multiprocessing demo for select sqlite db.

multiprocessing_db.py

import sqlite3
import multiprocessing as mp
from itertools import repeat


def build_db(db_name):
    conn = sqlite3.connect(db_name)
    cursor = conn.cursor()
    cursor.execute('''CREATE TABLE  IF NOT EXISTS EMPLOYEE (
       ID INTEGER PRIMARY KEY,
       FIRST_NAME VARCHAR(255),
       LAST_NAME VARCHAR(255),
       AGE int,
       SEX VARCHAR(255),
       INCOME int
    )''')
    cursor.execute('''INSERT INTO EMPLOYEE(
       FIRST_NAME, LAST_NAME, AGE, SEX, INCOME) VALUES 
       ('Ramya', 'Rama Priya', 27, 'F', 9000)''')
    cursor.execute('''INSERT INTO EMPLOYEE(
       FIRST_NAME, LAST_NAME, AGE, SEX, INCOME) VALUES 
       ('Vinay', 'Battacharya', 20, 'M', 6000)''')
    cursor.execute('''INSERT INTO EMPLOYEE(
       FIRST_NAME, LAST_NAME, AGE, SEX, INCOME) VALUES 
       ('Sharukh', 'Sheik', 25, 'M', 8300)''')
    cursor.execute('''INSERT INTO EMPLOYEE(
       FIRST_NAME, LAST_NAME, AGE, SEX, INCOME) VALUES 
       ('Sarmista', 'Sharma', 26, 'F', 10000)''')
    cursor.execute('''INSERT INTO EMPLOYEE(
       FIRST_NAME, LAST_NAME, AGE, SEX, INCOME) VALUES 
       ('Tripthi', 'Mishra', 24, 'F', 6000)''')
    conn.commit()
    conn.close()


def init_worker(function, db_name):
    conn = sqlite3.connect(db_name)
    function.cursor = conn.cursor()


def use_db(id):
    cursor = use_db.cursor
    query = cursor.execute('SELECT ID, FIRST_NAME FROM EMPLOYEE WHERE ID=?', (id,))
    result = query.fetchall()
    print(result[0])
    return result[0]


def use_db_two_args(id, db_name):
    conn = sqlite3.connect(db_name)
    cursor = conn.cursor()
    query = cursor.execute('SELECT ID, FIRST_NAME FROM EMPLOYEE WHERE ID=?', (id,))
    result = query.fetchall()
    print(result[0])
    return result[0]


result_list = []


def log_result(result):
    result_list.append(result)


def main():
    db_name = 'test.db'
    build_db(db_name)

    pool = mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name))
    result_list = []
    print('--- apply ---')
    for i in range(1, 6):
        res = pool.apply(use_db, (i,))
        result_list.append(res)
    pool.close()
    pool.join()
    print(result_list)

    pool = mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name))
    print('--- apply_async ---')
    for i in range(1, 6):
        pool.apply_async(use_db, (i,), callback=log_result)
    pool.close()
    pool.join()
    print(result_list)

    ## wrong
    print('--- apply_async wrong ---')
    with mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name)) as pool:
        for i in range(1, 6):
            pool.apply_async(use_db, (i,), callback=log_result)

    pool = mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name))
    print('--- map ---')
    res = pool.map(use_db, range(1, 6))
    pool.close()
    pool.join()
    print(res)

    pool = mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name))
    print('--- map_async ---')
    res = pool.map_async(use_db, range(1, 6))
    pool.close()
    pool.join()
    print(res.get())

    pool = mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name))
    result_list = []
    print('--- imap ---')
    for res in pool.imap(use_db, range(1, 6)):
        result_list.append(res)
    pool.close()
    pool.join()
    print(result_list)

    pool = mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name))
    result_list = []
    print('--- imap_unordered ---')
    for res in pool.imap_unordered(use_db, range(1, 6)):
        result_list.append(res)
    pool.close()
    pool.join()
    print(result_list)

    pool = mp.Pool(processes=2, initializer=init_worker, initargs=(use_db, db_name))
    print('--- imap_unordered2 ---')
    res_iter = pool.imap_unordered(use_db, range(1, 6))
    res = list(res_iter)
    pool.close()
    pool.join()
    print(res)

    pool = mp.Pool(processes=2)
    print('--- starmap ---')
    args1 = range(1, 6)
    res = pool.starmap(use_db_two_args, zip(args1, repeat(db_name)))
    pool.close()
    pool.join()
    print(res)

    pool = mp.Pool(processes=2)
    print('--- starmap_async ---')
    args1 = range(1, 6)
    res = pool.starmap_async(use_db_two_args, zip(args1, repeat(db_name)))
    pool.close()
    pool.join()
    print(res.get())


if __name__ == '__main__':
    main()