How to use Postgres JSON store to persist DDD aggregates

1. DDD, Rails and persistence
Michał Łomnicki
January, 2016
DRUG
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2. Inspiration
Blog
https://vaughnvernon.co
Ideal DDD Aggregate Store
Book
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3. Problem
I want DDD in my Rails project
I want fast and clean tests
I want to build my application around domain objects not around database schema
...but I struggle with persistence and ActiveRecord gets into my way all the time
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4. Model
classSquad
includeVirtus.model#optional,canbePORO
MAX_FIRST_SQUAD_PLAYERS=11
attribute:id, UUID
attribute:match_id, UUID
attribute:team_id, UUID
attribute:formation, Formation
attribute:first_squad,Set[Player]
attribute:bench, Set[Player]
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5. Model
defremove_from_first_squad(player)
raiseSquadErrorif!first_squad.member?(player)
first_squad.delete(player)
bench.add(player)
end
defadd_to_first_squad(player)
raiseSquadErrorif!bench.member?(player)
raiseSquadErroriffirst_squad.size==MAX_FIRST_SQUAD_PLAYERS
bench.remove(player)
first_squad.add(player)
end
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6. Model
defsubstitute(player_off,player_on)
remove_from_first_squad(player_off)
add_to_first_squad(player_on)
DomainEventPublisher.publish(
PlayerSubstituted.new(
squad_id: id,
player_off_id:player_off.id,
player_on_id: player_on.id
)
)
end
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7. Service
classSquadService
includeTransactionSupport
definitialize(squad_repository,player_repository)
@squad_repository =squad_repository
@player_repository=player_repository
end
defsubstitute(substitution_form)
transactiondo
DomainEventPublisher.subscribe(PlayerSubstituted,SomeHandler)
player_off=player_repository.find(substitution_form.player_off_id)
player_on =player_repository.find(substitution_form.player_on_id)
squad =squad_repository.find(substitution_form.squad_id)
squad.substitute(player_off,player_on)
squad_repository.save(squad)
end
end
defchange_formation(formation_form)
...
end
end
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8. Repository
classSquadRepository
defsave(squad)
ifsquad.id
update(squad)
else
create(squad)
end
end
defcreate(squad)
SquadAR.create(
match_id:squad.match_id,
formation:squad.formation.to_s,
squad_players:
squad.first_squad.map{|player|PlayerAR.new(first_squad:true,...)}
+squad.bench.map{|player|PlayerAR.new(first_squad:false,...)}
)
end
...
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9. Repository / Naive update
defupdate(squad)
record=SquadAR.find(squad.id)
record.formation=squad.formation.to_s
#deleteandre-createassociations
record.squad_players=
squad.first_squad.map{|player|PlayerAR.new(first_squad:true,...)}
+squad.bench.map{|player|PlayerAR.new(first_squad:false,...)}
record.save
end
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10. Repository / Naive update
defupdate(squad)
record=SquadAR.find(squad.id)
record.formation=squad.formation.to_s
#deleteandre-createassociations
record.squad_players=
squad.first_squad.map{|player|PlayerAR.new(first_squad:true,...)}
+squad.bench.map{|player|PlayerAR.new(first_squad:false,...)}
record.save
end
Hard to maintain
Error­prone
Poor performance
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11. Solution 1
Data Mapper
No mature Data Mapper for Ruby
ROM looks promising
...but is yet incomplete
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12. Solution 2
Events as a storage mechanism
Yes, that's a good solution
Big mental model change
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13. Solution 3
Postgres + JSON
Ideal DDD Aggregate store?
Aggregate data stored as JSON
One database row ­ one aggregate
create_table"squads"do|t|
t.jsonb:data,null:false
end
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14. DB schema
create_table"users"do|t|
t.jsonb:data,null:false
end
create_table"matches"do|t|
t.jsonb:data,null:false
end
create_table"teams"do|t|
t.jsonb:data,null:false
end
create_table"squads"do|t|
t.jsonb:data,null:false
end
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15. JSON Repository
classSquadRepository
defsave(squad)
ifsquad.id
update(squad)
else
create(squad)
end
end
deffind(squad_id)
Domain::Squad.new(SquadAR.find(squad_id))
end
private
defcreate(squad)
SquadAR.create(data:squad.as_json)
end
defupdate(squad)
SquadAR.where(id:squad.id).update_all(data:squad.as_json)
end
end
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16. Why not Mongo? This looks like NoSQL
Mongo is not ACID­compliant
Transactions only at the document level
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17. Postgres + JSON
JSON introduced in Postgres 9.3
JSONB introduced in Postgres 9.4
JSONB can be indexed
Postgres = ACID
No foreign keys and unique indexes
Data consistency ensured at application level
Introduce this approach in the existing database
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18. Postgres + JSON
JSON introduced in Postgres 9.3
JSONB introduced in Postgres 9.4
JSONB can be indexed
Postgres = ACID
No foreign keys and unique indexes
Data consistency ensured at application level
Introduce this approach in the existing database
CREATEINDEXONsquadsUSINGgin(data)
SELECT*FROMsquadsWHERE(data->>'match_id')::INT=12
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19. Locking
Some locking mechanism is required
Optimisic locking is preferred
a)Process1reads
b)Process2reads
c)Process1writes
d)Process2overwritesc)
SquadAR.where(id:squad.id,lock_version:current_version[squad]).update_all(
data: squad.as_json,
lock_version:current_version[squad]+1
)
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20. Lessons learnt
It works
Changes are easy to introduce
Fast and easy to store and load an entire aggregate
Code explains the application, not the DB schema
Migrations require more work
Most probably you will build a read model
De­normalized data needs synchronization
Avoid big aggregates
No help from the database (foreign keys, not null, etc)
Can't really fiddle in railsconsole
Squad.find(123).squad_players.update_all(...)
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21. Thank you
Resources:
https://vaughnvernon.co/?p=942
http://www.amazon.com/Implementing­Domain­Driven­Design­Vaughn­
Vernon/dp/0321834577
http://www.postgresql.org/docs/9.4/static/datatype­json.html
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