1 2Normally, when SQLite writes to a database file, it waits until the write 3operation is finished before returning control to the calling application. 4Since writing to the file-system is usually very slow compared with CPU 5bound operations, this can be a performance bottleneck. This directory 6contains an extension that causes SQLite to perform all write requests 7using a separate thread running in the background. Although this does not 8reduce the overall system resources (CPU, disk bandwidth etc.) at all, it 9allows SQLite to return control to the caller quickly even when writing to 10the database, eliminating the bottleneck. 11 12 1. Functionality 13 14 1.1 How it Works 15 1.2 Limitations 16 1.3 Locking and Concurrency 17 18 2. Compilation and Usage 19 20 3. Porting 21 22 23 241. FUNCTIONALITY 25 26 With asynchronous I/O, write requests are handled by a separate thread 27 running in the background. This means that the thread that initiates 28 a database write does not have to wait for (sometimes slow) disk I/O 29 to occur. The write seems to happen very quickly, though in reality 30 it is happening at its usual slow pace in the background. 31 32 Asynchronous I/O appears to give better responsiveness, but at a price. 33 You lose the Durable property. With the default I/O backend of SQLite, 34 once a write completes, you know that the information you wrote is 35 safely on disk. With the asynchronous I/O, this is not the case. If 36 your program crashes or if a power loss occurs after the database 37 write but before the asynchronous write thread has completed, then the 38 database change might never make it to disk and the next user of the 39 database might not see your change. 40 41 You lose Durability with asynchronous I/O, but you still retain the 42 other parts of ACID: Atomic, Consistent, and Isolated. Many 43 appliations get along fine without the Durablity. 44 45 1.1 How it Works 46 47 Asynchronous I/O works by creating a special SQLite "vfs" structure 48 and registering it with sqlite3_vfs_register(). When files opened via 49 this vfs are written to (using the vfs xWrite() method), the data is not 50 written directly to disk, but is placed in the "write-queue" to be 51 handled by the background thread. 52 53 When files opened with the asynchronous vfs are read from 54 (using the vfs xRead() method), the data is read from the file on 55 disk and the write-queue, so that from the point of view of 56 the vfs reader the xWrite() appears to have already completed. 57 58 The special vfs is registered (and unregistered) by calls to the 59 API functions sqlite3async_initialize() and sqlite3async_shutdown(). 60 See section "Compilation and Usage" below for details. 61 62 1.2 Limitations 63 64 In order to gain experience with the main ideas surrounding asynchronous 65 IO, this implementation is deliberately kept simple. Additional 66 capabilities may be added in the future. 67 68 For example, as currently implemented, if writes are happening at a 69 steady stream that exceeds the I/O capability of the background writer 70 thread, the queue of pending write operations will grow without bound. 71 If this goes on for long enough, the host system could run out of memory. 72 A more sophisticated module could to keep track of the quantity of 73 pending writes and stop accepting new write requests when the queue of 74 pending writes grows too large. 75 76 1.3 Locking and Concurrency 77 78 Multiple connections from within a single process that use this 79 implementation of asynchronous IO may access a single database 80 file concurrently. From the point of view of the user, if all 81 connections are from within a single process, there is no difference 82 between the concurrency offered by "normal" SQLite and SQLite 83 using the asynchronous backend. 84 85 If file-locking is enabled (it is enabled by default), then connections 86 from multiple processes may also read and write the database file. 87 However concurrency is reduced as follows: 88 89 * When a connection using asynchronous IO begins a database 90 transaction, the database is locked immediately. However the 91 lock is not released until after all relevant operations 92 in the write-queue have been flushed to disk. This means 93 (for example) that the database may remain locked for some 94 time after a "COMMIT" or "ROLLBACK" is issued. 95 96 * If an application using asynchronous IO executes transactions 97 in quick succession, other database users may be effectively 98 locked out of the database. This is because when a BEGIN 99 is executed, a database lock is established immediately. But 100 when the corresponding COMMIT or ROLLBACK occurs, the lock 101 is not released until the relevant part of the write-queue 102 has been flushed through. As a result, if a COMMIT is followed 103 by a BEGIN before the write-queue is flushed through, the database 104 is never unlocked,preventing other processes from accessing 105 the database. 106 107 File-locking may be disabled at runtime using the sqlite3async_control() 108 API (see below). This may improve performance when an NFS or other 109 network file-system, as the synchronous round-trips to the server be 110 required to establish file locks are avoided. However, if multiple 111 connections attempt to access the same database file when file-locking 112 is disabled, application crashes and database corruption is a likely 113 outcome. 114 115 1162. COMPILATION AND USAGE 117 118 The asynchronous IO extension consists of a single file of C code 119 (sqlite3async.c), and a header file (sqlite3async.h) that defines the 120 C API used by applications to activate and control the modules 121 functionality. 122 123 To use the asynchronous IO extension, compile sqlite3async.c as 124 part of the application that uses SQLite. Then use the API defined 125 in sqlite3async.h to initialize and configure the module. 126 127 The asynchronous IO VFS API is described in detail in comments in 128 sqlite3async.h. Using the API usually consists of the following steps: 129 130 1. Register the asynchronous IO VFS with SQLite by calling the 131 sqlite3async_initialize() function. 132 133 2. Create a background thread to perform write operations and call 134 sqlite3async_run(). 135 136 3. Use the normal SQLite API to read and write to databases via 137 the asynchronous IO VFS. 138 139 Refer to sqlite3async.h for details. 140 141 1423. PORTING 143 144 Currently the asynchronous IO extension is compatible with win32 systems 145 and systems that support the pthreads interface, including Mac OSX, Linux, 146 and other varieties of Unix. 147 148 To port the asynchronous IO extension to another platform, the user must 149 implement mutex and condition variable primitives for the new platform. 150 Currently there is no externally available interface to allow this, but 151 modifying the code within sqlite3async.c to include the new platforms 152 concurrency primitives is relatively easy. Search within sqlite3async.c 153 for the comment string "PORTING FUNCTIONS" for details. Then implement 154 new versions of each of the following: 155 156 static void async_mutex_enter(int eMutex); 157 static void async_mutex_leave(int eMutex); 158 static void async_cond_wait(int eCond, int eMutex); 159 static void async_cond_signal(int eCond); 160 static void async_sched_yield(void); 161 162 The functionality required of each of the above functions is described 163 in comments in sqlite3async.c. 164 165