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.

arman000/mcfly