I am trying to write a Python model which is capable of doing some processing in a PostgreSQL database using the multi-threading module and peewee. 

In single core mode the code works, however, when I try to run the code with multiple cores I am running into a SSL error. 

I would like to post the structure of my model in the hope that somebody can advice how to set of my model in a proper way. Currently, I have chosen to use an object oriented approach in which I make one connection which is shared in a pool. To clarify what I have done, I will now show the source code I have so far

I have three files: main.py, models.py and parser.py. The contents is the following

models.py defines the peewee postgresql table and makes a connection to the postgres server

```
import peewee as pw
from playhouse.pool import PooledPostgresqlExtDatabase

KVK_KEY = "id_number"
NAME_KEY = "name"
N_VOWELS_KEY = "n_vowels"

# initialise the data base
database = PooledPostgresqlExtDatabase(
    "testdb", user="postgres", host="localhost", port=5432, password="xxxx",
    max_connections=8, stale_timeout=300 )

class BaseModel(pw.Model):
    class Meta:
        database = database
        only_save_dirty = True

# this class describes the format of the sql data base
class Company(BaseModel):
    id_number = pw.IntegerField(primary_key=True)
    name = pw.CharField(null=True)
    n_vowels = pw.IntegerField(default=-1)
    processor = pw.IntegerField(default=-1)

def connect_database(database_name, reset_database=False):
    """ connect the database """
    database.connect()
    if reset_database:
        database.drop_tables([Company])
    database.create_tables([Company])

```

parser.py contains the CompanyParser class which is used as the engine of the code to do all the processing. It generates some artificial data which is stored to the postgresql database and then the *run* method is used to do some processing with the data already stored in the database

```
import pandas as pd
import numpy as np
import random
import string
import peewee as pw
from models import (Company, database, KVK_KEY, NAME_KEY)
import multiprocessing as mp

MAX_SQL_CHUNK = 1000

np.random.seed(0)

def random_name(size=8, chars=string.ascii_lowercase):
    """ Create a random character string of 'size' characters """
    return "".join(random.choice(chars) for _ in range(size))

def vowel_count(characters):
    """
    Count the number of vowels in the string 'characters' and return as an integer
    """
    count = 0
    for char in characters:
        if char in list("aeiou"):
            count += 1
    return count

class CompanyParser(mp.Process):
    def __init__(self, number_of_companies=100, i_proc=None,
                 number_of_procs=1,
                 first_id=None, last_id=None):
        if i_proc is not None and number_of_procs > 1:
            mp.Process.__init__(self)

        self.i_proc = i_proc
        self.number_of_procs = number_of_procs
        self.n_companies = number_of_companies
        self.data_df: pd.DataFrame = None

        self.first_id = first_id
        self.last_id = last_id

    def generate_data(self):
        """ Create a dataframe with fake company data and id's """
        id_list = np.random.randint(1000000, 9999999, self.n_companies)
        company_list = np.array([random_name() for _ in range(self.n_companies)])
        self.data_df = pd.DataFrame(data=np.vstack([id_list, company_list]).T,
                                    columns=[KVK_KEY, NAME_KEY])
        self.data_df.sort_values([KVK_KEY], inplace=True)

    def store_to_database(self):
        """
        Store the company data to a sql database
        """
        record_list = list(self.data_df.to_dict(orient="index").values())

        n_batch = int(len(record_list) / MAX_SQL_CHUNK) + 1

        with database.atomic():
            for cnt, batch in enumerate(pw.chunked(record_list, MAX_SQL_CHUNK)):
                print(f"writing {cnt}/{n_batch}")
                Company.insert_many(batch).execute()

    def run(self):
        print("Making query at {}".format(self.i_proc))
        query = (Company.
                 select().
                 where(Company.id_number.between(self.first_id, self.last_id)))
        print("Found {} companies".format(query.count()))

        for cnt, company in enumerate(query):
            print("Processing @ {} - {}:  company {}/{}".format(self.i_proc, cnt,
                                                                company.id_number,
                                                                company.name))
            number_of_vowels = vowel_count(company.name)
            company.n_vowels = number_of_vowels
            company.processor = self.i_proc
            print(f"storing number of vowels: {number_of_vowels}")
            company.save()

```

Finally, my main script load the class stored in the models.py and parser.py and launches the code. 

```
from models import (Company, connect_database)
from parser import CompanyParser

number_of_processors = 2
connect_database(None, reset_database=True)

# init an object of the CompanyParser and use the create database 
parser = CompanyParser()

company_ids = Company.select(Company.id_number)
parser.generate_data()
parser.store_to_database()

n_companies = company_ids.count()
n_comp_per_proc = int(n_companies / number_of_processors)
print("Found {} companies: {} per proc".format(n_companies, n_comp_per_proc))

for i_proc in range(number_of_processors):
    i_start = i_proc * n_comp_per_proc
    first_id = company_ids[i_start]
    last_id = company_ids[i_start + n_comp_per_proc - 1]

    print(f"Running proc {i_proc} for id {first_id} until id {last_id}")
    sub_parser = CompanyParser(first_id=first_id, last_id=last_id,
                               i_proc=i_proc,
                               number_of_procs=number_of_processors)

    if number_of_processors > 1:
        sub_parser.start()
    else:
        sub_parser.run()

```

In case that the *number\_of\_processors = 1* this script works perfectly fine. It generates artificial data, stores it to the PostgreSQL database and does some processing on the data (it counts the number of vowels in the name and stores it to the n\_vowels column)

However, in case I am trying to run this with 2 cores with *number\_of\_processors = 2*, I run into the following error

```
/opt/miniconda3/bin/python /home/eelco/PycharmProjects/multiproc_peewee/main.py
writing 0/1
Found 100 companies: 50 per proc
Running proc 0 for id 1020737 until id 5295565
Running proc 1 for id 5302405 until id 9891087
Making query at 0
Found 50 companies
Processing @ 0 - 0:  company 1020737/wqrbgxiu
storing number of vowels: 2
Making query at 1
Process CompanyParser-1:
Processing @ 0 - 1:  company 1086107/lkbagrbc
storing number of vowels: 1
Processing @ 0 - 2:  company 1298367/nsdjsqio
storing number of vowels: 2
Traceback (most recent call last):
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2714, in execute_sql
    cursor.execute(sql, params or ())
psycopg2.OperationalError: SSL error: sslv3 alert bad record mac

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/miniconda3/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/home/eelco/PycharmProjects/multiproc_peewee/parser.py", line 82, in run
    company.save()
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 5748, in save
    rows = self.update(**field_dict).where(self._pk_expr()).execute()
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1625, in inner
    return method(self, database, *args, **kwargs)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1696, in execute
    return self._execute(database)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2121, in _execute
    cursor = database.execute(self)
  File "/opt/miniconda3/lib/python3.7/site-packages/playhouse/postgres_ext.py", line 468, in execute
    cursor = self.execute_sql(sql, params, commit=commit)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2721, in execute_sql
    self.commit()
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2512, in __exit__
    reraise(new_type, new_type(*exc_args), traceback)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 186, in reraise
    raise value.with_traceback(tb)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2714, in execute_sql
    cursor.execute(sql, params or ())
peewee.OperationalError: SSL error: sslv3 alert bad record mac

Process CompanyParser-2:
Traceback (most recent call last):
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2714, in execute_sql
    cursor.execute(sql, params or ())
psycopg2.OperationalError: SSL error: decryption failed or bad record mac

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/miniconda3/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/home/eelco/PycharmProjects/multiproc_peewee/parser.py", line 72, in run
    print("Found {} companies".format(query.count()))
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1625, in inner
    return method(self, database, *args, **kwargs)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1881, in count
    return Select([clone], [fn.COUNT(SQL('1'))]).scalar(database)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1625, in inner
    return method(self, database, *args, **kwargs)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1866, in scalar
    row = self.tuples().peek(database)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1625, in inner
    return method(self, database, *args, **kwargs)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1853, in peek
    rows = self.execute(database)[:n]
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1625, in inner
    return method(self, database, *args, **kwargs)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1696, in execute
    return self._execute(database)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 1847, in _execute
    cursor = database.execute(self)
  File "/opt/miniconda3/lib/python3.7/site-packages/playhouse/postgres_ext.py", line 468, in execute
    cursor = self.execute_sql(sql, params, commit=commit)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2721, in execute_sql
    self.commit()
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2512, in __exit__
    reraise(new_type, new_type(*exc_args), traceback)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 186, in reraise
    raise value.with_traceback(tb)
  File "/opt/miniconda3/lib/python3.7/site-packages/peewee.py", line 2714, in execute_sql
    cursor.execute(sql, params or ())
peewee.OperationalError: SSL error: decryption failed or bad record mac

Process finished with exit code 0

```

Somehow something goes wrong as soon as the second thread start to do something with the database. Does somebody has advice to get this code working. I have tried the following already

* Try the PooledPostgresDatabase and normal PostgresqlDatabase to
connect to the database. This leads to the same error
* Try using sqlite in stead of postgres. This works for 2 cores, but only if the two processes are not interfering too much; otherwise I
can some locking problems. I was in the impression that postgres
would be better for doing multiprocessing then sqlite (is that true?)
* When putting a break after launching the first process(so effectively using only one core), the code works, showing that the *start* method is called correctly.

Hopefully somebody can advise.

Regards
Eelco