1. Two database findings
Tobias Schmidt
@dagrobie
github.com/grobie
Lightning talk @ Railsberry, Kraków - 19 April 2012

2. Know your usage patterns

3. pt-query-digest
# Profile
# Rank Query ID Response time Calls R/Call Apdx V/M Item
# ==== ================== ============== ===== ====== ==== ===== =========
# 1 0x5E796D5A4A7D1CA9 449.4200 40.6% 4763 0.0944 0.99 1.32 ADMIN STATISTICS
# 2 0xDEA06A23CC92009D 101.0927 9.1% 352 0.2872 0.95 1.00 SELECT users
# 3 0xE320654019E45018 99.4647 9.0% 648 0.1535 0.98 1.15 SELECT users
# 4 0x813031B8BBC3B329 28.6658 2.6% 192 0.1493 0.99 16.10 COMMIT
# 5 0xB7CFCFF53D7D16A7 25.2505 2.3% 278 0.0908 0.99 1.90 SELECT oauth2...
# ...

4. pt-query-digest
# Profile
# Rank Query ID Response time Calls R/Call Apdx V/M Item
# ==== ================== ============== ===== ====== ==== ===== =========
# 1 0x5E796D5A4A7D1CA9 449.4200 40.6% 4763 0.0944 0.99 1.32 ADMIN STATISTICS
# 2 0xDEA06A23CC92009D 101.0927 9.1% 352 0.2872 0.95 1.00 SELECT users
# 3 0xE320654019E45018 99.4647 9.0% 648 0.1535 0.98 1.15 SELECT users
# 4 0x813031B8BBC3B329 28.6658 2.6% 192 0.1493 0.99 16.10 COMMIT
# 5 0xB7CFCFF53D7D16A7 25.2505 2.3% 278 0.0908 0.99 1.90 SELECT oauth2...
# ...
40% of the time for
“ADMIN STATISTICS”

5. COM_STATISTICS
Asks the MySQL server to compile a text
message with some server statistics (uptime,
queries per second, etc.).This packet can be sent
with mysqladmin status. No arguments.
$ mysqladmin status
Uptime: 41729399 Threads: 892 Questions: ... Slow queries: ... ...
http://forge.mysql.com/wiki/MySQL_Internals_ClientServer_Protocol#COM_STATISTICS

6. ActiveRecord
# active_record/connection_adapters/mysql_adapter.rb
class ActiveRecord::ConnectionAdapters::MysqlAdapter
def active?
@connection.stat
if @connection.respond_to?(:errno)
@connection.errno.zero?
else
true
end
end
end

7. ActiveRecord
# ar/connection_adapters/abstract_adapter.rb
class AR::ConnectionAdapters::AbstractAdapter
# Checks whether the connection to the database is
# still active (i.e. not stale). This is done under
# the hood by calling <tt>active?</tt>.
# If the connection is no longer active, then this
# method will reconnect to the database.
def verify!(*ignored)
reconnect! unless active?
end
end

8. ActiveRecord
# action_controller/dispatcher.rb
class ActionController::Dispatcher
def self.cleanup_application
# ...
ActiveRecord::Base.clear_reloadable_connections!
end
end

9. Optimistic health check
# config/initializers/active_record_extensions.rb
class ActiveRecord::ConnectionAdapters::MysqlAdapter
def active?
true
end
end
additional info: http://www.mysqlperformanceblog.com/2010/05/05/checking-for-a-live-database-connection-considered-harmful/

10. Know your tools

11. Lhm
Large hadron migrator
github.com/soundcloud/large-hadron-migrator

12. Lhm
• Online schema change tool
• migrate tables with (almost) no downtime
• Replication safe
• Ruby 1.8 / 1.9, ActiveRecord 2.3 / 3.x and
mysql / mysql2 compatible

13. Lhm
users

14. Lhm - prepare
create temporary table
users lhmn_users

15. Lhm - alter
require "lhm"
alter table
class AddRailsberryToUsers < ActiveRecord::Migration
def up
Lhm.change_table :users do |u|
u.add_column :railsberry, "BOOLEAN DEFAULT true" lhmn_users
u.add_index :railsberry
end
end
end

16. Lhm - entangle
create trigger
users lhmn_users
after insert
after update
after delete

17. Lhm - copy
copy all rows
users lhmn_users

18. Lhm - rename
rename
lhma_users users

19. Know your usage patterns
Know your tools

20. soundcloud.com/jobs