sharing a :memory: database between different thre

Question

I would like to create a :memory: database in python and access it from different threads. Essentially something like:

class T(threading.Thread):
    def run(self):
        self.conn = sqlite3.connect(':memory:')
        # do stuff with the database

for i in xrange(N):
    T().start()

and have all the connections referring to the same database.

I am aware of passing check_same_thread=True to the connect function and sharing the connection between threads but would like to avoid doing that if possible. Thanks for any help.

EDIT: corrected a typo. I originally said "have all the connections referring to the same thread" substituting thread for database.

Answer 1

Without hacking sqlite3 library itself you cannot reuse :memory: database, because it's guaranteed to be exclusive and private for every connection. To hack access to it, look closer at src/pager.c in sqlite3 distribution (not Python module distribution). Maybe, most convenient way to implement this would be make :memory:00, :memory:something, :memory:okay_hai etc. aliases to address different pPager->memDb pointers through some simple C-side mapping.

Answer 2

SQLite had improved over last 4 years, so now shared in-memory databases are possible. Check the following code:

import sqlite3

foobar_uri = 'file:foobar_database?mode=memory&cache=shared'
not_really_foobar_uri = 'file:not_really_foobar?mode=memory&cache=shared'

# connect to databases in no particular order
db2 = sqlite3.connect(foobar_uri, uri=True)
db_lol = sqlite3.connect(not_really_foobar_uri, uri=True)
db1 = sqlite3.connect(foobar_uri, uri=True)

# create cursor as db2
cur2 = db2.cursor()

# create table as db2
db2.execute('CREATE TABLE foo (NUMBER bar)')

# insert values as db1
db1.execute('INSERT INTO foo VALUES (42)')
db1.commit()

# and fetch them from db2 through cur2
cur2.execute('SELECT * FROM foo')
print(cur2.fetchone()[0])  # 42

# test that db_lol is not shared with db1 and db2
try:
    db_lol.cursor().execute('SELECT * FROM foo')
except sqlite3.OperationalError as exc:
    print(exc)  # just as expected

Database accesses are entangled intentionally, to show that two connections to the in-memory database with the same name are the same from SQLite's point of view.

References:

1. SQLite URIs
2. SQLite shared cache

---

Unfortunately, connection by URI is only available since Python 3.4. However, if you have Python 2.6 or later (but not Python 3), builtin sqlite3 module is still capable of importing APSW connections, which can be used to achieve same effect. Here goes the drop-in sqlite3 module replacement:

from sqlite3 import *
from sqlite3 import connect as _connect
from apsw import Connection as _ApswConnection
from apsw import SQLITE_OPEN_READWRITE as _SQLITE_OPEN_READWRITE
from apsw import SQLITE_OPEN_CREATE as _SQLITE_OPEN_CREATE
from apsw import SQLITE_OPEN_URI as _SQLITE_OPEN_URI

# APSW and pysqlite use different instances of sqlite3 library, so initializing
# APSW won't help pysqlite. Because pysqlite does not expose any way to
# explicitly call sqlite3_initialize(), here goes an ugly hack. This only has
# to be done once per process.
_connect(':memory:').close()

def connect(database, timeout=5.0, detect_types=0, isolation_level=None,
            check_same_thread=True, factory=Connection, cached_statements=100,
            uri=False):
    flags = _SQLITE_OPEN_READWRITE | _SQLITE_OPEN_CREATE

    if uri:
        flags |= _SQLITE_OPEN_URI

    db = _ApswConnection(database, flags, None, cached_statements)
    conn = _connect(db, timeout, detect_types, isolation_level, 
                    check_same_thread, factory, cached_statements)

    return conn