This document discusses database sharding in Rails applications. It begins with an introduction and agenda, then discusses whether Rails can scale and what scaling means. It covers scaling a Rails application by adding web workers, background jobs, and sharding the database. It provides examples of sharding database tables in Rails and gems that can be used. Finally, it discusses how one company implemented database sharding step-by-step and offers pros and cons of sharding.

1. DATABASE SHARDING
IN RAILS APPLICATIONS
RUBY MEDITATION #23
September 2018

2. Work: @Talkable
Ruby: since 2012
Rails: since v3.2
Open Source: about 0 contributions
@justmacc
@vitalikdanchenko
ABOUT ME

3. ABOUT ME

4. Hobby: Running
Marathon Finisher: 4 times
Marathon PB: 3:05:10
ABOUT ME

5. A LITTLE BIT ABOUT SCALING
DATABASE SHARDING
A LOT OF DIAGRAMS
ALMOST NO CODE
AGENDA

6. CAN RAILS SCALE?

7. CAN RAILS SCALE?

8. — @busterarm at Hacker News
"TWITTER SCALE" WASN'T
SO MUCH A RUBY/RAILS
PROBLEM AS IT WAS AN
RDBMS
”
“
https://news.ycombinator.com/item?id=17217210
CAN RAILS SCALE?

9. SCALING

10. SCALABILITY VS PERFORMANCE
SCALING

11. SCALING

12. Rails Application
WEB WORKERS
DATABASE
SCALING

13. Rails Application
WEB WORKERS
BACKGROUND
JOBS
DATABASE
SCALING

14. Rails Application
WEB WORKERS
BACKGROUND JOBS
DATABASE
SCALING

15. Rails Application
WEB WORKERS
BACKGROUND JOBS
DATABASE
SCALING

16. Rails Application
WEB WORKERS
BACKGROUND JOBS DATABASE
SCALING

17. Rails Application
WEB WORKERS
DATABASE
BACKGROUND JOBS
SCALING

18. Rails Application
WEB
WORKERS
DATABASE
WEB
WORKERS
WEB
WORKERS
WEB
WORKERS
JOBS JOBS
JOBS JOBS
SCALING

19. SCALING

20. DATABASE
DATABASE SCALING

21. DATABASE
MASTER
DATABASE
SLAVE
DATABASE SCALING

22. DATABASE
MASTER
DATABASE
SLAVE
DATABASE
SLAVE
DATABASE
SLAVE
DATABASE SCALING

23. DATABASE
PARTIAL DATA
DATABASE
PARTIAL DATA
DATABASE
PARTIAL DATA
DATABASE SHARDING

24. FUNCTIONAL EXPRESSIONAL METADATA
Separate
Modules
id % N
year
country
row -> shard
DATABASE SHARDING

25. THEORETICAL PART
PRACTICAL PART

26. DATABASE SHARDING IN RAILS
MULTIPLE DATABASES MODEL
1 class ShardedBase < ActiveRecord::Base
2 establish_connection SHARD_DB_CONFIG
3 self.abstract_class = true
4 end
5
6 class Post < ShardedBase
7 end
8
9 class Comment < ShardedBase
10 end

27. DATABASE SHARDING IN RAILS
GEMS
• thiagopradi/octopus
• hsgubert/rails-sharding
• zendesk/active_record_shards
1 ActiveRecord::Base.on_shard(:number_two) do
2 Post.find_by(author: author)
3 end

28. DATABASE SHARDING IN TALKABLE
STEP BY STEP

29. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard?

30. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard?
BY CUSTOMER

31. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata

32. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded?

33. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded?
THE LARGEST

34. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1 class Visitor < ShardedRecord
2 has_many :purchases
3 end
4
5 class Purchase < ShardedRecord
6 belongs_to :person
7 has_one :referral
8 end
9
10 class Referral < ShardedRecord
11 has_many :rewards
12 end
13
14 class Reward < ShardedRecord
15 belongs_to :incentive
16 end

35. DATABASE SHARDING IN TALKABLE
STEP BY STEP
DATABASE
NOT SHARDED
CONFIG DATA
DATABASE
SHARD #1
REFERRAL DATA
DATABASE
SHARD #2
REFERRAL DATA

36. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins

37. 1 module ShardingJoinFinder
2 TABLE_REFERENCE = /(^|s+)(join|from)s+`?([a-z_]+)`?/i
3
4 def execute(query, name = nil)
5 ensure_sharding_match(query)
6 super(query, name)
7 end
8
9 def select_all(query, *args)
10 ensure_sharding_match(query)
11 super
12 end
13
14 def ensure_sharding_match(query)
15 query.scan(TABLE_REFERENCE).map(&:last)
16 sharded, unsharded = klasses.partition(&:is_sharded?)
17 raise Error if sharded.any? && unsharded.any?
18 end
19 end
20
21 ActiveRecord::ConnectionAdapters::Mysql2Adapter.prepend(ShardingJ
22 ActiveRecord::Relation.prepend(ShardingJoinFinder)

38. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins — joins detector

39. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins — joins detector
4. Introduce connection management

40. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins — joins detector
4. Introduce connection management — gem

41. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins — joins detector
4. Introduce connection management — gem
5. Add utility helpers

42. 1 class DatabaseUtils
2 class SlaveError < StandardError; end
3
4 class << self
5 def use_shard(shard, &block); end
6 def current_shard; end
7 def uses_slave?; end
8 def config_for(slave: uses_slave?, shard: current_shard); end
9 def connection(slave: uses_slave?, shard: current_shard); end
10 def sharded_tables; end
11 def on_all_shards(&block); end
12 def processlist; end
13 def slave_lag_for(connection); end
14 end
15 end
1 Rails.application.console do
2 def shard(shard)
3 shard = shard.database_shard if shard.respond_to?(:database_shard)
4 DatabaseUtils.use_shard!(shard)
5 end
6 end

43. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins — joins detector
4. Introduce connection management — gem
5. Add utility helpers — depends on individual needs

44. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins — joins detector
4. Introduce connection management — gem
5. Add utility helpers — depends on individual needs
6. Migrate

45. 1 production:
2 host: 'master.production.com'
3 database: 'primary'
4 slave:
5 host: 'slave.production.com'
6 database: 'primary'
7
8 shards:
9 1:
10 host: 'master.production.com'
11 database: 'shard_1'
12 slave:
13 host: 'slave.production.com'
14 database: 'shard_1'
15 2:
16 host: 'shard_2.production.com'
17 database: 'shard_2'
18 slave:
19 host: 'slave.shard_2.production.com'
20 database: 'shard_2'

46. 1 production:
2 host: 'master.production.com'
3 database: 'primary'
4 slave:
5 host: 'slave.production.com'
6 database: 'primary'
7
8 shards:
9 1:
10 host: 'shard1.production.com'
11 database: 'shard_1'
12 slave:
13 host: 'slave.shard_1.production.com'
14 database: 'shard_1'
15 2:
16 host: 'shard_2.production.com'
17 database: 'shard_2'
18 slave:
19 host: 'slave.shard_2.production.com'
20 database: 'shard_2'

47. DATABASE SHARDING IN TALKABLE
STEP BY STEP
1. How to shard? — expressional + metadata
2. Which tables have to be sharded? — config vs referral
3. Get rid of joins — joins detector
4. Introduce connection management — gem
5. Add utility helpers — depends on individual needs
6. Migrate — profit!!!

48. MySQL PostgreSQL
Vites Citus
Jetpants ???
ScaleBase
DATABASE SHARDING SOLUTIONS

49. PROS CONS
Easier to Manage Not Easy to Implement
Smaller and Faster Lost Abilities
Reduces Costs
DATABASE SHARDING
CONCLUSION

50. DATABASE SHARDING
CONCLUSION
• Do not shard your data until you really need it
• Choose a sharding model that fits your context (project)
• Each additional shard increases the chance of a random
crash

51. When: 08:00 am
Where: 46°28'44.5"N 30°45'41.0"E
How far: doesn’t matter
How fast: doesn’t matter
SUNDAY MORNING RUN

52. Q&A