guanghuange / django-sequences

Generate gap-less sequences of integer values.

django-sequences

The problem

Django's default, implicit primary keys aren't guaranteed to be sequential.

If a transaction inserts a row and then is rolled back, the sequence counter isn't rolled back for performance reasons, creating a gap in primary keys.

This can cause compliance issues for some use cases such as accounting.

This risk isn't well known. Since most transactions succeed, values look sequential. Gaps will only be revealed by audits.

The solution

django-sequences provides just two function, get_next_value and get_next_n_value, which is designed to be used as follows:

from django.db import transaction

from sequences import get_next_value

from invoices.models import Invoice

with transaction.atomic():
    Invoice.objects.create(number=get_next_value('invoice_numbers'))

The guarantees of django-sequences only apply if you call get_next_value and save its return value to the database within the same transaction!

Installation

Install django-sequences:

cat requirements.txt
--trusted-host=pypi.luojilab.com
--index-url http://pypi.luojilab.com/rock2018/pypi

django-sequences=0.1.5    # 请安装最新版本
pip install -r requirements.txt
$ pip install django-sequences

Add it to the list of applications in your project's settings:

INSTALLED_APPS = [
'...',
'sequences',

]

Run migrations:

$ ./manage.py migrate

API

get_next_value generates a gap-less sequence of integer values:

>>> get_next_value()
1
>>> get_next_value()
2
>>> get_next_value()
3

It supports multiple independent sequences:

>>> get_next_value('PO')
1
>>> get_next_value('PO')
2
>>> get_next_value('PO')
1
>>> get_next_value('PO')
2

It supports multiple values sequences:

>>> get_next_n_value(5, 'PO')
[0, 1, 2, 3, 4]
>>> get_next_value(5, 'PO')
[5, 6, 7, 8, 9]
>>> get_next_n_value(5, 'SO')
[0, 1, 2, 3, 4]
>>> get_next_value(5, 'SO')
[5, 6, 7, 8, 9]