JavaZone 2013 - Datomic vs EventStore

vs
Immutable data stores
onsdag 25 september 13

Immutable data stores
Only add, never remove or change
Can retrieve old values

Source code
Version controlled
Keep all versions

Example domain

Rock - Paper - Scissors

http://rock-paper-scissors.com/
The future Facebook of Rock Paper Scissors
Millions of users
Many games per user

Playing the game
Player A
Player B
Server

Playing the game
Player A
Player B
Server
Game 123

Playing the game
Player A
Player B
Server
paper
Game 123

Playing the game
Player A
Player B
Server
paper
Player B: paper
Game 123

Playing the game
Player A
Player B
Server
rock
Player B: paper
Player A: rock
Game 123

Playing the game
Player A
Player B
Server
Player B: paper
Player A: rock
Game 123
Game 123
winner: Player B
loser: Player A

Playing the game
Player A
Player B
Server
Player B: paper
Player A: rock
Game 123
Game 123
winner: Player B
loser: Player A
CREATED WAITING
GAME WON
GAME TIED
any move
other move
(victory)
other move
(tie)

Entities

Entities
Player
+ id
+ name
+ email

Entities
Game
+ id
+ state
+ players
+ moves
+ winner
+ loser
Player
+ id
+ name
+ email

Entities
Game
+ id
+ state
+ players
+ moves
+ winner
+ loser
Move
+ move
+ player
Player
+ id
+ name
+ email

Actions
Create game
Make move
Get game result
POST /games
POST /games/123
move=rock
GET /games/123

Actions
Create game
Make move
Get game result
POST /games
POST /games/123
move=rock
GET /games/123
1. Form (make move)
2. Waiting
3. Game result
4. Error

Event Sourcing
with
Event Store

Event Sourcing
Events that have happened
Ordered in time
Source of truth

http://geteventstore.com
Runs on .NET and Mono
Free (store) or Commercial (HA)
API
Custom TCP (c#)
ATOM over HTTP (xml/json)
Features
Event streams, projections, generate new events (CEP)
€1500/year

Event example
Server EventStore

Event example
Server
CreateGameCommand{:game 123 :creator "player-1"}
EventStore

Event example
Server
GameCreatedEvent{:game 123 :creator "player-1"}
EventStore

Event example
Server
MakeMoveCommand{:game 123 :player "player-1" :move "rock"}
MoveMadeEvent{:game 123 :player "player-1" :move "rock"}
EventStore

Event example
Server
MakeMoveCommand{:game 123 :player "player-2" :move "paper"}
MoveMadeEvent{:game 123 :player "player-1" :move "rock"}
MoveMadeEvent{:game 123 :player "player-2" :move "paper"}
GameWonEvent{:game 123 :winner "player-2" :loser "player-1"}
EventStore

GameCreatedEvent{:game 123, :creator "player-1", :time 1}
MoveMadeEvent{:game 123, :player "player-1", :move "rock", :time 2}
Current state?

CREATED WAITING
GAME WON
GAME TIED
any move
if victory
other move
if tie
Current state?

CREATED WAITING
GAME WON
GAME TIED
any move
if victory
other move
if tie
GameCreatedEvent
Current state?
CREATED

CREATED WAITING
GAME WON
GAME TIED
any move
if victory
other move
if tie
Current state?
WAITING
MoveMadeEvent

Aggregates
Scope of consistency when handling a command
Game
+ id
+ state
+ players
+ moves
+ winner
+ loser
Move
+ id
+ move
+ player
Player
+ id
+ name
+ email

Aggregates as Event Streams
event1 GameCreatedEvent{:game 1...}
event2 MoveMadeEvent{:game 1....}
stream = game-1
version = 2

stream = game-1
version = 2
event4 GameWonEvent{:game 2....}
stream = game-2
version = 4

stream = game-1
version = 2
event4 GameWonEvent{:game 2....}
stream = game-2
version = 4
curl "http://127.0.0.1:2113/streams/game-1"
-d @eventdata.json
-H "Content-Type:application/json"
-H "ES-ExpectedVersion: 2"

Projections
GameCreatedEvent{:game 1...}
MoveMadeEvent{:game 1....}
GameWonEvent{:game 2....}

Projections
for (Event events : allEvents) {
if (event instanceof GameCreatedEvent) {
created++;
}
}
return created;

Projections
created = 2

Projections
(state, event) -> {
if (event instanceof GameCreatedEvent) {
return state++;
}
rerturn state;
}
created = 2

Projections in Javascript
fromAll()
.when({
$init: function () {
return { created: 0, completed:0 };
},
});

fromAll()
.when({
},
});
GameCreatedEvent: function(s, e) {
s.created++;
return s;
},

fromAll()
.when({
},
});
s.created++;
return s;
},
GameWonEvent: function(s, e) {
s.completed++;
return s;
},

fromAll()
.when({
},
});
s.created++;
return s;
},
s.completed++;
return s;
},
GameTiedEvent: function(s, e) {
s.completed++;
return s;
}

fromAll()
.when({
},
});
s.created++;
return s;
},
s.completed++;
return s;
},
s.completed++;
return s;
}
/projections/games-counter
{
created: 15,
completed: 10
}

External Projections
events
Business logic

events events
Business logic Projections

Consistent
Available
events events
Business logic Projections

Commands Queries
Consistent
Available
events events

System with EventStore

Command
Handler

1.command Command
Handler

1.command Event
Store
Command
Handler
2.get events

1.command Event
Store
Command
Handler
2.get events
3.add events

1.command Event
Store
Command
Handler
2.get events
3.add events
4.update
projections

1.command Event
Store
Command
Handler
2.get events
3.add events
4.update
projections
5.get events
or projections
Some
service

1.command Event
Store
Command
Handler
2.get events
6.update state
3.add events
4.update
projections
5.get events
or projections
Some
service

1.command Event
Store
Command
Handler
2.get events
6.update state
3.add events
4.update
projections
7.query
5.get events
or projections
Some
service

Datomic

http://www.datomic.com/
Runs on JVM
Free (3 peers, local storage) or Commercial (HA, distrib)
Features
ACID transactions, declarative query engine
€2300

Fact
Something known to have happened or existed

Fact
2004-10-01 the email of Jan was ”jan.kronquist@jayway.se”

Fact
2004-10-01 the email of Jan was ”jan.kronquist@jayway.se”
2007-01-01 the email of Jan was ”jan.kronquist@jayway.com”

Atomic fact (datom)
Entity
Attribute
Value
Time

Atomic fact (datom)
Entity
Attribute
Value
Time
The person Jan Kronquist
email
”jan.kronquist@jayway.se”
2004-10-01

Data perception
Facts
Add facts
Add more facts
Even more facts
time

Data perception
Facts
Add facts
Add more facts
Even more facts
time
query

Data perception
Facts
Add facts
Add more facts
Even more facts
time
query
query

Database deconstructed

Transactor
Consistency by serializing transactions

Transactor
Peer
Your code + Query engine

Transactor
Peer
Your code + Query engine
Storage service
Key/value store
Scalable reads and writes
eg DynamoDB, InﬁniSpan, Riak + ZooKeeper, SQL(!)

System with Datomic
Transactor
Distributed Storage Service
Peer APeer A

System with Datomic
Transactor
Peer APeer A
1.command

System with Datomic
Transactor
Peer APeer A
2.read and cache
1.command

System with Datomic
Transactor
Peer A
3.transact
Peer A
2.read and cache
1.command

System with Datomic
Transactor
Peer A
3.transact
Peer A
4.write
2.read and cache
1.command

System with Datomic
Transactor
Peer A
3.transact
Peer A
4.write
2.read and cache
Peer B
5.notify
1.command

Datomic example
Peer Transactor

Datomic example
Peer
Transactor

Datomic example
Peer
[123 :creator "player-1" T1]
[123 :state :created T1]
Transactor

Datomic example
Peer
Transactor
["player-1" :email "player@gmail.com" T0]

Datomic example
Peer
Transactor

Datomic example
Peer
MakeMoveCommand{:game 123 :player "player-1" :move :rock}
[123_1 :move :rock T2]
[123_1 :player "player-1" T2]
[123 :moves 123_1 T2]
[123 :state :waiting T2]
Transactor

Datomic example
Peer
MakeMoveCommand{:game 123 :player "player-2" :move :paper}
[123_1 :move :rock T2]
[123 :moves 123_1 T2]
[123 :state :waiting T2]
[123_2 :move :paper T3]
[123 :moves 123_2 T3]
[123 :state :won T3]
[123 :winner "player-2" T3]
[123 :loser "player-1" T3]
Transactor

Aggregates?
Only by convention
cas (compare and set)
isComponent on relation
Game
+ id
+ state
+ players
+ moves
+ winner
+ loser
Move
+ id
+ move
+ player

What to ﬁnd
Where clauses [entity attribute value]
Implicit joins
Call arbitrary function
Query language

Query example - basic
[:find ?email
:where
[_ :email ?email]]

Query example - basic
[:find ?email
:where
[_ :email ?email]]
[["jan.kronquist@jayway.com]
["player-1@gmail.com"]
["player-2@gmail.com"]]

Query example - aggregation
[:find ?state (?count game)
:where
[?game :state ?state]]

:where
[[:created 4]
[:waiting 1]
[:won 7]
[:tied 3]]

:where
[[:created 4]
[:waiting 1]
[:won 7]
[:tied 3]]
inProgess = created + waiting

:where
[[:created 4]
[:waiting 1]
[:won 7]
[:tied 3]]
inProgess = created + waiting
fromAll()
.when({
},
s.created++;
return s;
},
s.completed++;
return s;
},
s.completed++;
return s;
}
});
/projections/games-counter
{
created: 15,
completed: 10
}

Query example - join
[:find ?email (count ?game)
:where
[?player :email ?email]
[?game :winner ?player]]

Query at any time
(q ’[:find ?email
:where
[_ :email ?email]]
db)

Query at any time
(q ’[:find ?email
:where
[_ :email ?email]]
db)
(q ’[:find ?email
:where
[_ :email ?email]]
(as-of db #inst "2013-09-02"))

Query over time
(q ’[:find ?e (count ?tx)
:where
[?e :email _ ?tx]]
db)

Query over time
:where
[?e :email _ ?tx]]
db)
:where
[?e :email _ ?tx]]
(history db))

Java API
List<Object> results =
Peer.q(someQuery, conn.db());
Future<Map> txResult = conn.transact(data_tx);

Comparison

Comparison
Data model
Schema evolution
Queries
Recommendation

EventStore Events
✓ Understandable by normal people
✓ Designed for integration
✓ Forces aggregate design (scalability)
✓ Capture intent
๏ Require design effort to get right
๏ Complect transaction and query

Datomic Facts
✓ Support partial information
✓ Normalization support
✓ CRUD out of the box
๏ Static schema

✓ New events
✓ New event attributes
✓ Event attributes rename (change deserialization)
✓ Projected state change (rebuild projection)
๏ Cross event refactoring
EventStore schema evolution

✓ New or removed attributes
✓ New or removed entity types
✓ Move attributes between entities
๏ Fact attributes rename not possible
๏ Fact attribute type change not possible
Datomic schema evolution

✓ Projections use JavaScript
✓ Persistent
๏ Require projection
EventStore queries

✓ Declarative
✓ Logic-based
✓ Allows calling out to your own code
๏ New language to learn
Datomic queries

When to use Event Sourcing?
Domains where events seem natural
Different teams with different models

When to use Datomic?
Data can be modelled using entities and attributes
History is or may be interesting

More details
http://www.jayway.com/author/jankronquist/
https://github.com/jankronquist

Questions?

JavaZone 2013 - Datomic vs EventStore

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