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sqlalchemy_audit 0.1.0

sqlalchemy-audit provides an easy way to set up revision tracking for your data.

sqlalchemy-audit provides an easy way to set up revision tracking for your data. It is inspired by SQLAlchemy’s versioned_history example, but uses mapper events instead of session events.

Example

Share your DBSession with Versioned:

DBSession = ...
Versioned.versioned_session(DBSession)

Then simply declare your class as usual and have it inherit Versioned:

class Reservation(Versioned, Base):
  __tablename__ = 'reservation'
  id = Column(Integer, primary_key=True)
  name = Column(String(50))
  date = Column(Date)
  time = Column(Time)
  party = Column(Integer)
  last_modified = Column(DateTime)

Reservation.broadcast_crud()  # todo: handle this automagically

Note

You can also sub-class Versioned from your declarative base class.

Normal usage remains the same:

# make new reservation
steve_reservation = Reservation(name='Steve',
                                date=datetime.date(2015, 04, 15),
                                time=datetime.time(19, 00),
                                party=6)
session.add(steve_reservation)
session.commit()

# change reservation to party of 4
steve_reservation.party = 4
session.commit()

# cancel the reservation
session.delete(steve_reservation)
session.commit()

Plus, you could access its revision history.

>>> DBSession.query(ReservationRev).all()
[ ReservationRev(rev_id='c74d5bce...', rev_created=1427995346.0, rev_isdelete=False, id=1, name='Steve', date='2015-04-15', time='19:00', party=6, last_modified='2015-04-02 13:22:26.291670'),
  ReservationRev(rev_id='f3f5091d...', rev_created=1428068391.0, rev_isdelete=False, id=1, name='Steve', date='2015-04-15', time='19:00', party=4, last_modified='2015-04-03 09:39:51.098798'),
  ReservationRev(rev_id='3cf1394b...', rev_created=1428534191.0, rev_isdelete=True, id=1, name=None, date=None, time=None, party=None, last_modified=None)
]

How it works

Suppose you have a reservations table.

id name date time party last_modified
1 Steve 2015-04-15 19:00 4 2015-04-08 13:22:26.291670
2 Phil 2015-05-01 18:30 3 2015-04-13 09:38:01.060898

Behind the scenes, we create an revision class ReservationRev mapped to table reservations_rev. It has the same schema with three additional columns:

rev_id : string (uuid)
Surrogate key for the revision table.
rev_created : timestamp
Timestamp (seconds since the epoch as a floating point number) of when the revision was created. (See Use of rev_created.)
rev_isdelete : boolean
Whether the entry was deleted. (See Use of rev_isdelete.)

Whenever you write to the reservations table, we will insert a new row into the reservations_rev table. This allows your usage of reservations to remain unchanged. If need, you could reference the reservations_rev to get the revision timelime.

Example

For the following timeline:

  • On 2015-04-02, Steve makes a reservation for party of 6 on 2015-04-15 at 19:30.
  • On 2015-04-03, Steve changes the reservation to 4 people.
  • On 2015-04-08, Steve cancels the reservation.

reservations_rev will have the following

rev_id rev_created rev_isdelete id name date time party last_modified
c74d5bce… 1427995346.0 False 1 Steve 2015-04-15 19:00 6 2015-04-02 13:22:26.291670
f3f5091d… 1428068391.0 False 1 Steve 2015-04-15 19:00 4 2015-04-03 09:39:51.098798
3cf1394b… 1428534191.0 True 1 (null) (null) (null) (null) (null)

Design Decisions

Writing to revision table for all writes

There are several advantages by writing to the revision table for all writes:

  1. complete transaction history in the revision table for easy reads (no joins required)
  2. complete timeline even if the original table doesn’t have a last modified column

However, this approach has a particular drawback with INSERT statements with dynamic defaults (such as sequences or auto-datetime). At the time of the insert, the revision table does not have the dynamic values. We recommend the following workarounds:

  1. generate dynamic defaults during object instantiation instead using database defaults
  2. strictly use client-side defaults in the ORM
  3. create server-side database triggers to copy values to revision table for inserts
  4. perform a write-read-write transaction for inserts, which is sub-optimal due to the performance hit

Use of rev_created

To re-create the revision timeline, we are relying on the use of timestamps. While we recognize there could be clock drift or desynchronization across different servers, there are solutions to these problems. Hence we opt to proceed with timestamp’s simplicity.

Use of rev_isdelete

The rev_isdelete is a fast and convenient way to determined that a row has been deleted without inspecting the entries. It also allows for entries with all nulls.

Requirement of primary/compound keys

TBD

Requirement of association objects for many-to-many relationships

TBD

 
File Type Py Version Uploaded on Size
sqlalchemy_audit-0.1.0.tar.gz (md5) Source 2016-12-04 9KB