如何在多线程Python应用程序中共享单个SQLite连接

Question

我正在尝试编写一个多线程Python应用程序,其中在线程之间共享单个SQlite连接.我无法让这个工作.真正的应用程序是一个令人讨厌的Web服务器,但以下简单的代码演示了我的问题.

下面成功运行示例代码需要做哪些更改或更改？

当我运行THREAD_COUNT设置为1的程序时,它工作正常,我的数据库按照我的预期更新(即,字母"X"被添加到SectorGroup列中的文本值).

当我在THREAD_COUNT设置为高于1的任何值的情况下运行它时,除1之外的所有线程都会过早地终止与SQLite相关的异常.不同的线程抛出不同的异常(没有可辨别的模式),包括:

OperationalError: cannot start a transaction within a transaction 

(发生在UPDATE声明中)

OperationalError: cannot commit - no transaction is active 

(发生在.commit()调用上)

InterfaceError: Error binding parameter 0 - probably unsupported type. 

(发生在UPDATE和SELECT声明中)

IndexError: tuple index out of range

(这个让我完全不解,它发生在语句中group = rows[0][0] or '',但只有在多个线程运行时)

这是代码:

CONNECTION = sqlite3.connect('./database/mydb', detect_types=sqlite3.PARSE_DECLTYPES, check_same_thread = False)
CONNECTION.row_factory = sqlite3.Row

def commands(start_id):

    # loop over 100 records, read the SectorGroup column, and write it back with "X" appended.
    for inv_id in range(start_id, start_id + 100):

        rows = CONNECTION.execute('SELECT SectorGroup FROM Investment WHERE InvestmentID = ?;', [inv_id]).fetchall()
        if rows:
            group = rows[0][0] or ''
            msg = '{} inv {} = {}'.format(current_thread().name, inv_id, group)
            print msg
            CONNECTION.execute('UPDATE Investment SET SectorGroup = ? WHERE InvestmentID = ?;', [group + 'X', inv_id])

        CONNECTION.commit()

if __name__ == '__main__':

    THREAD_COUNT = 10

    for i in range(THREAD_COUNT):
        t = Thread(target=commands, args=(i*100,))
        t.start()

Answer 1

在线程之间共享连接是不安全的; 至少你需要使用一个锁来序列化访问.还请阅读http://docs.python.org/2/library/sqlite3.html#multithreading,因为较旧的SQLite版本仍有更多问题.

check_same_thread在这方面,该选项似乎故意记录不足,请参阅http://bugs.python.org/issue16509.

您可以使用每个线程的连接,或者查看SQLAlchemy以获取连接池(以及非常有效的工作声明和排队系统).

Answer 2

我在编写一个简单的 WSGI 服务器以获取乐趣和学习时遇到了 Sqlite 线程问题。WSGI 在 Apache 下运行时本质上是多线程的。以下代码似乎对我有用：

import sqlite3
import threading

class LockableCursor:
    def __init__ (self, cursor):
        self.cursor = cursor
        self.lock = threading.Lock ()

    def execute (self, arg0, arg1 = None):
        self.lock.acquire ()

        try:
            self.cursor.execute (arg1 if arg1 else arg0)

            if arg1:
                if arg0 == 'all':
                    result = self.cursor.fetchall ()
                elif arg0 == 'one':
                    result = self.cursor.fetchone ()
        except Exception as exception:
            raise exception

        finally:
            self.lock.release ()
            if arg1:
                return result

def dictFactory (cursor, row):
    aDict = {}
    for iField, field in enumerate (cursor.description):
        aDict [field [0]] = row [iField]
    return aDict

class Db:
    def __init__ (self, app):
        self.app = app

    def connect (self):
        self.connection = sqlite3.connect (self.app.dbFileName, check_same_thread = False, isolation_level = None)  # Will create db if nonexistent
        self.connection.row_factory = dictFactory
        self.cs = LockableCursor (self.connection.cursor ())

使用示例：

if not ok and self.user:    # Not logged out
    # Get role data for any later use
    userIdsRoleIds = self.cs.execute ('all', 'SELECT role_id FROM users_roles WHERE user_id == {}'.format (self.user ['id']))

    for userIdRoleId in userIdsRoleIds:
        self.userRoles.append (self.cs.execute ('one', 'SELECT name FROM roles WHERE id == {}'.format (userIdRoleId ['role_id'])))

另一个例子：

self.cs.execute ('CREATE TABLE users (id INTEGER PRIMARY KEY, email_address, password, token)')         
self.cs.execute ('INSERT INTO users (email_address, password) VALUES ("{}", "{}")'.format (self.app.defaultUserEmailAddress, self.app.defaultUserPassword))

# Create roles table and insert default role
self.cs.execute ('CREATE TABLE roles (id INTEGER PRIMARY KEY, name)')
self.cs.execute ('INSERT INTO roles (name) VALUES ("{}")'.format (self.app.defaultRoleName))

# Create users_roles table and assign default role to default user
self.cs.execute ('CREATE TABLE users_roles (id INTEGER PRIMARY KEY, user_id, role_id)') 

defaultUserId = self.cs.execute ('one', 'SELECT id FROM users WHERE email_address = "{}"'.format (self.app.defaultUserEmailAddress)) ['id']         
defaultRoleId = self.cs.execute ('one', 'SELECT id FROM roles WHERE name = "{}"'.format (self.app.defaultRoleName)) ['id']

self.cs.execute ('INSERT INTO users_roles (user_id, role_id) VALUES ({}, {})'.format (defaultUserId, defaultRoleId))

使用此结构的完整程序可从以下网址下载：http : //www.josmith.org/

注意上面的代码是实验性的，当将它与（许多）并发请求（例如作为 WSGI 服务器的一部分）一起使用时，可能存在（基本）问题。性能对我的应用程序并不重要。最简单的事情可能是只使用 MySql，但我喜欢尝试一些，而 Sqlite 的零安装对我很有吸引力。如果有人认为上面的代码存在根本缺陷，请回复，因为我的目的是学习。如果没有，我希望这对其他人有用。