Safer Rails database migrations with Soundcloud’s Large Hadron Migrator
When I first started using Rails years ago I fell in love with the concept of database migrations. Perhaps because at the time I was working on commercial projects in C# which lacked this and I could feel the difference. The fact that for many years the concept remained almost the same with some minor improvements speaks for itself. Ruby on Rails continues evolving all the time but migrations remain simple.
However, there is an elephant in the room.
Some DDL operations on MySQL database (such as adding or removing columns) are locking the whole affected table. It means that no other process will be able to add or update a record at that time and will wait until lock is released or timeout occurs. The list of operations that can be performed online (without lock) constantly increases with every new MySQL release so make sure to check the version of your database and consult its documentation. In particular, this has been very much improved in MySQL 5.6.
With lower number of records, offline DDL operations are not problematic. You can live with 1s lock. Your application and background workers will not do anything in that time, some customers might experience slower response times. But in general, nothing harmful very much.
However, when the table has millions of records changing it can lock the table for many seconds or even minutes. Soundcloud even says an hour, although I personally haven’t experienced it.
Anyway, there are tables in our system of utter importance such as orders or payments and locking them for minutes would mean that customers can’t buy, merchants can’t sell and we don’t earn a dime during that time.
For some time our solution was to run the costly migrations around 1 am or 6 am when there was not much traffic and a few minutes of downtime was not a problem. But with the volume of purchases constantly increasing, with having more merchants from around the whole world, there is no longer a good hour to do maintenance anymore.
Not to mention that everyone loves to sleep and waking up earlier just to run a migration is pointless. We needed better tools and better robots to solve this problem.
We decided to use Large Hadron Migrator created by Soundcloud.
How does it work?
- It creates a new version of table
- It installs triggers that make updates in old table to appear in a new table
- It copies in batches data from old table to new table
- It switches atomically old and new tables when the whole process is finished.
That’s the idea behind it.
The syntax is not as easy as with standard Rails migrations because you will need to restore to using SQL syntax a bit more often.
require 'lhm' class MigrateUsers < ActiveRecord::Migration def up Lhm.change_table :users do |m| m.add_column :arbitrary_id, "INT(12)" m.add_index [:arbitrary_id, :created_at] end end def down Lhm.change_table :users do |m| m.remove_index [:arbitrary_id, :created_at] m.remove_column :arbitrary end end end
If you need to migrate big tables without downtime in MySQL you can use LHM or upgrade MySQL to 5.6 :)
If you are still worried how to safely do Continuous Deployment and handle migrations please read our other blog posts as well: