This is a proof of concept module to implement an "expand" and "contract"
method of database migrations.

There are two main sources of downtime when using traditional migration
(such as sqlalchemy-migrate or alembic):

1) ALTER TABLE can and often will take a lock on the table for the
   duration of the operation, which can be long depending on the row count
2) The length of time it takes to migrate the data after the schema change
   is done can be long. Such as moving data to new tables, etc

Also, to a lesser extent, the length of time code needs to be stopped
can cause extra downtime. This is necessary because the old code can
only use the old schema and the new code can only use the new schema.

This software tackles the latter problem of the length of time it takes
to migrate the data, not the downtime from the schema change (see below
for possible solutions).

It tackles the problem by splitting the process into an "expand" phase
which adds new columns (never modifying existing columns) and then a
"contract" phase which removes unused columns.

In between the "expand" and "contract", as software reads data from the
database, the data is migrated to the new columns/tables that have been
added.

ALTER TABLE downtime
--------------------
ALTER TABLE will often cause locks to be obtained for the duration of the
operation. This can happen even when adding a new column with InnoDB.
This is a potential source of downtime as the database updates all rows
with the write lock held.

Possible solutions:
https://www.facebook.com/notes/mysql-at-facebook/online-schema-change-for-mysql/430801045932
http://code.google.com/p/openarkkit/
https://www.percona.com/doc/percona-toolkit/2.1/pt-online-schema-change.html
Upgrade to MySQL 5.6, which appears to have better code to handle online
schema changes with minimal downtime