Mcfly
Mcfly is a database table versioning system. It's useful for tracking and auditing changes to database tables. It's also very easy to access the state of Mcfly tables at any point in its history.
Features
-
All row versions are stored in the same table.
-
Different row versions are accessed through scoping.
-
Applications can use Mcfly to time-warp all tables to previous points in time.
-
Table queries for points in time are symmetric. i.e. queries to access data in the present look just like queries available in any particular point in time.
-
Implemented as database triggers. So, the versioning system is language/platform agnostic.
Installation
$ gem install mcfly
Or add it to your Gemfile
, etc.
Usage
To create Mcfly enabled tables, they need to be created using
Mcfly::McflyMigration
or Mcfly::McflyAppendOnlyMigration
instead
of the usual ActiveRecord::Migration
.
class CreateSecurityInstruments < McflyAppendOnlyMigration
def change
create_table :security_instruments do |t|
t.string :name, null: false
t.string :settlement_class, limit: 1, null: false
end
end
end
class CreateMarketPrices < McflyMigration
def change
create_table :market_prices do |t|
t.references :security_instrument, null: false
t.decimal :coupon, null: false
t.integer :settlement_mm, null: false
t.integer :settlement_yy, null: false
t.decimal :price
end
end
end
These migrations add the necessary versioning triggers for INSERT, UPDATE and DELETE operations. The append-only migration disallows updates. As such, append-only Mcfly tables allow rows to be INSERTed or DELETEed, but not modified.
When you declare has_mcfly
in your model, Mcfly adds some basic
functionality to the class.
class SecurityInstrument < ActiveRecord::Base
has_mcfly append_only: true
validates_presence_of :name, :settlement_class
mcfly_validates_uniqueness_of :name
mcfly_lookup :lookup, sig: 2 do
|pt, name|
find_by_name(name)
end
mcfly_lookup :lookup_all, sig: 1 do
|pt| all
end
end
The has_mcfly
declaration provides the mcfly_lookup
generator which scopes queries to the proper timeline. Also, mcfly_validates_uniqueness_of
is Mcfly's scoped version of ActiveRecord's validates_uniqueness_of
.
Setting/Finding Responsible Party For A Change
TODO: discuss using current_user
method in ApplicationController
. Also, setting Mcfly.whodunnit
.
Implementation
TODO
Limitations/Requirements
Currently, Mcfly only works with PostgreSQL databases. The following
line must be added to the postgresql.conf
file. Mcfly uses the
PostgreSQL session variable mcfly.whodunnit
to store the current
user id.
custom_variable_classes = 'mcfly'
History
The database table versioning mechanism used in Mcfly was originally developed at TWINSUN. It has since been modified and enhanced at PENNYMAC.
License
Mcfly has been released under the MIT license. Please check the LICENSE file for more details.