django-sequences-py2 0.2

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 one function, get_next_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:

$ pip install django-sequences-py2

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

INSTALLED_APPS += ['sequences.apps.SequencesConfig']

Run migrations:

$ django-admin 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('cases')
1
>>> get_next_value('cases')
2
>>> get_next_value('invoices')
1
>>> get_next_value('invoices')
2

The first value defaults to 1. It can be customized:

>>> get_next_value('customers', initial_value=1000)  # pro growth hacking

The initial_value parameter only matters when get_next_value is called for the first time for a given sequence — assuming the corresponding database transaction gets committed; as discussed above, if the transaction is rolled back, the generated value isn’t consumed. It’s also possible to initialize a sequence in a data migration and not use initial_value in actual code.

Sequences can loop:

>>> get_next_value('seconds', initial_value=0, reset_value=60)

When the sequence reaches reset_value, it restarts at initial_value. In other works, it generates reset_value - 2, reset_value - 1, initial_value, initial_value + 1, etc. In that case, each call to get_next_value must provide initial_value and reset_value.

Database transactions that call get_next_value for a given sequence are serialized. In other words, when you call get_next_value in a database transaction, other callers which attempt to get a value from the same sequence will block until the transaction completes, either with a commit or a rollback. You should keep such transactions short to minimize the impact on performance.

Passing nowait=True will cause get_next_value to raise an exception instead of blocking. This will rarely be useful. Also it doesn’t work for the first call. (Arguably this is a bug. Patches welcome.)

Calls to get_next_value for distinct sequences don’t interact with one another.

Finally, passing using='...' allows selecting the database on which the current sequence value is stored. When this parameter isn’t provided, the current value is stored in the default database for writing to models of the sequences application. See below for details.

To sum up, the complete signature of get_next_value is:

get_next_value(sequence_name='default', initial_value=1, reset_value=None,
               *, nowait=False, using=None)

Under the hood, it relies on the database’s transactional integrity to guarantee that each value will be returned exactly once.

Contributing

You can run tests with:

$ make test

If you’d like to contribute, please open an issue or a pull request on GitHub!

Other databases

INTEGER PRIMARY KEY AUTOINCREMENT fields on SQLite don’t have this problem.

The author doesn’t know if this problem can happens on MySQL or Oracle. If it does, then the current implementation of django-sequences should work. If you test this, please open an issue on GitHub to report your findings. Note that MySQL won’t support the nowait parameter.

Multiple databases

Since django-sequences relies on the database to guarantee transactional integrity, the current value for a given sequence must be stored in the same database as models containing generated values.

In a project that uses multiple databases, you must write a suitable database router to create tables for the sequences application on all databases storing models containing sequential numbers.

Each database has its own namespace: a sequence with the same name stored in two databases will have independent counters in each database.

Changelog