python中的SQlite WAL模式。一位作者和一位读者的并发 [英] SQlite WAL-mode in python. Concurrency with one writer and one reader

问题描述

我正在尝试在一个写入器进程和一个读取器进程之间共享一个sqlite3数据库。但是，它不起作用，在我看来，example.db中什么也没写。

I'm trying to share a sqlite3 database between one writer process, and one reader process. However, it does not work, and it seems to me that nothing is being written in example.db.

reader.py

reader.py

import sqlite3
from time import sleep
conn = sqlite3.connect('example.db', isolation_level=None)

c = conn.cursor()
while True:
    c.execute("SELECT * FROM statistics")
    try:
        print '**'
        print c.fetchone()
    except:
        pass
    sleep(3)

writer.py

writer.py

import sqlite3
from time import sleep
import os

if os.path.exists('example.db'):
    os.remove('example.db')

conn = sqlite3.connect('example.db', isolation_level=None)
c = conn.cursor()
c.execute('PRAGMA journal_mode=wal')
print c.fetchone()
c.execute("CREATE TABLE statistics (stat1 real, stat2 real)")

stat1 = 0
stat2 = 0.0
while True:
    stat1 += 1
    stat2 += 0.1
    cmd = "INSERT INTO statistics VALUES (%d,%f)" % (stat1, stat2)
    print cmd
    c.execute(cmd)
    #conn.commit()
    c.execute("PRAGMA wal_checkpoint=FULL")
    sleep(0.25)

我在后台进程中运行writer.py，然后运行reader.py。它显示了以下内容：

I run writer.py in a background process, and then I run reader.py. This is what it shows:

**
(1.0, 0.1)
**
(1.0, 0.1)
**
(1.0, 0.1)

（...）

用一个读者和一个作家来设置此框架的正确（也是最佳）方法是什么？

What is the correct (and best) way to set this framework with one reader and one writer?

推荐答案

您期望什么？您只选择表的第一行。并且这总是同一行。 PRAGMA语句在这里没有可见的效果。

What do you expect? You only select the first row of the table. And this is always the same row. The "PRAGMA"-statements have no visible effects here.

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