况下相处得很好。

1.1 它是如何工作的

异步 I/O 的工作原理是创建一个 SQLite VFS 对象并将其注册到 sqlite3_vfs_register（）。当文件通过此 VFS 被写入（使用 vfs xWrite（）方法），数据不是直接写入磁盘，但被放置在“写入队列”中由后台线程处理。

从中读取使用异步 VFS 打开的文件时（使用 vfs xRead（）方法），从磁盘和写入队列，因此从 vfs 读取器 xWrite（）似乎已经完成。

异步 I/O VFS 通过调用 API 函数 sqlite3async_initialize（）和 sqlite3async_shutdown（）。有关详细信息，请参阅下面的“编译和使用”部分。

1.2 限制

为了获得有关异步的主要思想的经验 IO，此实现特意保持简单。附加将来可能会添加功能。

例如，按照当前实现的，如果写入发生在超过后台编写器的 I/O 能力的稳定流线程，则挂起的写入操作队列将无限制地增长。如果这种情况持续足够长的时间，主机系统可能会耗尽内存。一个更复杂的模块可以跟踪挂起的写入，并在队列挂起的写入变得太大。

1.3 锁定和并发

来自使用此功能的单个进程中的多个连接异步 IO 的实现可以访问单个数据库文件并发。从用户的角度来看，如果全部连接来自单个进程，没有区别在“普通”SQLite 和 SQLite 提供的并发性之间使用异步后端。

如果启用了文件锁定（默认情况下启用），则连接还可以从多个进程读取和写入数据库文件。但是，并发性降低如下：

当使用异步 IO 的连接启动数据库时事务，数据库立即被锁定。然而，在所有相关操作之后，锁不会释放在写入队列中已刷新到磁盘。这意味着（例如）数据库可能在某些情况下保持锁定状态发出“COMMIT”或“ROLLBACK”后的时间。
如果使用异步 IO 的应用程序执行事务在快速连续的情况下，其他数据库用户可以有效地锁定在数据库之外。这是因为当执行 BEGIN 时，会立即建立数据库锁。但当发生相应的 COMMIT 或 ROLLBACK 时，锁定直到写入队列的相关部分才被释放已被冲洗。因此，如果遵循 COMMIT 在刷新写入队列之前，通过 BEGIN 将数据库永不解锁，阻止其他进程访问数据库。

可以在运行时使用 sqlite3async_control（）禁用文件锁定 API（见下文）。这可能会提高 NFS 或其他 NFS 或其他网络文件系统，因为与服务器的同步往返是避免了建立文件锁所需的条件。但是，如果多个连接在文件锁定时尝试访问同一数据库文件被禁用，应用程序崩溃和数据库损坏是可能的结果。

2.0 编译与使用

异步 IO 扩展由 C 代码的单个文件组成（sqlite3async.c）和头文件（sqlite3async.h），位于 SQLite 源代码树的 ext/async/ 子文件夹中，用于定义应用程序用于激活和控制模块的 C API 功能性。

若要使用异步 IO 扩展，请将 sqlite3async.c 编译为使用 SQLite 的应用程序的一部分。然后使用定义的 API 在 sqlite3async.h 中初始化和配置模块。

异步 IO VFS API 在注释中进行了详细描述。 sqlite3async.h. 使用 API 通常包括以下步骤：

通过调用 sqlite3async_initialize（）函数。
创建后台线程以执行写入操作和调用 sqlite3async_run（）。
使用普通的 SQLite API 通过以下方式读取和写入数据库异步 IO VFS。

有关详细信息，请参阅 sqlite3async.h 头文件中的注释。或本文编写时最新代码如下：


#ifndef __SQLITEASYNC_H_
#define __SQLITEASYNC_H_ 1

/*
** Make sure we can call this stuff from C++.
*/
#ifdef __cplusplus
extern "C" {
#endif

#define SQLITEASYNC_VFSNAME "sqlite3async"

/*
** THREAD SAFETY NOTES:
**
** Of the four API functions in this file, the following are not threadsafe:
**
**   sqlite3async_initialize()
**   sqlite3async_shutdown()
**
** Care must be taken that neither of these functions is called while 
** another thread may be calling either any sqlite3async_XXX() function
** or an sqlite3_XXX() API function related to a database handle that
** is using the asynchronous IO VFS.
**
** These functions:
**
**   sqlite3async_run()
**   sqlite3async_control()
**
** are threadsafe. It is quite safe to call either of these functions even
** if another thread may also be calling one of them or an sqlite3_XXX()
** function related to a database handle that uses the asynchronous IO VFS.
*/

/*
** Initialize the asynchronous IO VFS and register it with SQLite using
** sqlite3_vfs_register(). If the asynchronous VFS is already initialized
** and registered, this function is a no-op. The asynchronous IO VFS
** is registered as "sqlite3async".
**
** The asynchronous IO VFS does not make operating system IO requests 
** directly. Instead, it uses an existing VFS implementation for all
** required file-system operations. If the first parameter to this function
** is NULL, then the current default VFS is used for IO. If it is not
** NULL, then it must be the name of an existing VFS. In other words, the
** first argument to this function is passed to sqlite3_vfs_find() to
** locate the VFS to use for all real IO operations. This VFS is known
** as the "parent VFS".
**
** If the second parameter to this function is non-zero, then the 
** asynchronous IO VFS is registered as the default VFS for all SQLite 
** database connections within the process. Otherwise, the asynchronous IO
** VFS is only used by connections opened using sqlite3_open_v2() that
** specifically request VFS "sqlite3async".
**
** If a parent VFS cannot be located, then SQLITE_ERROR is returned.
** In the unlikely event that operating system specific initialization
** fails (win32 systems create the required critical section and event 
** objects within this function), then SQLITE_ERROR is also returned.
** Finally, if the call to sqlite3_vfs_register() returns an error, then 
** the error code is returned to the user by this function. In all three
** of these cases, intialization has failed and the asynchronous IO VFS
** is not registered with SQLite.
**
** Otherwise, if no error occurs, SQLITE_OK is returned.
*/ 
int sqlite3async_initialize(const char *zParent, int isDefault);

/*
** This function unregisters the asynchronous IO VFS using 
** sqlite3_vfs_unregister().
**
** On win32 platforms, this function also releases the small number of 
** critical section and event objects created by sqlite3async_initialize().
*/ 
void sqlite3async_shutdown(void);

/*
** This function may only be called when the asynchronous IO VFS is 
** installed (after a call to sqlite3async_initialize()). It processes
** zero or more queued write operations before returning. It is expected
** (but not required) that this function will be called by a different 
** thread than those threads that use SQLite. The "background thread"
** that performs IO.
**
** How many queued write operations are performed before returning 
** depends on the global setting configured by passing the SQLITEASYNC_HALT
** verb to sqlite3async_control() (see below for details). By default
** this function never returns - it processes all pending operations and 
** then blocks waiting for new ones.
**
** If multiple simultaneous calls are made to sqlite3async_run() from two
** or more threads, then the calls are serialized internally.
*/
void sqlite3async_run(void);

/*
** This function may only be called when the asynchronous IO VFS is 
** installed (after a call to sqlite3async_initialize()). It is used 
** to query or configure various parameters that affect the operation 
** of the asynchronous IO VFS. At present there are three parameters 
** supported:
**
**   * The "halt" parameter, which configures the circumstances under
**     which the sqlite3async_run() parameter is configured.
**
**   * The "delay" parameter. Setting the delay parameter to a non-zero
**     value causes the sqlite3async_run() function to sleep for the
**     configured number of milliseconds between each queued write 
**     operation.
**
**   * The "lockfiles" parameter. This parameter determines whether or 
**     not the asynchronous IO VFS locks the database files it operates
**     on. Disabling file locking can improve throughput.
**
** This function is always passed two arguments. When setting the value
** of a parameter, the first argument must be one of SQLITEASYNC_HALT,
** SQLITEASYNC_DELAY or SQLITEASYNC_LOCKFILES. The second argument must
** be passed the new value for the parameter as type "int".
**
** When querying the current value of a paramter, the first argument must
** be one of SQLITEASYNC_GET_HALT, GET_DELAY or GET_LOCKFILES. The second 
** argument to this function must be of type (int *). The current value
** of the queried parameter is copied to the memory pointed to by the
** second argument. For example:
**
**   int eCurrentHalt;
**   int eNewHalt = SQLITEASYNC_HALT_IDLE;
**
**   sqlite3async_control(SQLITEASYNC_HALT, eNewHalt);
**   sqlite3async_control(SQLITEASYNC_GET_HALT, &eCurrentHalt);
**   assert( eNewHalt==eCurrentHalt );
**
** See below for more detail on each configuration parameter.
**
** SQLITEASYNC_HALT:
**
**   This is used to set the value of the "halt" parameter. The second
**   argument must be one of the SQLITEASYNC_HALT_XXX symbols defined
**   below (either NEVER, IDLE and NOW).
**
**   If the parameter is set to NEVER, then calls to sqlite3async_run()
**   never return. This is the default setting. If the parameter is set
**   to IDLE, then calls to sqlite3async_run() return as soon as the
**   queue of pending write operations is empty. If the parameter is set
**   to NOW, then calls to sqlite3async_run() return as quickly as 
**   possible, without processing any pending write requests.
**
**   If an attempt is made to set this parameter to an integer value other
**   than SQLITEASYNC_HALT_NEVER, IDLE or NOW, then sqlite3async_control() 
**   returns SQLITE_MISUSE and the current value of the parameter is not 
**   modified.
**
**   Modifying the "halt" parameter affects calls to sqlite3async_run() 
**   made by other threads that are currently in progress.
**
** SQLITEASYNC_DELAY:
**
**   This is used to set the value of the "delay" parameter. If set to
**   a non-zero value, then after completing a pending write request, the
**   sqlite3async_run() function sleeps for the configured number of 
**   milliseconds.
**
**   If an attempt is made to set this parameter to a negative value,
**   sqlite3async_control() returns SQLITE_MISUSE and the current value
**   of the parameter is not modified.
**
**   Modifying the "delay" parameter affects calls to sqlite3async_run() 
**   made by other threads that are currently in progress.
**
** SQLITEASYNC_LOCKFILES:
**
**   This is used to set the value of the "lockfiles" parameter. This
**   parameter must be set to either 0 or 1. If set to 1, then the
**   asynchronous IO VFS uses the xLock() and xUnlock() methods of the
**   parent VFS to lock database files being read and/or written. If
**   the parameter is set to 0, then these locks are omitted.
**
**   This parameter may only be set when there are no open database
**   connections using the VFS and the queue of pending write requests
**   is empty. Attempting to set it when this is not true, or to set it 
**   to a value other than 0 or 1 causes sqlite3async_control() to return
**   SQLITE_MISUSE and the value of the parameter to remain unchanged.
**
**   If this parameter is set to zero, then it is only safe to access the
**   database via the asynchronous IO VFS from within a single process. If
**   while writing to the database via the asynchronous IO VFS the database
**   is also read or written from within another process, or via another
**   connection that does not use the asynchronous IO VFS within the same
**   process, the results are undefined (and may include crashes or database
**   corruption).
**
**   Alternatively, if this parameter is set to 1, then it is safe to access
**   the database from multiple connections within multiple processes using
**   either the asynchronous IO VFS or the parent VFS directly.
*/
int sqlite3async_control(int op, ...);

/*
** Values that can be used as the first argument to sqlite3async_control().
*/
#define SQLITEASYNC_HALT          1
#define SQLITEASYNC_GET_HALT      2
#define SQLITEASYNC_DELAY         3
#define SQLITEASYNC_GET_DELAY     4
#define SQLITEASYNC_LOCKFILES     5
#define SQLITEASYNC_GET_LOCKFILES 6

/*
** If the first argument to sqlite3async_control() is SQLITEASYNC_HALT,
** the second argument should be one of the following.
*/
#define SQLITEASYNC_HALT_NEVER 0       /* Never halt (default value) */
#define SQLITEASYNC_HALT_NOW   1       /* Halt as soon as possible */
#define SQLITEASYNC_HALT_IDLE  2       /* Halt when write-queue is empty */

#ifdef __cplusplus
}  /* End of the 'extern "C"' block */
#endif
#endif        /* ifndef __SQLITEASYNC_H_ */

3.0 移植

目前，异步 IO 扩展与 win32 系统兼容以及支持 pthreads 接口的系统，包括 Mac OS X、Linux、和其他 Unix 变体。

若要将异步 IO 扩展移植到另一个平台，用户必须为新平台实现互斥锁和条件变量基元。目前没有外部可用的接口可以允许这样做，但是修改 sqlite3async.c 中的代码以包含新平台并发原语相对容易。在 sqlite3async.c 中搜索有关详细信息，请使用注释字符串“PORTING FUNCTIONS”。然后实施以下各项的新版本：

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);

描述了上述每个功能所需的功能在 sqlite3async.c 的注释中。