Can two processes access in-memory (:memory:) sqli

问题:

Is it possible to access database in one process, created in another? I tried:

IDLE #1

import sqlite3
conn = sqlite3.connect(':memory:')
c = conn.cursor()
c.execute("create table test(testcolumn)")
c.execute("insert into test values('helloooo')")
conn.commit()
conn.close()

IDLE #2

import sqlite3
conn = sqlite3.connect(':memory:')
c = conn.cursor()
c.execute("select * from test")

Error:

Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    q = c.execute("select * from test")
sqlite3.OperationalError: no such table: test

回答1:

No, they cannot ever access the same in-memory database from different processes Instead, a new connection to :memory: always creates a new database.

From the SQLite documentation:

Every :memory: database is distinct from every other. So, opening two database connections each with the filename ":memory:" will create two independent in-memory databases.

This is different from an on-disk database, where creating multiple connections with the same connection string means you are connecting to one database.

Within one process it is possible to share an in-memory database if you use the file::memory:?cache=shared URI:

conn = sqlite3.connect('file::memory:?cache=shared', uri=True)

but this is still not accessible from other another process.

回答2:

of course I agree with @Martijn because doc says so, but if you are focused on unix like systems, then you can make use of shared memory:

If you create file in /dev/shm folder, all files create there are mapped directly to RAM, so you can use to access the-same database from two-different processes.

#/bin/bash
rm -f /dev/shm/test.db
time bash -c $'
FILE=/dev/shm/test.db
sqlite3 $FILE "create table if not exists tab(id int);"
sqlite3 $FILE "insert into tab values (1),(2)"
for i in 1 2 3 4; do sqlite3 $FILE "INSERT INTO tab (id) select (a.id+b.id+c.id)*abs(random()%1e7) from tab a, tab b, tab c limit 5e5"; done; #inserts at most 2'000'000 records to db.
sqlite3 $FILE "select count(*) from tab;"'

it takes that much time:

FILE=/dev/shm/test.db
real    0m0.927s
user    0m0.834s
sys 0m0.092s

for at least 2 million records, doing the same on HDD takes (this is the same command but FILE=/tmp/test.db):

FILE=/tmp/test.db
real    0m2.309s
user    0m0.871s
sys 0m0.138s

so basically this allows you accessing the same databases from different processes (without loosing r/w speed):

Here is demo demonstrating this what I am talking about:

xterm -hold -e 'sqlite3 /dev/shm/testbin "create table tab(id int); insert into tab values (42),(1337);"' &
xterm -hold -e 'sqlite3 /dev/shm/testbin "insert into tab values (43),(1338); select * from tab;"' &
;